Board Thread:Lore Discussion/@comment-24583421-20140302161145/@comment-13446185-20140304012136

Martin was the person who became the dragon,  and Sheogorath never says he mantled Akatosh. "Well, except for that Martin fellow, but he turned into a dragon god,"

-Sheogorath

Sheogorath says exactly what Martin did. He turned, temporarily, into a "dragon god" by fusing with the hundreds of dragon souls contained within the Chim-el Adabal. Since we know the Dragonblood comes from Akatosh, the emperor oversoul formed a dragon that resembled an avatar of Akatosh.