Imperius sounds possible if u think of it, i mean the council voted to destroy mankind a long time ago. Now when humans are becoming as powerful as your character is he may try again, we all know the other members of the council would vote no again, so i think he will go mad, grab a bunch of likeminded angels and attack Sanctuary. Not a good story i know, but that could be a good distraction, something like while humans and angels were trying to stop him some new powerful demons were born... most likely someone, somewhere willingly or by accident, managed to resurrect the prime evils.