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On February 24, 2026, Sompo International Holdings Ltd., a Bermuda exempted company limited by shares (“Sompo International”, and, together with its subsidiaries, the “Sompo International Group”), through its direct wholly owned subsidiary, Endurance Specialty Insurance Ltd., completed the acquisition of Aspen Insurance Holdings Limited, a Bermuda exempted company limited by shares (the “Company”, and together with its subsidiaries, the “Aspen Group”). As part of the continued integration of the Aspen Group into the Sompo International Group, on August 14, 2026, the Company completed a series of transactions to simplify its corporate structure and harmonize its operations with those of the Sompo International Group. In connection with these transactions, the Company contributed all of its shares in Aspen Bermuda Limited, a Bermuda exempted company limited by shares (“ABL”) and direct wholly owned subsidiary of the Company, through a series of contributions to another of the Company’s indirect wholly owned subsidiaries, Aspen Re America, Inc., a Delaware corporation. Thereafter, Aspen (UK) Holdings Limited, a company incorporated in England and Wales (“AUKH”) and an indirect wholly owned subsidiary of the Company, transferred all of its shares in Aspen U.S. Holdings, Inc., a Delaware corporation (“AUSH”) and an indirect wholly owned subsidiary of the Company, to Endurance U.S. Holdings Corp., a Delaware corporation (“EUSH”) and an indirect wholly owned subsidiary of Sompo International. EUSH issued $2.722 billion of intragroup debt to AUKH, pursuant to which EUSH is obligated to make payments to AUKH on the terms set forth therein. In addition, in connection with the transfer of AUSH to EUSH, AUSH merged with and into EUSH, with EUSH surviving the merger. As a result of the merger of AUSH and EUSH, all of the pre-merger subsidiaries of AUSH (including Aspen American Insurance Company, Aspen Specialty Insurance Company and ABL) are now owned by EUSH.

In connection with the foregoing, Sompo International and the Company entered into (i) the Seventh Supplemental Indenture with Deutsche Bank Trust Company Americas as trustee, pursuant to which Sompo International agreed to guarantee all payment obligations of the Company with respect to the Company’s 5.750% Senior Notes due 2030 and other guaranteed obligations (as defined therein) under the Indenture, dated as of August 16, 2004, as amended, and (ii) a Keepwell and Financial Support Agreement pursuant to which Sompo International agreed to provide certain cash payments to the Company if necessary to maintain its capital and surplus and to support any payments to holders of the Company's outstanding Preference Shares if and when the board of directors of the Company determines to declare dividends on the Preference Shares.

This Report of Foreign Private Issuer on Form 6-K contains forward-looking statements within the meaning of the “safe harbor” provisions of the Private Securities Litigation Reform Act of 1995 and other applicable securities laws. The forward-looking statements in this report are subject to numerous risks, uncertainties, estimates and assumptions. Given these risks and uncertainties, you should not place undue reliance on forward-looking statements as a prediction of actual results. Accordingly, any such statements are qualified in their entirety by reference to, and are accompanied by, important factors included in “Part I-Item 3.D.-Risk Factors” in our Annual Report on Form 20-F for the year ended December 31, 2025 and in any subsequent reports on Form 6-K (in addition to any assumptions and other factors referred to specifically in connection with such forward-looking statements).