The Ancient Lands: The Sundering
Atlantis was a small island-continent in the ocean west of Thuria, and east of the Pictish Islands. It is the home of the Atlanteans, enemies of the Picts and of the civilized Seven Empires of Thuria. They established colonies on Thuria. It was the home of Kull. Priests of Xultha were sent from Na-hor to Atlantis.
In the Cataclysm that ended the Thurian Age, Atlantis sank beneath the sea but some escaped the destruction on ships, and joined their continental colonies. There, the survivors are forced back into the stone age by lack of resources. They soon enter a war with the surviving Picts and lose to their superior numbers and stone-age military technology. The survivors of the war devolve back into apes. They eventually re-evolve into humans to become the Cimmerians of the Hyborian Age.