From Podpedia

The Demonweb is the holy city of Lolth, the goddess of nightmares. It is largely populated by both drow and humans, but mostly spiders. The spiders are attracted to Lolth and provide the structure of the Demonweb as well as its name.

The Demonweb is shaped in the form of a funnel web resting on rocks jutting from the Astral Sea.

When the diplomatic envoy from the Pentatheon came to the Demonweb during the Celestial Crusade, they were faced with nightmares involving the failures of Torq, Ket H'zard, Seven-Owls Wise, and Albrecht Ghostbeard before a dreamlike meeting with Lolth. It is unknown whether they ever set foot in the Demonweb as they woke on board their ship.