SUMMARY

This summary highlights selected information included in this joint proxy statement/prospectus and does not contain all of the information that may be important to you. You should read this joint proxy statement/prospectus and its annexes carefully and in their entirety and the other documents to which Diamondback and Energen refer before you decide how to vote with respect to the proposals to be considered and voted on at the special meeting for your company. In addition, Diamondback and Energen incorporate by reference important business and financial information about Diamondback and Energen into this joint proxy statement/prospectus, as further described in the sections entitled “Where You Can Find More Information” and “Information Incorporated by Reference,” each beginning on page 196. You may obtain the information incorporated by reference into this joint proxy statement/prospectus without charge by following the instructions in the sections entitled “Where You Can Find More Information” and “Information Incorporated by Reference,” each beginning on page 196. Each item in this summary includes a page reference directing you to a more complete description of that item in this joint proxy statement/prospectus.

Information About the Companies (page 57)

Diamondback Energy, Inc.

500 West Texas

Suite 1200

Midland, Texas 79701

Phone: (432) 221-7400

Diamondback Energy, Inc. was incorporated in Delaware on December 30, 2011. Based in Midland, Texas, Diamondback is an independent oil and natural gas company focused on the acquisition, development, exploration and exploitation of unconventional, onshore oil and natural gas reserves in the Permian Basin in West Texas. Diamondback’s activities are primarily directed at the horizontal development of the Wolfcamp and Spraberry formations in the Midland Basin and the Wolfcamp and Bone Spring formations in the Delaware Basin. Diamondback intends to continue to develop its reserves and increase production through development drilling and exploitation and exploration activities on its multi-year inventory of identified potential drilling locations and through acquisitions that meet its strategic and financial objectives, targeting oil-weighted reserves. Substantially all of its revenues are generated through the sale of oil, natural gas liquids and natural gas production.

Energen Corporation

605 Richard Arrington Jr. Boulevard North

Birmingham, Alabama 35203

Phone: (205) 326-2700

Energen is an oil and natural gas exploration and production company engaged in the exploration, development and production of oil, natural gas liquids and natural gas. Energen’s operations are conducted through its subsidiary, Energen Resources Corporation (which we refer to as “Energen Resources”) and primarily occur within the Midland Basin, the Delaware Basin and the Central Basin Platform areas of the Permian Basin in west Texas and New Mexico. Energen was incorporated in Alabama in 1978 in connection with a corporate reorganization which resulted in Energen becoming the parent company to a predecessor company of Energen Resources. Energen’s corporate headquarters are based in Birmingham, Alabama.

Sidewinder Merger Sub Inc.

c/o Diamondback Energy, Inc.

500 West Texas

Suite 1200

Midland, Texas 79701

Phone: (432) 221-7400

Sidewinder Merger Sub Inc., or Merger Sub, is a direct, wholly owned subsidiary of Diamondback. Upon the completion of the merger, Merger Sub will cease to exist. Merger Sub was incorporated in Alabama on August 14, 2018 for the sole purpose of effecting the merger.

The Merger and the Merger Agreement (page 71)

The terms and conditions of the merger are contained in the merger agreement, which is attached to this document as Annex A and is incorporated by reference herein in its entirety. Diamondback and Energen encourage you to read the merger agreement carefully and in its entirety, as it is the legal document that governs the merger.

The Diamondback board of directors and the Energen board of directors each has unanimously approved and adopted the merger agreement and the transactions contemplated by the merger agreement. Pursuant to the terms and subject to the conditions included in the merger agreement, Diamondback has agreed to acquire Energen by means of a merger of Merger Sub with and into Energen, with Energen surviving the merger as a wholly owned subsidiary of Diamondback.

Merger Consideration (page 133)

As a result of the merger, each eligible share of Energen common stock (other than shares held in treasury by Energen, shares owned by Diamondback or Merger Sub and, in each case, not held on behalf of third parties (which we refer to collectively as the “cancelled shares”), or shares with respect to which dissenters’ rights have been validly exercised in accordance with Alabama law (which we refer to as the “dissenting shares”)) issued and outstanding immediately prior to the effective time of the merger will be converted into the right to receive 0.6442 of a share of Diamondback common stock, with cash in lieu of any fractional shares (which we refer to as the “merger consideration”).

Energen shareholders will not be entitled to receive any fractional shares of Diamondback common stock in the merger, and no Energen shareholders will be entitled to dividends, voting rights or any other rights in respect of any fractional shares of Diamondback common stock. Each holder of shares of Energen common stock exchanged pursuant to the merger who would otherwise have been entitled to receive a fraction of a share of Diamondback common stock (after taking into account all certificates and book-entry shares delivered by such holder) shall receive, in lieu thereof, cash (without interest) in an amount equal to the product of (i) such fractional part of a share of Diamondback common stock multiplied by (ii) the volume weighted average price of Diamondback common stock for the five consecutive trading days ending on the date that is two business days prior to the closing date as reported by The Wall Street Journal.

Risk Factors (page 43)

The merger and an investment in Diamondback common stock involve risks, some of which are related to the transactions contemplated by the merger agreement. You should carefully consider the information about these risks set forth under the section entitled “Risk Factors” beginning on page 43, together with the other information included or incorporated by reference in this joint proxy statement/prospectus, particularly the risk

factors contained in Diamondback’s and Energen’s Annual Reports on Form 10-K, Quarterly Reports on Form 10-Q, Current Reports on Form 8-K and other filings they make with the SEC. Energen shareholders should carefully consider the risks set out in that section before deciding how to vote with respect to the merger proposal and non-binding compensation advisory proposal to be considered and voted on at the Energen special meeting, and Diamondback stockholders should carefully consider the risks set out in that section before deciding how to vote with respect to the Diamondback issuance proposal to be considered and voted on at the Diamondback special meeting. For additional information, see the sections entitled “Where You Can Find More Information” and “Information Incorporated by Reference,” each beginning on page 196.

Treatment of Energen Equity Awards (page 134)

Energen Options

At the effective time of the merger, each option to purchase shares of Energen common stock that is outstanding immediately prior to the effective time of the merger shall be converted into a fully vested option to purchase (i) that number of whole shares of Diamondback common stock (rounded down to the nearest whole share) equal to the product of (A) the total number of shares of Energen common stock subject to such option immediately prior to the effective time of the merger multiplied by (B) the exchange ratio, (ii) at an exercise price per share of Diamondback common stock (rounded up to the nearest whole cent) equal to the quotient of (A) the exercise price per share of Energen common stock of such option immediately prior to the effective time divided by (B) the exchange ratio.

Energen SARs

At the effective time of the merger, each outstanding stock appreciation right in respect of Energen common stock that is outstanding immediately prior to the effective time of the merger shall be converted into a fully vested stock appreciation right in respect of (i) that number of whole shares of Diamondback common stock (rounded down to the nearest whole share) equal to the product of (A) the total number of shares of Energen common stock subject to such stock appreciation right immediately prior to the effective time of the merger multiplied by (B) the exchange ratio, (ii) at an exercise price per share of Diamondback common stock (rounded up to the nearest whole cent) equal to the quotient of (A) the exercise price per share of Energen common stock of such stock appreciation right immediately prior to the effective time of the merger divided by (B) the exchange ratio.

Energen RSU Awards

At the effective time of the merger, each outstanding time-vesting RSU award in respect of shares of Energen common stock that is outstanding immediately prior to the effective time of the merger shall be converted into an award of RSUs in respect of that number of whole shares of Diamondback common stock (rounded to the nearest whole share) equal to the product of (i) the total number of shares of Energen common stock subject to such time-vesting RSU award immediately prior to the effective time of the merger multiplied by (ii) the exchange ratio.

Accelerating Energen Performance Share Awards

At the effective time of the merger, each performance share award in respect of shares of Energen common stock subject to a performance period scheduled to terminate on December 31, 2018 (each, an “accelerating performance share award”) that is outstanding immediately prior to the effective time of the merger shall fully vest and shall be cancelled and converted automatically into the right to receive the merger consideration in respect of each share of Energen common stock underlying such accelerating performance share award, less

applicable employment and withholding tax. The accelerating performance share awards will be deemed to be achieved at the greater of the target level and the actual level of performance as of the effective time of the merger, as determined by Energen’s compensation committee prior to the effective time of the merger.

Rollover Energen Performance Share Awards

At the effective time of the merger, each performance share award in respect of shares of Energen common stock that is outstanding immediately prior to the effective time of the merger (other than an accelerating performance share award) shall be converted into an award of RSUs in respect of that number of whole shares of Diamondback common stock (rounded to the nearest whole share) equal to the product of (i) the total number of shares of Energen common stock subject to such performance share award immediately prior to the effective time of the merger multiplied by (ii) the exchange ratio. The performance share awards will be deemed to be achieved at the greater of the target level and the actual level of performance as of the effective time of the merger, as determined by Energen’s compensation committee prior to the effective time of the merger and will be subject solely to time-based vesting over the originally scheduled vesting period of the corresponding performance share awards.

Recommendation of the Diamondback Board of Directors and Diamondback’s Reasons for the Merger (page 76)

The Diamondback board unanimously recommends that you vote “FOR” the Diamondback issuance proposal. For the factors considered by the Diamondback board in reaching this decision and additional information on the recommendation of the Diamondback board, see the section entitled “The Merger—Recommendation of the Diamondback Board of Directors and Diamondback’s Reasons for the Merger” beginning on page 76.

Recommendation of the Energen Board of Directors and Energen’s Reasons for the Merger (page 90)

The Energen board unanimously recommends that you vote “FOR” the merger proposal and “FOR” the non-binding compensation advisory proposal. For the factors considered by the Energen board in reaching this decision and additional information on the recommendation of the Energen board, see the section entitled “The Merger—Recommendation of the Energen Board of Directors and Energen’s Reasons for the Merger” beginning on page 90.

Opinions of Financial Advisors (page 80)

Opinion of Citigroup Global Markets Inc., Diamondback’s financial advisor

In connection with the merger, Diamondback’s financial advisor, Citigroup Global Markets Inc. (which we refer to as “Citi”), delivered a written opinion, dated August 14, 2018, to the Diamondback board of directors as to the fairness, from a financial point of view and as of the date of the opinion, to Diamondback of the exchange ratio provided for pursuant to the merger agreement.

The full text of Citi’s written opinion, dated August 14, 2018, which describes the assumptions made, procedures followed, matters considered and limitations and qualifications on the review undertaken, is attached as Annex B to this joint proxy statement/prospectus and is incorporated into this joint proxy statement/prospectus by reference. The description of Citi’s opinion set forth in this joint proxy statement/prospectus is qualified in its entirety by reference to the full text of Citi’s opinion. Citi’s opinion was provided for the information of the Diamondback board of directors (in its capacity as such) in connection with its evaluation of the exchange ratio from a financial point of view to Diamondback and did not address any other terms, aspects or implications of the merger. Citi expressed no view as to, and its opinion did not address, the underlying business decision of Diamondback to effect or enter into the merger, the relative merits of the merger as compared to any alternative business strategies that might

exist for Diamondback or the effect of any other transaction which Diamondback might engage in or consider. Citi’s opinion is not intended to be and does not constitute a recommendation to any securityholder as to how such securityholder should vote or act on any matters relating to the proposed merger or otherwise. For further information, see the section of this joint proxy statement/prospectus entitled “The Merger—Opinion of Citi, Diamondback’s Financial Advisor” and Annex B.

Opinion of J.P. Morgan, Energen’s financial advisor

The Energen board retained J.P. Morgan Securities LLC (which we refer to as “J.P. Morgan”) as financial advisor to Energen in connection with the merger. At the meeting of the Energen board on August 14, 2018, J.P. Morgan rendered its oral opinion to the Energen board that, as of such date and based upon and subject to the factors and assumptions set forth in its opinion, the exchange ratio in the merger was fair, from a financial point of view, to the holders of Energen common stock. J.P. Morgan has confirmed its August 14, 2018 oral opinion by delivering its written opinion to the Energen board, dated August 14, 2018, that, as of such date, the exchange ratio in the merger was fair, from a financial point of view, to the holders of Energen common stock.

The full text of the written opinion of J.P. Morgan dated August 14, 2018, which sets forth the assumptions made, matters considered and limits on the review undertaken, is attached as Annex C to this joint proxy statement/prospectus and is incorporated herein by reference. The summary of the opinion of J.P. Morgan set forth in this joint proxy statement/prospectus is qualified in its entirety by reference to the full text of such opinion. Energen’s shareholders are urged to read the opinion in its entirety. J.P. Morgan’s written opinion was addressed to the Energen board (in its capacity as such) in connection with and for the purposes of its evaluation of the merger, was directed only to the exchange ratio in the merger and did not address any other aspect of the merger. J.P. Morgan expressed no opinion as to the fairness of any consideration to be paid in connection with the merger to the holders of any other class of securities, creditors or other constituencies of Energen or as to the underlying decision by Energen to engage in the merger. The issuance of J.P. Morgan’s opinion was approved by a fairness committee of J.P. Morgan. The opinion does not constitute a recommendation to any shareholder of Energen as to how such shareholder should vote with respect to the merger or any other matter. For a description of the opinion that the Energen board received from J.P. Morgan, see “The Merger—Opinion of J.P. Morgan, Energen’s Financial Advisor” and Annex C.

Opinion of TPH, Energen’s financial advisor

The Energen board engaged Tudor Pickering Holt & Co Advisors LP (which we refer to as “TPH”) to act as Energen’s financial advisor in connection with the merger. On August 14, 2018, at a meeting of the Energen board held to evaluate the merger, TPH rendered its oral opinion, subsequently confirmed in writing, that, as of August 14, 2018 and based on and subject to the assumptions, limitations and qualifications set forth in the opinion and based on other matters as TPH considered relevant, the exchange ratio in the merger was fair, from a financial point of view, to the holders of the Eligible Shares (as defined under “The Merger—Opinion of TPH, Energen’s Financial Advisor”) of Energen.

The full text of TPH’s written opinion, dated August 14, 2018, is attached as Annex D to this joint proxy statement/prospectus and is incorporated herein by reference in its entirety. We encourage you to read the opinion carefully in its entirety for a description of, among other things, the assumptions made, procedures followed, factors considered and qualifications and limitations on the review undertaken. TPH’s opinion was provided to the Energen board in connection with the board’s consideration of the transactions, does not address any other aspect of the merger agreement or the transactions and does not constitute a recommendation as to how any holder of interests in Energen should vote with respect to the

transactions or any other matter. For further information, see the section of this joint proxy statement/prospectus entitled “The Merger—Opinion of TPH, Energen’s Financial Advisor” and Annex D.

Special Meeting of Diamondback Stockholders (page 58)

Date, Time, Place and Purpose of the Diamondback Special Meeting

The Diamondback special meeting will be held on                , 2018, at                , Central Time, at                 . The purpose of the Diamondback special meeting is to consider and vote on the Diamondback issuance proposal. Approval of the Diamondback issuance proposal by Diamondback stockholders is a condition to the obligation of Diamondback and Energen to complete the merger.

Record Date and Outstanding Shares of Diamondback Common Stock

Only holders of record of issued and outstanding shares of Diamondback common stock as of the close of business on                , 2018 (which we refer to as the “Diamondback record date”) , are entitled to notice of, and to vote at, the Diamondback special meeting or any adjournment or postponement of the Diamondback special meeting.

As of the close of business on the Diamondback record date, there were                shares of Diamondback common stock issued and outstanding and entitled to vote at the Diamondback special meeting. You may cast one vote for each share of Diamondback common stock that you held as of the close of business on the Diamondback record date.

A complete list of Diamondback stockholders entitled to vote at the Diamondback special meeting will be available for inspection at Diamondback’s principal place of business during regular business hours for a period of no less than 10 days before the Diamondback special meeting and during the Diamondback special meeting at                 .

Quorum; Abstentions and Broker Non-Votes

A quorum of Diamondback stockholders is necessary for Diamondback to hold a valid meeting. The presence at the Diamondback special meeting, in person or by proxy, of the holders of a majority of the outstanding shares of Diamondback common stock entitled to vote at the Diamondback special meeting constitutes a quorum.

If you submit a properly executed proxy card, even if you do not vote for the proposal or vote to “abstain” in respect of the proposal, your shares of Diamondback common stock will be counted for purposes of determining whether a quorum is present for the transaction of business at the Diamondback special meeting. Diamondback common stock held in “street name” with respect to which the beneficial owner fails to give voting instructions to the broker, bank or other nominee, and Diamondback common stock with respect to which the beneficial owner otherwise fails to vote, will not be considered present and entitled to vote at the Diamondback special meeting for the purpose of determining the presence of a quorum.

A broker non-vote will result if your broker, bank or other nominee returns a proxy but does not provide instruction as to how shares should be voted on a particular matter. It is not expected that there will be any broker non-votes at the Diamondback special meeting. However, if there are any broker non-votes, the shares will not be considered present and entitled to vote at the Diamondback special meeting for the purpose of determining the presence of a quorum.

Executed but unvoted proxies will be voted in accordance with the recommendation of the Diamondback board.

Required Vote to Approve the Diamondback Issuance Proposal

Approval of the Diamondback issuance proposal requires the affirmative vote of a majority of votes cast by Diamondback stockholders present in person or by proxy at the Diamondback special meeting and entitled to vote on this proposal. Abstentions, failure to vote in person or by proxy and broker non-votes will not be counted as votes cast “FOR” or “AGAINST” this proposal and, as a result, will have no effect on the outcome of the vote on this proposal.

The Diamondback issuance proposal is described in the section entitled “Diamondback Proposal” beginning on page 63.

Voting by Diamondback Directors and Executive Officers

As of the Diamondback record date, Diamondback directors and executive officers, and their affiliates, as a group, owned and were entitled to vote                  shares of Diamondback common stock, or approximately         % of the total outstanding shares of Diamondback common stock as of the Diamondback record date.

Diamondback currently expects that all of its directors and executive officers will vote their shares “FOR” the Diamondback issuance proposal.

Adjournment

If a quorum is not present or if there are not sufficient votes for the approval of the Diamondback issuance proposal, Diamondback expects that the Diamondback special meeting will be adjourned by the chairman of the Diamondback special meeting to solicit additional proxies. At any subsequent reconvening of the Diamondback special meeting, all proxies will be voted in the same manner as the manner in which such proxies would have been voted at the original convening of the Diamondback special meeting, except for any proxies that have been validly revoked or withdrawn prior to the subsequent meeting.

Special Meeting of Energen Shareholders (page 64)

Date, Time, Place and Purpose of the Energen Special Meeting

The Energen special meeting will be held on                 , 2018, at                 , Central Time, at                 . The purpose of the Energen special meeting is to consider and vote on the merger proposal and the non-binding compensation advisory proposal. Approval of the merger proposal is a condition to the obligation of Energen and Diamondback to complete the merger. Approval of the non-binding compensation advisory proposal is not a condition to the obligation of either Energen or Diamondback to complete the merger.

Record Date and Outstanding Shares of Energen Common Stock

Only holders of record of issued and outstanding shares of Energen common stock as of the close of business on                 , 2018 (which we refer to as the “Energen record date”) are entitled to notice of, and to vote at, the Energen special meeting or any adjournment or postponement of the Energen special meeting.

As of the close of business on the Energen record date, there were                  shares of Energen common stock issued and outstanding and entitled to vote at the Energen special meeting. You may cast one vote for each share of Energen common stock that you held as of the close of business on the Energen record date.

A complete list of Energen shareholders entitled to vote at the Energen special meeting will be available for inspection at Energen’s principal office at 605 Richard Arrington Jr. Boulevard North, Birmingham, Alabama 35203-2707 during regular business hours beginning two business days after notice of the meeting is given and continuing through the Energen special meeting and during the Energen special meeting.

Quorum; Abstentions and Broker Non-Votes

A quorum of Energen shareholders is necessary for Energen to hold a valid meeting. The presence of the holders of a majority of the outstanding shares of Energen common stock entitled to vote at the special meeting, present in person or represented by proxy, constitutes a quorum.

If you submit a properly executed proxy card, even if you do not vote for the proposal or vote to “abstain” in respect of the proposal, your shares of Energen common stock will be counted for purposes of determining whether a quorum is present for the transaction of business at the Energen special meeting. Energen common stock held in “street name” with respect to which the beneficial owner fails to give voting instructions to the broker, bank or other nominee, and Energen common stock with respect to which the beneficial owner otherwise fails to vote, will not be considered present and entitled to vote at the Energen special meeting for the purpose of determining the presence of a quorum. A broker non-vote will result if your broker, bank or other nominee returns a proxy but does not provide instruction as to how shares should be voted on a particular matter. It is not expected that there will be any broker non-votes at the Energen special meeting. However, if there are any broker non-votes, the shares will not be considered present and entitled to vote at the Energen special meeting for the purpose of determining the presence of a quorum.

Executed but unvoted proxies will be voted in accordance with the recommendations of the Energen board.

Required Vote to Approve the Merger Proposal

Approval of the merger proposal requires the affirmative vote of two-thirds of the outstanding shares of Energen common stock entitled to vote on the proposal. Abstentions and broker non-votes will have the same effect as a vote “AGAINST” the merger proposal. Failure to vote on the merger proposal will also have the same effect as a vote “AGAINST” the merger proposal.

The merger proposal is described in the section entitled “Energen Proposals” beginning on page 69.

Required Vote to Approve the Non-Binding Compensation Advisory Proposal

Approval of the non-binding compensation advisory proposal requires the affirmative vote of a majority of the shares of Energen common stock present in person or represented by proxy at the Energen special meeting and entitled to vote on the proposal. Abstentions will have the same effect as a vote “AGAINST” the proposal, and broker non-votes will have no effect on the outcome of the vote.

The non-binding compensation advisory proposal is described in the section entitled “Energen Proposals” beginning on page 69.

Voting by Energen Directors and Executive Officers

As of the Energen record date, Energen directors and executive officers, and their affiliates, as a group, owned and were entitled to vote                 shares of Energen common stock, or approximately         % of the total outstanding shares of Energen common stock as of the Energen record date.

Energen currently expects that all of its directors and executive officers will vote their shares “FOR” the merger proposal and “FOR” the non-binding compensation advisory proposal.

Adjournment

If a quorum is not present or if there are not sufficient votes for the approval of the merger proposal, Energen expects that the Energen special meeting will be adjourned by the chairman of the meeting to solicit additional proxies in accordance with the merger agreement. At any subsequent reconvening of the Energen special meeting, all proxies will be voted in the same manner as the manner in which such proxies would have been voted at the original convening of the Energen special meeting, except for any proxies that have been validly revoked or withdrawn prior to the subsequent reconvening of the Energen special meeting.

Interests of Diamondback Directors and Executive Officers in the Merger (page 123)

In considering the recommendation of the Diamondback board with respect to the Diamondback issuance proposal, Diamondback stockholders should be aware that the directors and executive officers of Diamondback have interests in the merger that may be different from, or in addition to, the interests of Diamondback stockholders generally. These interests are described in more detail in the section entitled “The Merger—Interests of Diamondback Directors and Executive Officers in the Merger” beginning on page 123. The members of the Diamondback board were aware of and considered these interests, among other matters, in evaluating and negotiating the merger agreement, in approving the merger agreement and in determining to recommend that Diamondback stockholders approve the Diamondback issuance proposal.

Board of Directors and Management of Diamondback Following Completion of the Merger (page 123)

Upon closing of the merger, Diamondback’s board and executive management will remain unchanged. Additionally, Diamondback will continue to be headquartered in Midland, Texas.

Interests of Energen Directors and Executive Officers in the Merger (page 123)

In considering the recommendation of Energen’s board of directors, Energen shareholders should be aware that the directors and executive officers of Energen have certain interests in the merger that may be different from, or in addition to, the interests of Energen shareholders generally. Energen’s board of directors was aware of these interests and considered them, among other matters, in making its recommendation that Energen shareholders vote to approve the merger proposal.

These interests include:

For a more complete description of these interests, see “The Merger—Interests of Energen Directors and Executive Officers in the Merger.”

Conditions to the Completion of the Merger (page 161)

Each party’s obligation to complete the merger is subject to the satisfaction or waiver of the following mutual conditions:

The obligations of Diamondback and Merger Sub to complete the merger are subject to the satisfaction or waiver of further conditions, including:

The obligation of Energen to complete the merger is subject to the satisfaction or waiver of the following additional conditions:

No Solicitation (page 150)

No Solicitation by Diamondback

Diamondback has agreed that, from and after August 14, 2018, Diamondback will, and will cause its subsidiaries, and its and their respective directors, officers and representatives to, immediately cease, and cause to be terminated, any solicitation, encouragement, discussion or negotiations that commenced prior to and were ongoing as of August 14, 2018 with respect to a Diamondback competing proposal (as defined in the section entitled “The Merger Agreement—No Solicitation; Changes of Recommendation—Definitions of Competing Proposals” beginning on page 152).

Diamondback has also agreed that, from and after August 14, 2018 until the effective time of the merger or the termination of the merger agreement in accordance with the terms thereof, Diamondback will not, and will cause its affiliates and subsidiaries, and its and their respective directors and officers, and will cause its representatives not to, directly or indirectly:

Notwithstanding the agreements described above, prior to, but not after, the time the issuance proposal has been approved by Diamondback stockholders, Diamondback may engage in the first, second and third bullets directly above with any person if Diamondback receives a bona fide written Diamondback competing proposal that did not arise from a breach of the obligations described directly above and in the section entitled “The Merger Agreement—No Solicitation; Changes of Recommendation—Definitions of Competing Proposals” beginning on page 152; provided, however, that:

Notwithstanding the agreements described above, prior to, but not after, the time the issuance proposal has been approved by Diamondback stockholders, Diamondback may seek clarification from (but not engage in negotiations with or provide non-public information to) any person that has made a Diamondback competing proposal solely to clarify and understand the terms and conditions of such proposal to provide adequate information for the Diamondback board of directors or any committee thereof to make an informed determination under the relevant provisions of the merger agreement.

No Solicitation by Energen

Energen has agreed that, from and after August 14, 2018, Energen will, and will cause its subsidiaries, and its and their respective directors, officers and representatives to, immediately cease, and cause to be terminated, any solicitation, encouragement, discussion or negotiations that commenced prior to and were ongoing as of August 14, 2018 with respect to an Energen competing proposal (as defined in the section entitled “The Merger Agreement—No Solicitation; Changes of Recommendation—Definitions of Competing Proposals” beginning on page 152).

Energen has also agreed that, from and after August 14, 2018 until the effective time of the merger or the termination of the merger agreement in accordance with the terms thereof, Energen will not, and will cause its affiliates and subsidiaries, and its and their respective directors and officers, and will cause its representatives not to, directly or indirectly:

Notwithstanding the agreements described above, prior to, but not after, the time the merger proposal has been approved by Energen shareholders, Energen may engage in the first, second and third bullets directly above with any person if Energen receives a bona fide written Energen competing proposal that did not arise from a breach of the obligations described directly above and in the section entitled “The Merger Agreement—No Solicitation; Changes of Recommendation—Definitions of Competing Proposals” beginning on page 152; provided, however, that:

Notwithstanding the agreements described above, prior to, but not after, the time the merger proposal has been approved by Energen shareholders, Energen or any of its representatives may seek clarification from any person that has made an Energen competing proposal solely to clarify and understand the terms and conditions of such proposal to provide adequate information for Energen board of directors or any committee thereof to make an informed determination under the relevant provisions of the merger agreement.

Changes of Recommendation (page 145)

Diamondback Restrictions on Changes of Recommendation

Subject to certain exceptions described below, the Diamondback board of directors may not effect a Diamondback recommendation change (as defined in the section entitled “The Merger Agreement—No Solicitation; Changes of Recommendation—Diamondback: Restrictions on Changes of Recommendation” beginning on page 148).

Energen Restrictions on Changes of Recommendation

Subject to certain exceptions described below, the Energen board of directors may not effect an Energen recommendation change (as defined in the section entitled “The Merger Agreement—No Solicitation; Changes of Recommendation—Energen: Restrictions on Changes of Recommendation” beginning on page 148).

Diamondback: Permitted Changes of Recommendation and Permitted Termination to Enter into a Diamondback Superior Proposal

Prior to, but not after, the time that the Diamondback issuance proposal has been approved by Diamondback stockholders, in response to a bona fide written Diamondback competing proposal from a third party that is conditioned upon the termination of the merger agreement or the failure of the transactions contemplated thereby to be consummated and that did not arise from a breach of the “no solicitation” obligations described above and in the section entitled “The Merger Agreement—No Solicitation; Changes of Recommendation—No Solicitation by Diamondback,” the Diamondback board of directors may effect a Diamondback recommendation change or terminate the merger agreement if:

Diamondback: Permitted Changes of Recommendation in Connection with Intervening Events

Prior to, but not after, the time that the Diamondback issuance proposal has been approved by Diamondback stockholders, if a Diamondback intervening event (as defined in the section entitled “The Merger Agreement—No Solicitation; Changes of Recommendation—Diamondback: Permitted Changes of Recommendation in Connection with Intervening Events” beginning on page 149) occurs or arises after August 14, 2018, Diamondback may effect a Diamondback recommendation change if:

Energen: Permitted Changes of Recommendation and Permitted Termination to Enter into an Energen Superior Proposal

Prior to, but not after, the merger proposal has been approved by Energen shareholders, in response to a bona fide written Energen competing proposal from a third party that did not arise from a breach of the “no solicitation” obligations described above and in the section entitled “The Merger Agreement—No Solicitation; Changes of Recommendation—No Solicitation by Energen,” the Energen board of directors may effect an Energen recommendation change or terminate the merger agreement if:

Energen: Permitted Changes of Recommendation in Connection with Intervening Events

Prior to, but not after, the time the merger proposal has been approved by Energen shareholders, in response to an Energen intervening event (as defined in the section entitled “The Merger Agreement—No Solicitation; Changes of Recommendation—Energen: Permitted Changes of Recommendation in Connection with Intervening Events” beginning on page 151) that occurs or arises after August 14, 2018, Energen may effect an Energen recommendation change if:

Termination (page 163)

Diamondback and Energen may terminate the merger agreement and abandon the merger at any time prior to the effective time of the merger by mutual written consent of Diamondback and Energen.

The merger agreement may also be terminated by either Diamondback or Energen at any time prior to the effective time of the merger in any of the following situations:

In addition, the merger agreement may be terminated by Diamondback:

Further, the merger agreement may be terminated by Energen:

Expenses and Termination Fees (page 164)

Termination Fees Payable by Diamondback

The merger agreement requires Diamondback to pay Energen a termination fee of $400 million (which we refer to as the “reverse termination fee”) if:

In no event will Diamondback be required to pay the reverse termination fee on more than one occasion.

Termination Fees Payable by Energen

The merger agreement requires Energen to pay Diamondback a termination fee of $250 million (which we refer to as the “termination fee”) if:

In no event will Energen be required to pay the termination fee on more than one occasion.

Expenses

If the merger agreement is terminated because of a failure of Energen’s shareholders or Diamondback’s stockholders to approve the proposals required to complete the merger, Energen and Diamondback, as applicable, may be required to reimburse the other party for its actual transaction expenses in an amount not to exceed $40 million, in the case of Energen’s expenses, and $25 million, in the case of Diamondback’s expenses. In no event will either party be entitled to receive more than one termination fee, net of any expense reimbursement.

Appraisal Rights/Dissenters’ Rights in the Merger (page 137)

Appraisal rights, which are also sometimes known as dissenters’ rights, are statutory rights that, if applicable under law, enable shareholders to dissent from an extraordinary transaction, such as a merger, and to demand that the corporation pay the fair value for their shares as determined immediately prior to the effective time of the merger) in cash, instead of receiving the consideration offered to shareholders in connection with the extraordinary transaction.

Under Delaware law, Diamondback stockholders are not entitled to appraisal rights in connection with the issuance of shares of Diamondback common stock as contemplated by the merger agreement. Diamondback stockholders may vote against the Diamondback issuance proposal if they do not favor the merger.

The holders of Energen common stock are entitled to dissenters’ rights in the merger under the Alabama Business and Non-Profit Entity Code (which we refer to as “ABNEC”). An Energen shareholder wishing to exercise dissenters’ rights must (1) not vote in favor of the merger, (2) deliver to Energen before the vote on the merger proposal is taken written notice of such shareholder’s intent to demand payment for his or her shares under the dissenters’ rights statute, (3) submit a payment demand to Energen following the merger within the time frame specified by a notice that Energen will provide to such shareholder who provided notification under clause (2), and (4) surrender the stock certificate to Energen that represents the shares subject to the appraisal. For more information, see “The Merger—Appraisal Rights/Dissenters’ Rights in the Merger.”

Material U.S. Federal Income Tax Consequences of the Merger (page 168)

Energen and Diamondback intend for the merger to qualify as a “reorganization” within the meaning of Section 368(a) of the Code. It is a condition to Diamondback’s obligation to complete the merger that Diamondback receive a written opinion from Diamondback tax counsel to the effect that, on the basis of facts, representations and assumptions set forth or referred to in such opinion, the merger will qualify as a “reorganization” within the meaning of Section 368(a) of the Code, and it is a condition to Energen’s obligation to complete the merger that Energen receive a written opinion from Energen tax counsel to the effect that, on the basis of facts, representations and assumptions set forth or referred to in such opinion, the merger will qualify as a “reorganization” within the meaning of Section 368(a) of the Code.

If the merger so qualifies, then U.S. holders (as defined in the section entitled “Material U.S. Federal Income Tax Consequences of the Merger”) of shares of Energen common stock generally will not recognize any gain or loss for U.S. federal income tax purposes upon receipt of Diamondback common stock in exchange for Energen common stock in the merger (other than gain or loss, if any, with respect to any cash received in lieu of a fractional share of Diamondback common stock). The material U.S. federal income tax consequences of the merger are discussed in more detail in the section entitled “Material U.S. Federal Income Tax Consequences

of the Merger” beginning on page 168. The discussion of the material U.S. federal income tax consequences contained in this joint proxy statement/prospectus is intended to provide only a general discussion and is not a complete analysis or description of all potential U.S. federal income tax consequences of the merger that may vary with, or are dependent on, individual circumstances. In addition, it does not address the effects of any foreign, state or local tax laws or any U.S. federal tax laws other than U.S. federal income tax laws.

TAX MATTERS ARE COMPLICATED AND THE TAX CONSEQUENCES OF THE MERGER WILL DEPEND ON THE FACTS OF YOUR OWN SITUATION. YOU SHOULD CONSULT YOUR OWN TAX ADVISOR AS TO THE SPECIFIC TAX CONSEQUENCES AS A RESULT OF THE MERGER TO YOU IN YOUR PARTICULAR CIRCUMSTANCES.

Comparison of Rights of Stockholders of Diamondback and Shareholders of Energen (page 184)

The rights of Energen shareholders who receive shares of Diamondback common stock in the merger will be governed by the Amended and Restated Certificate of Incorporation of Diamondback, as amended (which we refer to as the “Diamondback certificate of incorporation”), and the Amended and Restated Bylaws of Diamondback, as amended (which we refer to as the “Diamondback bylaws”), which are governed by Delaware law, rather than by the Restated Certificate of Incorporation of Energen, as amended (which we refer to as the “Energen certificate of incorporation”), and the Bylaws of Energen, as amended (which we refer to as the “Energen bylaws”), which are governed by Alabama law. As a result, Energen shareholders will have different rights once they become Diamondback stockholders due to the differences in the organizational documents of Energen and Diamondback and the differences between Alabama and Delaware law. The key differences are described in the section entitled “Comparison of Rights of Stockholders of Diamondback and Shareholders of Energen” beginning on page 184.

Listing of Diamondback Common Stock; Delisting and Deregistration of Energen Shares (page 128)

If the merger is completed, the shares of Diamondback common stock to be issued in the merger will be listed for trading on Nasdaq, shares of Energen common stock will be delisted from the NYSE and deregistered under the Exchange Act, and Energen will no longer be required to file periodic reports with the SEC pursuant to the Exchange Act.

SELECTED HISTORICAL CONSOLIDATED FINANCIAL DATA OF DIAMONDBACK

The following table presents selected historical consolidated financial data for Diamondback (1) as of and for the years ended December 31, 2017, 2016, 2015, 2014 and 2013 and (2) as of and for the six months ended June 30, 2018 and 2017. The consolidated financial data for each of the years ended December 31, 2017, 2016 and 2015, and as of December 31, 2017 and 2016 have been derived from Diamondback’s selected financial data and audited consolidated financial statements included in its Annual Report on Form 10-K for the fiscal year ended December 31, 2017 filed on February 15, 2018, which is incorporated by reference herein in its entirety. The selected historical consolidated financial data of Diamondback for each of the years ended December 31, 2014 and 2013 and as of December 31, 2015, 2014 and 2013 have been derived from Diamondback’s selected financial data and audited consolidated financial statements for such years, which have not been incorporated by reference herein. The selected historical consolidated financial data for the six months ended June 30, 2018 and 2017 and as of June 30, 2018 have been derived from Diamondback’s unaudited consolidated financial data included in its Quarterly Report on Form 10-Q for the quarter ended June 30, 2018 filed August 9, 2018, which is incorporated by reference herein in its entirety. The selected balance sheet data as of June 30, 2017 has been derived from Diamondback’s unaudited consolidated financial statements as of June 30, 2017, which have not been incorporated by reference herein.

The information set forth below is only a summary and is not necessarily indicative of the results of future operations of Diamondback nor does it include the effects of the merger. This summary should be read together with the other information contained in Diamondback’s Annual Report on Form 10-K for the fiscal year ended December 31, 2017 and Quarterly Report on Form 10-Q for the quarter ended June 30, 2018, including the sections entitled “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and the consolidated financial statements and related notes therein. For additional information, see the sections entitled “Where You Can Find More Information” and “Information Incorporated by Reference,” in each case beginning on page 196.

Non-GAAP Financial Measures

Reconciliation of Net Income (Loss) to Consolidated Adjusted EBITDA

Consolidated Adjusted EBITDA (as defined below) is a supplemental non-GAAP financial measure that is used by Diamondback’s management and external users of Diamondback’s financial statements, such as industry analysts, investors, lenders and rating agencies.

For purposes of the Selected Historical Consolidated Financial Data of Diamondback, Consolidated Adjusted EBITDA is defined as net income (loss) plus non-cash (gain) loss on derivative instruments, net; interest expense, net; depreciation, depletion and amortization expense, impairment of oil and natural gas properties, non-cash equity-based compensation expense, capitalized equity-based compensation expense, asset retirement obligation accretion expense, income tax (benefit) provision and EBITDA attributable to non-controlling interest gain on revaluation of investment and loss on extinguishment of debt. Consolidated Adjusted EBITDA is not a measure of net income (loss) as determined by United States’ generally accepted

accounting principles (which we refer to as “GAAP”). Diamondback’s management believes Consolidated Adjusted EBITDA is useful because it allows it to more effectively evaluate Diamondback’s operating performance and compare the results of Diamondback’s operations from period to period without regard to Diamondback’s financing methods or capital structure. Diamondback adds the items listed above to net income (loss) in arriving at Consolidated Adjusted EBITDA because these amounts can vary substantially from company to company within Diamondback’s industry depending upon accounting methods and book values of assets, capital structures and the method by which the assets were acquired. Consolidated Adjusted EBITDA should not be considered as an alternative to, or more meaningful than, net income (loss) as determined in accordance with GAAP or as an indicator of Diamondback’s operating performance or liquidity. Certain items excluded from Consolidated Adjusted EBITDA are significant components in understanding and assessing a company’s financial performance, such as a company’s cost of capital and tax structure, as well as the historic costs of depreciable assets, none of which are components of Consolidated Adjusted EBITDA. Diamondback’s computations of Consolidated Adjusted EBITDA may not be comparable to other similarly titled measure of other companies or to such measure in Diamondback’s revolving credit facility or any of Diamondback’s other contracts, including the indentures governing its outstanding senior notes. Diamondback has included a reconciliation of Consolidated Adjusted EBITDA to net income, the most directly comparable GAAP financial measure, below for the periods indicated:

SELECTED HISTORICAL CONSOLIDATED FINANCIAL DATA OF ENERGEN

The following table sets forth Energen’s selected consolidated historical financial information that has been derived from Energen’s consolidated financial statements (1) as of and for the years ended December 31, 2017, 2016, 2015, 2014 and 2013 and (2) as of and for the six months ended June 30, 2018 and June 30, 2017. The information set forth below is only a summary and is not necessarily indicative of the results of future operations of Energen nor does it include the effects of the merger. You should read this financial information together with Energen’s consolidated financial statements, the related notes and the sections entitled “Management’s Discussion and Analysis of Financial Condition and Results of Operations” in its Annual Report on Form 10-K for the year ended December 31, 2017 filed on February 28, 2018, and Quarterly Reports on Form 10-Q for the quarter ended March 31, 2018 filed on May 9, 2018 and for the quarter ended June 30, 2018 filed on August 8, 2018, each of which is incorporated into this joint proxy statement/prospectus by reference. The selected statement of operations data and cash flow data for the years ended December 31, 2014 and 2013, and selected balance sheet data as of December 31, 2015, 2014 and 2013 have been derived from Energen’s audited consolidated financial statements for such years, which have not been incorporated into this joint proxy statement/prospectus by reference. The selected historical consolidated financial data for the six months ended June 30, 2018 and 2017 and as of June 30, 2018 have been derived from Energen’s unaudited consolidated financial data included in its Quarterly Report on Form 10-Q for the quarter ended June 30, 2018, which is incorporated into this joint proxy statement/prospectus by reference. The selected balance sheet data as of June 30, 2017 has been derived from Energen’s unaudited consolidated financial statements as of June 30, 2017, which have not been incorporated by reference herein. For additional information, see the sections entitled “Where You Can Find More Information” and “Information Incorporated by Reference,” each beginning on page 196.

Selected Financial and Common Stock Data

Selected Historical Business Data

SUMMARY UNAUDITED PRO FORMA CONDENSED COMBINED FINANCIAL DATA

The following summary unaudited pro forma condensed combined balance sheet data gives effect to the proposed merger as if it had occurred on June 30, 2018, while the unaudited pro forma condensed combined statement of operations data for the year ended December 31, 2017 and the six months ended June 30, 2018 is presented as if the merger had occurred on January 1, 2017. The following summary unaudited pro forma condensed combined financial information has been prepared for illustrative purposes only and is not necessarily indicative of what the combined company’s financial position or results of operations actually would have been had the merger occurred as of the dates indicated. In addition, the unaudited pro forma condensed combined financial information does not purport to project the future financial position or operating results of the combined company. Future results may vary significantly from the results reflected because of various factors, including those discussed in the section entitled “Risk Factors” beginning on page 43. The following summary unaudited pro forma condensed combined financial information should be read in conjunction with the section titled “Unaudited Pro Forma Condensed Combined Financial Statements” beginning on page 171 and the related notes.

SUMMARY PRO FORMA OIL, NATURAL GAS AND NGL RESERVE INFORMATION

The following tables present the estimated pro forma combined net proved developed and undeveloped oil, natural gas and NGL reserves as of December 31, 2017. The pro forma reserve information set forth below gives effect to the merger as if the merger had been completed on January 1, 2017. The following summary pro forma reserve information has been prepared for illustrative purposes only and is not intended to be a projection of future results of the combined company. Future results may vary significantly from the results reflected because of various factors, including those discussed in the section entitled “Risk Factors” beginning on page 43. The summary pro forma reserve information should be read in conjunction with the section titled “Unaudited Pro Forma Condensed Combined Financial Statements” beginning on page 171 and the related notes included in this joint proxy statement/prospectus.

COMPARATIVE HISTORICAL AND UNAUDITED PRO FORMA PER SHARE DATA

The following table presents Diamondback’s and Energen’s historical and pro forma per share data for the year ended December 31, 2017 and as of and for the six months ended June 30, 2018. The pro forma per share data for the year ended December 31, 2017 and as of and for the six months ended June 30, 2018 is presented as if the merger had been completed on January 1, 2017. Except for the historical information for the year ended December 31, 2017, the information provided in the table below is unaudited. This information should be read together with the historical consolidated financial statements and related notes of Diamondback and Energen, filed by each with the SEC, and incorporated by reference in this joint proxy statement/prospectus, and with the unaudited pro forma condensed combined financial statements included in the section entitled “Unaudited Pro Forma Condensed Combined Financial Statements” beginning on page 171.