Underchasm

The Underchasm is a massive chasm created by the events of the Spellplague in 1300 DR, running roughly from the borders of Vilhon Reach to Halruaa. When the Underchasm opened, it took countless lives, swallowing entire cities and villages whole. The city of Underhome was located in this region originally, home to almost 50,000 Gold Dwarves. It was a beautiful stronghold with massive golden gates, and the Gold Dwarves inhabiting it were proud and stalwart. When the Underchasm opened, the entire city was lost to its depths. The Underchasm itself is a scar that serves as a brutal reminder of this and so many other tragedies that occurred during the Spellplague.
The Underchasm leads to the Underdark, which is a vast network of underground caverns and tunnels beneath all of Toril. The Underdark is home to the Drow and a wide array of abominations and other dangerous beasts lurking within the depths.
The land near the Underchasm has become a dangerous place for travelers. It is where the beats of the Underdark can find their way to the surface, as the opening has created an easier passage between the surface world and the Underdark. Along the caves and cliffs within the Underchasm, all manner of flying beasts have made their home there, such as dragons.

The Underdark's underground landscape varies from small subterranean fissures almost impossible for adventurers to crawl through, to wide expanses of deep water that hid coral caves, to hollowed out sections of ice in which creatures lived, and to places where fungus, bones, or even pure force formed caves.

The Underdark is mainly without any source of light, save for the occasional luminous rocks or fungi. Large portions of it are also without food or water, making well-stocked food supplies a must-have when exploring uncharted terrain underground. Lack of light combined with a tomb-like silence made traveling something only for the strong-hearted. The nerve-wracking silence made even experienced adventurers grow agitated after a few days.