A link between eras.

swords clashed, the end of the world arrived, many went through the portal, and some hid underground. The portal was destroyed, and life on the surface became impossible. Everyone was forced to live underground, and when it became possible to live on the surface again, the elves revived nature, but many of them refused to leave their burrows and eventually became dark elves. Mages (Tower) didn't find snow in the new world and started living in the desert. Humans also took a long time to restore their former glory; they survived because of their faith in God. And the undead and demons felt at home in the new world. Well... it's vague, but it's something.