Board Thread:Lore Discussion/@comment-62.31.43.199-20131118141224/@comment-24854318-20140426175802

Simple answer; Auri-El was a champion of the Elves during the Dawn Era, and left Mundus following the the defeat of Lorkhan so that his followers might learn how to escape the mortal plane. Auri-El didn't betray him because he was no longer active on Nirn to begin with.

(Yes, I'm aware that it is commonly accepted that Auri-El is simply the elven name for Akatosh, but in my opinion there are several reasons to believe that they are not the same being. Namely, while Auri-El was a Champion of the Elves against Lorkhan and the races of men, Akatosh has acted on behalf of man and often against the elves time and time again. Furthermore, according to the Elves Auri-El left Mundus following the defeat of Lorkhan, while Akatosh has been active on Nirn even as recently as the Oblivion Crisis.)