The Restoration Team

After the Reckoning, that's exactly what happened. The thing is, not all of Enroth was destroyed (in fact, the idea of the Restoration is that Ashan is a world of Enroth, restored by one of the Ancients). The catastrophe couldn't have destroyed all habitable lands in one fell swoop. But their area was shrinking very quickly. And the struggle for these lands was very fierce. That's why the swamp dwellers had to retreat.
The moment of the birth of the new civilization of Enroth-Ashan will be described in more detail in the campaigns for the fifth game.