The Demonweb Pits are the malevolent plane in the multiverses of Dungeons & Dragons that serves as the home of Lolth, Demon Princess of Spiders and Goddess of the Drow, as well as her fellow deities in the Dark Seldarine. Its precise location varies depending on which cosmology you look at; whilst it was presented as a layer of the Abyss in the Great Wheel, and maintained this state in the World Axis, it is considered its own unique plane in the World Tree of the Forgotten Realms - originally, it was an Abyssal layer there, too, but after the events of the Silence of Lolth, it was created anew elsewhere, although the original version can still be reached from the Abyss and it contains a portal to the new version.
The Demonweb Pits are a huge inverted cone of webbing formed of planar materials and inhabited by spider fiends, drow petitioners and the Spider Queen Lolth. The goddess dwells in an enormous temple of black stone, fashioned in the shape of a giant black widow spider with a beautiful yet cruel drow woman's face for a head, which conceals the entrance. The temple moves about the web, spinning the stuff of the plane as it goes.
The freed Demonweb Pits are a rocky plane of pits, craters, chasms and gorges, littered with the petrified forms of giant spiders, with comparatively smaller spiders (notably songspiders) crawling everywhere, leaving webbing all over the place. It has a distinct day and night cycle; by day, a distant red star balefully glowers over the landscape, whilst at night, a cluster of eight bright red stars, arranged like the eyes of a spider, hangs over everything. Lolth resides in an iron citadel in the Infinite Web.
Ghaunadaur lurks in a fungus-caked, slimy pit somewhere on the plane.
Both Kiaransalee and Vhaeraun begrudgingly reside in territories within the Demonweb Pits - Kiaransalee is especially bitter, since she used to rule the Abyssal layer of Thanatos by right of conquest before Orcus came back from the dead and booted her ass off of his throne.
Eilistraee technically has holdings here, but for obvious reasons, she prefers to stay away from her mom.