Doth'Mordun is an enormous valley in the frozen wastes at the southern end of the world, surrounded by impossibly tall mountains. It is here where Lolth and the drow have been imprisoned for the last three thousand years. The prison was magically locked with five seals embedded in Winter's Gate, each linked to an artifact that was scattered somewhere throughout the world. About thirty years ago, the Cult of the Spider began actively seeking and destroying these artifacts in an attempt to weaken the Doth'Mordun's warding spells. They successfully destroyed two of the artifacts and their corresponding seals, weakening the wards enough that the drow were able to use a fragment of the Negative Energy Plane to create portals out of the prison. Eventually, they were thwarted when a band of heroes entered one of these portals and destroyed the planar fragment. Following this, Boccob cast new warding spells to reinforce the destroyed seals.
During the Deity War, the negative energy plane poured through the hole left by the fragment's destruction into the plane of shadow, creating the Shadowfell. There is now a massive planar fissure in Doth'Mordun where the Shadowfell opens directly into the Material Plane, though the guards watching the prison say there is no danger of the drow escaping through it.