The Elven Kingdoms are Ravaged

Military action

Belahir summons a more powerful demon prince and tells it to "destroy the elven kingdoms". The demon prince goes to the battlefield and curses the land just to the north of the mountains. Instantly, the ground turns to lava and volcanoes start rising. This creates the Lava Barren, and the volcanoes spew ash. The ash gets caught in some breezes and floats North, covering the elven kingdoms and creating the Ashland. The demon prince is unsummoned after he tries to escape the bonds of the summoning.  Many Elves escape through naturally occurring portals to the Shadowfell, taking with them High Centaurs and they push the Night Hags out, replacing them and getting corrupted into dark Elves and Shadow Centaurs over the next century.