Xanathar Guild

The Xanathar's Guild is a group of thieves and cutthroats operating out of the bowels of Waterdeep. From its main offices in Skullport, the Xanathar's Guild has access to several portals, with prominent ones leading to an alley in Waterdeep, a rock outcropping near Calimport, and a small Guild-owned warehouse in Westgate.
The Guild is led by an elder beholder called The Xanathar. It is at least the second such creature to claim the title, since it destroyed its predecessor. Few outside the inner circle of Masters know of The Xanathar's nature, because it is reclusive and guarded by undead beholders. Eleven Masters each control one of the Guild's businesses: assassination, blackmail, book-keeping, enforcement, extortion, information gathering, magical defenses, mercenaries, slavery, smuggling, and thievery. Most of the Guild's work involves slavery or thievery.
Members of the Guild are well trained and skilled in stealth, tactics, and discretion. They have excellent arms and armor, and those who need to use portals as part of their work are usually multiclass spellcasters or at least trained in rudimentary spellcasting.

Structure

Members of the guild were trained in stealth, discretion, and tactics with fighters, rogues, sorcerers, wizards, and experts which comprised most of its numbers. An ever-growing organization, it utilized multiple mouthpieces to protect and conceal the identity of the Xanathar. To join, a member had to prove their ruthlessness, obedience, and loyalty by committing a serious crime - killing or selling a friend or family member into slavery, as well as paying 300 gp. The typical agent earned dues of 60 gp a month. Support generally came in these ways:

Any member given an assignment would receive a bodyguard or assistant - such as a bugbear, wererat, kenku, or gazer - with secret instructions to spy on and assassinate the agent in case of failure.
The guild provided safe refuges - cellars in the sewers.