Thank you. I did think that the players could just take the Godhead away with them somewhere and spread its mutating power with them, treating it like a magic item. I think it can be used as a magical weapon, and thrown 20' to mutate anything in the area, as it has an area of effect when damaged. it has hit points, so players won't know exactly when the last throw will be. The head itself does not want to go back to the temple.
but then the mountain would eventually explode when the Godbody goes critical mass, yeah. maybe the players can freeze the body, and take it elsewhere, basically carrying a ticking nuclear bomb, or they could come up with another way of vanquishing it, a wormhole or spell or magic item that I haven't accounted for, or they could just evacuate the town.
the simplest ending is to place the Godhead back on the body and then it goes dormant and does not explode, but I'd like to see what else people come up with.