The Xanathar is a title held by the leader of the Xanathar's Guild. It primarily is held by beholders, however there have been other races to wield the title from time to time. The current Xanathar is a halfling sorcerer named Cade Underbough--who reigns as the 16th Xanathar.
Defeated the Xanathar with the help of the Lords' Alliance, and assumed his name in order to integrate into the guild easier. The guild didn't know of the new Xanathar aside from a few buried rumors.
Defeated the 2nd Xanathar with the help of a beholder named Uthh. Assumed the identity in order to integrate into the guild easier. Officially turned the name "Xanathar" into a title. Was eventually slain by the next Xanathar.
The 13th Xanathar. Eventually grew bored and retired--handing off the title to the young beholder, Zushaxx.