earthbound angels maybe? not fallen persay, but part of the angels that wouldn't fight, so they're made to be trapped on earth. Maybe even as souls stripped of memories and forced to born again and again into different bodies that are somehow awakened (they would be weakened, but full of knowledge) and its demons that have to kill them to make them be reborn again before they regain more power than what there human bodies can work with? It's not exactly what you're looking for maybe, but just ideas that maybe give you some inspiration.