The Elemental Split

Geological / environmental event

The elementals rifts opened up

Because of the wars, the barrier was weak enough between the planes for elementals to start coming through. This made the rifts wider the more elementals that came through. The deities knew they could no longer contain them, so the hero group (now known as Rust Force) was sent to do what they do best, talk there way out for the good of the prime plane. They made a bargain which set the course for the elemental thrones and the split of the world.