User blog comment:Tholanar/The mightiest god in The Elder Scrolls!/@comment-5871119-20121219011502/@comment-3492791-20121219013705

Magic comes from Aetherius. The stars are considered to be small holes into Aetherius created by the Aedra that fleed after realising the cost of creating Nirn. Magnus was the first to leave, and thus was the Aedra least weakened by the project. Being such he created the largest hole to Aetherius. The sun.

This being the case he is credited with "creating" magic.