Filed by Energen Corporation

Pursuant to Rule 425 under the Securities Act of 1933

and deemed filed pursuant to Rule 14a-12

under the Securities Exchange Act of 1934

Subject Company: Energen Corporation

Commission File No. for Registration Statement

on Form S-4 filed by Diamondback Energy, Inc.: 333-227328

November 7, 2018

Dear Fellow Shareholder:

We have previously sent to you proxy material for the special meeting of shareholders of Energen Corporation to be held on November 27, 2018, in connection with the proposed merger with Diamondback Energy, Inc. Your Board of Directors unanimously recommends that you vote FOR both proposals on the agenda.

Since approval of the merger proposal requires the affirmative vote of at least two-thirds of the shares of common stock outstanding and entitled to vote, your vote is important, no matter how many or how few shares you may own. Failing to vote is effectively a vote against the proposed merger. If you have not already done so, please vote TODAY by telephone, via the Internet, or by signing, dating and returning the enclosed proxy card in the envelope provided.

Very truly yours,

John K. Molen

Secretary

ADDITIONAL INFORMATION AND WHERE TO FIND IT

This communication does not constitute an offer to buy or sell or the solicitation of an offer to buy or sell any securities or a solicitation of any vote or approval. This communication relates to a proposed business combination between Diamondback Energy, Inc. (“Diamondback”) and Energen Corporation (“Energen”). In connection with the proposed transaction, Diamondback has filed with the Securities and Exchange Commission (the “SEC”) a registration statement on Form S-4 that includes a joint proxy statement of Diamondback and Energen that also constitutes a prospectus of Diamondback. The registration statement was declared effective on October 24, 2018. Each of Diamondback and Energen also plan to file other relevant documents with the SEC regarding the proposed transaction. No offering of securities shall be made except by means of a prospectus meeting the requirements of Section 10 of the U.S. Securities Act of 1933, as amended. The definitive joint proxy statement/prospectus was first mailed to stockholders of Diamondback and shareholders of Energen on or about October 26, 2018. INVESTORS AND SECURITY HOLDERS OF DIAMONDBACK AND ENERGEN ARE URGED TO READ THE REGISTRATION STATEMENT, JOINT PROXY STATEMENT/PROSPECTUS AND OTHER DOCUMENTS THAT MAY BE FILED WITH THE SEC CAREFULLY AND IN THEIR ENTIRETY IF AND WHEN THEY BECOME AVAILABLE BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION ABOUT THE PROPOSED TRANSACTION. Investors and security holders may obtain free copies of these documents and other documents containing important information about Diamondback and Energen, once such documents are filed with the SEC through the website maintained by the SEC at http://www.sec.gov. Copies of the documents filed with the SEC by Diamondback are available free of charge on Diamondback’s website at http://www.diamondbackenergy.com or by contacting Diamondback’s Investor Relations Department by email at IR@Diamondbackenergy.com, alawlis@diamondbackenergy.com, or by phone at 432-221-7467. Copies of the documents filed with the SEC by Energen are available free of charge on Energen website at http://www.energen.com or by phone at 205-326-2634.

CERTAIN INFORMATION CONCERNING PARTICIPANTS

Diamondback, Energen and certain of their respective directors and executive officers may be deemed to be participants in the solicitation of proxies in respect of the proposed transaction. Information about the directors and executive officers of Energen is set forth in Energen’s proxy statement for its 2018 annual meeting of shareholders, which was filed with the SEC on March 22, 2018. Information about the directors and executive officers of Diamondback is set forth in its proxy statement for its 2018 annual meeting of shareholders, which was filed with the SEC on April 27, 2018. These documents can be obtained free of charge from the sources indicated above. Other information regarding the participants in the proxy solicitations and a description of their direct and indirect interests, by security holdings or otherwise, are contained in the joint proxy statement/prospectus and other relevant materials filed with the SEC. Investors should read the joint proxy statement/prospectus carefully before making any voting or investment decisions. You may obtain free copies of these documents from Diamondback or Energen using the sources indicated above.