** Bermuda-based specialty insurer Aspen Insurance AHL.N set to begin trading on the New York Stock Exchange on Thursday under ticker symbol "AHL"

** Aspen parent Apollo Global Management APO.N late Wednesday sold 13.25 million shares at $30 apiece in Aspen's upsized IPO to raise $397.5 million

** Apollo had initially offered 11 million shares priced between $29 and $31 each

** Aspen originally went public in 2003 and traded on the NYSE until 2019, when Apollo acquired it for $2.60 billion

** At offer price, Aspen fetched a $2.76 billion valuation

** Under Apollo, Aspen returned to profitability as it shrunk catastrophe exposure and exited a number of insurance and reinsurance offerings

** Goldman Sachs, Citigroup and Jefferies led a 16-firm underwriting syndicate for the offering

** Apollo will continue to control a majority of Aspen's voting power post-IPO

(Reporting by Arasu Kannagi Basil in Bengaluru)

((ArasuKannagi.Basil@thomsonreuters.com;))