The Underdark Rappel

As part of the events leading up to the Transition of Magic, the city of Solhom was destroyed - its vast protective pillar of sunlight dispelled by the machinations of The First Red, one of the mythical Children of Davaris.
Within moments of the pillar being dispelled, the city was assaulted by a party of adventurers, and during their assault, a vast magical working was unleashed, killing nearly every inhabitant of the city.
As the city lay empty, it was unsure whether the scant survivors would stay or depart - but it was not long before the choice was taken from them. Not two weeks after the magical bombing, parties of Driders - holy soldiers of the Spider-Goddess Lolth - were seen on the outskirts of Solhom. Before any kind of defence or investigation could be launched, the Driders had completed a ritual circle around the nearly-empty city, and the entire city was pulled underground.
In its place now is the Underdark Rappel, a colossal yawning chasm that seems to stretch down to the very depths of the earth. The few explorers who have investigated it report that it is guarded by arachnids and Drow, and that you can see both the crashed ruins of Solhom at the bottom - as well as what looks like another ruined city that it was pulled down atop.