Xanathar is a Beholder, or rather a line of Beholders sharing a title, and a recurring antagonist of the Dungeons & Dragons setting of Forgotten Realms.
He is the boss of the underground Xanathar's Thieves Guild in Skullport.
Story
Xanathar was originally simply known as The Eye before becoming a crime lord. He is a elderly, greedy and paranoid Beholder from the Graypeaks Hive.
The Eye arrived in Skullport in 1205 DR, already a slaver in search of a greater market. In time, his business and his organization grew. At one point, he re-organized his organization into a semi-autonomous model and retreated into the background, spreading rumors about his demise or retirement. Soon, most people forgot about the beholder slaver and in 1304 DR the Agents of the Eye were simply a large slaving ring in Skullport with no association with beholders.
Meanwhile, the Eye gathered information about the mysterious Xanathar Thieves' Guild, a powerful underground thieves guild, and in time discovered that they were also led by a beholder, a beholder of a different hive. Thus the Eye decided to take control, by killing and taking the position of Xanathar. The Eye discovered all of the original Xanathar's secrets and at the same time manipulated another beholder, Uthh, into wanting to kill Xanathar. Eventually, Uthh fought Xanathar in his lair; Xanathar slew Uthh but was too weak to resist the surprise attack of the Eye. The Eye killed Xanathar and took his place as the leader of the Xanathar Thieves' Guild. The Eye pretended to be his predecessor but, in order to avoid problems, said that "Xanathar" was just a title. Afterward, the Eye gathered together his agents and the Xanathar's Guild.
In time more and more beholders assumed the Xanathar's mantle, some for a brief time. Until 1479 DR ruled a beholder previously called Izulktur. That year Izulktur grew bored and decided to retire. The new master was Zushaxx, a young and ambitious beholder of the same beholder cluster of Izulktur. With the support of the drow henchman Kal'dir Zushaxx quickly established himself as the new Xanathar.
Personality
The various Xanathars never ventured out from the Guild's lairs, preferring to work exclusively through intermediaries and assassins. They are usually attended by swarms of lurking stranglers and beholderkin.
As usual for Beholders, all the Xanathars viewed the humanoid races as inferior creatures only worthy of being slaves, but some of them always interrogated their prisoners for useful information with a affable demeanor, killing only when it is necessary.