UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): March 4, 2015

DUNE ENERGY, INC.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (713) 229-6300

NOT APPLICABLE

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2 below):

On March 4, 2015, Dune Energy, Inc. (the “Company”) notified Eos Petro, Inc., a Nevada corporation (“Eos”), that, effective March 4, 2015, it was terminating the Agreement and Plan of Merger, dated as of September 17, 2014 (as amended through such date, the “Merger Agreement”) among the Company, Eos and Eos Merger Sub, Inc., a Delaware corporation and a wholly owned subsidiary of Eos (“Merger Subsidiary”), due to the breach and failure to perform by Eos and the Merger Subsidiary of the Merger Agreement. Pursuant to the Merger Agreement, Merger Subsidiary had agreed to acquire any and all outstanding shares of common stock of the Company (the “Shares”) validly tendered and not properly withdrawn through a cash tender offer (the “Offer”) of $0.30 per Share, to be followed by a merger to acquire all remaining outstanding Shares at the same price per share paid in the Offer. For a summary of the material terms of the Merger Agreement and the Offer, please see the Company’s Forms 8-Ks filed with the Securities and Exchange Commission on September 18, 2014, November 21, 2014, December 23, 2014, January 6, 2015, January 16, 2015, January 26, 2015, February 3, 2015, February 9, 2015, February 17, 2015, February 23, 2015 and February 25, 2015 and the Company’s Schedule 14D-9 and related amendments filed with the Securities and Exchange Commission on October 9, 2014, October 10, 2014, October 23, 2014, November 7, 2014, November 21, 2014, December 23, 2014, January 16, 2015, January 26, 2015, February 2, 2015, February 9, 2015, February 17, 2015, February 23, 2015, February 25, 2015 and March 4, 2015.

On December 16, 2014, Eos informed the Company that Eos could not complete the Merger and Offer on the terms originally set forth in the Merger Agreement and Schedule TO. Accordingly, subsequent to December 16, the parties, along with the Company’s senior lenders and the holders of the Company’s outstanding Senior Notes, spent substantial time negotiating in an effort to reach an agreement on the parameters of a transaction acceptable to all parties and for which Eos may obtain financing to consummate the merger and the transactions contemplated by the Merger Agreement; however, the parties were unable to consummate a transaction on alternate terms.

As the Company previously announced, the Offer expired at midnight New York City time, on Friday, February 27, 2015 (the “Extended Expiration Time”). According to American Stock Transfer & Trust Company, the depositary for the Offer, as of the expiration of the Offer at midnight New York City time, on February 27, 2015, a total of 72,078,305 Shares were validly tendered and not properly withdrawn pursuant to the Offer, representing 98.73261% of all outstanding Shares, which is sufficient to satisfy the Minimum Tender Condition.

After the Extended Expiration Time, Eos informed the Company through counsel that it was unable to complete the Offer because Eos and Merger Subsidiary had been unable to secure sufficient financing to fund the Offer as of the Extended Expiration Time, and that none of the Shares validly tendered and not properly withdrawn would be purchased in the Offer.

The Company believes that all of the conditions to the Offer had been satisfied as of the Extended Expiration Time. Accordingly, Eos and Merger Subsidiary were obligated to purchase all Shares validly tendered and not properly withdrawn as of the Extended Expiration Time by midnight New York City time, on February 27, 2015, or be in breach of the Merger Agreement. As Eos and Merger Subsidiary did not purchase all Shares validly tendered and not properly withdrawn as of the Extended Expiration Time, the Company believes Eos and Merger Subsidiary have breached the Merger Agreement and failed to perform their obligations thereunder and thus the Company is entitled to terminate the Merger Agreement pursuant to Section 8.1(c)(i) of the Merger Agreement and to receive the Parent Termination Fee and reimbursement for certain expenses incurred by the Company.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.