Hm, I thought of something. Maybe the monsters realize what is happening once the gods are unleashed, and sacrifice themselves to power the party up? In the first war, they never really knew the creators, but after being sealed away with them for all those years, they knew who to blame for their condition. Something about the humans giving the monsters hope for a better future. Perhaps most of the monsters get wiped out, but some survive, and either learn to coexist with humans, or live in hiding and in peace.