A link between eras.

swords clashed, the end of the world came, many went through the portal, and some hid under the ground. The portal was destroyed, and life on the surface became impossible. And everyone was forced to live underground, and when it became possible to live on the surface again, the elves revived nature, but many of the elves refused to come out of their burrows and eventually became dark elves,
mages (tower) did not find snow in the new world and began to live in the desert
people 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 at least it's something.