Undermountain

Originally a mine and dwelling for Dwarves.
Dwarves fought in here against the drow and started losing.
Home of the Xanathar guild. Goes to the underdark.
Houses Drow, Duergar, powerful wizards.

Skullport

In the underdark under undermountain.

Xanathar

The most powerful thieves guild around Waterdeep || The beholder that runs the guild.
Operates from Skullport, underneath WD. Slaves, Smuggled goods, and information. Assassins for hire. Driven out competing organisations via links to rulers.
Always run by a beholder (The Xanathar). There used to be a canonical one, but it was slain by another one who then claimed it is a title (competing thieves guilds early on).

Managed to enstate one of its agents into the lords of waterdeep. Took over skullport entirely. They eye was killed later on. Another eye took over. Several beholders swapped the title, either through fight or retirement (one got bored). Then Zushacks(?) took over. Had many ties drow name Kaldeer, illithid named Zethul + an agent of Asmodeus. Also supported by the masked lords of waterdeep (they want to drive Zhentarim out of waterdeep).

11 masters of the guild. Each in charge of one of:
Assassination, blackmail, bookkeeping, enforcement, extortion, information gathering, magical defences, mercenaries, slavery, smuggling, and thievery.

Each master have subordinates below them to keep them insulated. Subordinates also insulated. Communication only goes vertically in this hierarchy.

Treat like a king of a corrupt empire. They also are in the pockets of some of the most powerful pockets of waterdeep. You can’t beat them with just righteousness.