Atlanteans
The Atlanteans were once a powerful empire that spread across the Midlands. They employed magical tools and technological wonders that few understood. So powerful and wide was their influence, that they actually began to change the cultures and religions of the world. However, they spoke out against the gods, and began enslaving people to their strange ideologies. Thus, the gods of the known world united against them and smited their main island and capitol of Atlantis, cursing it's people and burying the island deep beneath the sea.
Or so the legends go. Little proof of Atlantis exists save for nearly everyone having myths and legends of them.