Storm Kings Thunder
Crime boss in Waterdeep
The adventurers first heard rumors of The Xanathar when they visited Old Jack at the Decoys Dower and later found out more when they visited the Yawning Portal.
The Xanathar is the name of an old crime boss from the past century. The Xanathar ruled over crime for many years using vast knowledge of the Undermountain to his advantage. Eventually the Blackstaff, at that time Kelban Arunsun, took charge and found and dispatched The Xanathar himself.