Annulii Mountains

Known collectively as the Annulii Mountains, a range of titanic cloud-piercing peaks separates the Inner and Outer Elven Kingdoms on the island of Ulthuan. The highest valleys and plateau of this region disappear into a strange, glittering mist of raw magic so strong it becomes visible even to those without the wizard sight. 1

Within this realm the stuff of dreams and nightmares can coalesce from the very air. The sun is never visible, it is eternal twilight. Time flows strangely and travelers become lost for years although they think they have been walking only hours. Even the use of magic becomes unpredictable in the mountains. This unstable region of Ulthuan has a strange other-worldly quality more akin to the realms of Chaos than to mortal lands.

The Mountains are riven with magic because Ulthuan itself acts as focal point for the winds of magical energy which blow across the known world from the Northern Wastes. These drifting energies are drawn to Ulthuan, like water in a whirlpool, forming a vortex of magic. In this way, Ulthuan drains magic out of the known world and prevents the tide of it overwhelming everything and turning it into a seething realm of Chaos.

The Annulii is the most impossible to cross save by certain passes and tunnels guarded by massive fortified gates. Many wild and evil creatures are spawned in the mountains or drawn there on the winds of magic. For the most party they are confined there by the same spell which draws magic to Ulthuan, but some things manage to find their way down to ravage the lands below. Griffons, chimera, and other monsters find their way into the hinterlands of the Inner Kingdoms where they are hunted for sport of captured as war mounts by the High Elves.

So few passes breach the near impenetrable mountains that most communication between the inner and outer realms takes place by sea. Thus, the Inner Kingdoms are mostly unspoiled wilderness, covered by huge, ancient forests or grassy, rolling plains, utterly free of the stamp of habitation.

Source
Warhammer Armies: High Elves (4th Edition) -- pg. 5