I think you're confusing the Alessian Rebellion with the storyline of Oblivion.The former had no daedric hordes and was won without Akatosh (unless you count Pelinal as partly Akatosh, which I know you don't
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I dimly remember reading or being told this story at some point in Oblivion, though. Ayleids used daedra to bolster their armies, Alessia made some sort of deal with Akatosh, which caused him to seal Oblivion from Nirn. This was why the Amulet/dragonfires were so important to ending the Oblivion Crisis, IIRC.