Most of the gods that had lots of souls and noticed that their loose souls were going to the slugpeople would immediately put down their quarrels with each other to try to remove the drain on their souls.
But the other less powerful gods and gods without as many souls in their possession resent them, and side with Methus.
And Methus tells them how to make mortals. And the other gods make a lot of different species.
And so it's 200 gods and their armies against 400 weaker gods and their mortals they're protecting.