Qi'sztovir:

Location: lower underdark, beneath present-day Charu Mountains
Rise: circa -8,600SG
Fall: circa -5,700SG

Qi'sztovir was originally known as the dwarven citadel Karak Delvar, “the Eternal Underhome”. After more than a millennia of the drow clans surviving as mere scavengers and renegades of their new underdark home, they gathered sufficient forces to launch an attack upon this, one of the greatest dwarven settlements of the time, and not far distant from their original homeland above. With the aid of duergar allies, the city fell and the drow took it as their own.

The city itself stood for nearly two millennia, as the drow rebuilt it in their own image and rose to become a feared force within that stretch of underdark. But sooner or later, with less and less foes to oppose, they fell to fighting amongst themselves, eventually branching out into blatant war. This culminated in a series of magical explosions that destroyed most of the city, and sundered the earth above with such force that it shaped the Charu highlands/mountains, and opened the rift that is now known as the Wailing Chasm. The surviving drow, now hunted viciously by their dwarven foes of millennia earlier, fled in all directions to establish other, smaller settlements.