It's probably impossible to know, what with how secretive they are. Have it their way, no one would even know he was a beholder, let alone the latest in a long line of beholder crime lords.
The official lore is convoluted, more rumours and glints of truth in tales of conflicting information. Years are pretty much undecided about what happened when.
Apparently it's been over 140 years since the original Xanathar founded his kingdom of crime in the sewers of Waterdeep. Lore specifically states the original Xanathar was slain in 1358 DR by a rival beholder, who also ran a slave trade business in Skullport.
This rival beholder, Kirukeskai (aka. the Eye), had been plotting Xanathar's downfall for a while and titled himself "the Xanathar" to better ease the Xanathar guild into his own service.
By 1370, apparently, Xanathar II (Kirukeskai) began to severely feel the deterioration of old age, but had been trying to stave off its effects with potions and magic. Another piece of information is that apparently, he had other beholders in his service in Undermountain, though it doesn't specify how many or why. This is before later lore begins to claim all beholders hate one another's guts I guess, but maybe they are lesser beholders, and therefore acceptable? Anyway, the main point is that he had plenty of underlings and biological children to replace him.
Xanathar II (Kirukeskai) was apparently later slain by one of his lich servants, Avaereene.
After this, things get vague.
Between the death of Xanathar II and 1379, several beholders apparently held the title of Xanathar, or fought with each other for it, though of course, no sources specify their identities or provide any dates to elaborate on this.
In 1379, the most recent Xanathar, named Izulktur, became bored of the position and retired. I can only imagine him in a nursing home somewhere, knitting woolly hats and reading novels telekinetically, a great monocle over his main eye. 😂
Then came the current Xanathar, Zushaxx, a fairly young beholder, and rather quirky too. This is the Xanathar who owns and adores Sylgar. Oh, according to the wiki, Zushaxx was at least the fourteenth beholder to use the title Xanathar, and because he keeps the corpses of his predecessors in glass tanks.
Though the wiki says Zushaxx is the fourteenth, and Dragon Heist may hint at it, this is of course, unlikely, seeing as though not all the previous beholders may have been around to have been put in tanks, if they've been slain by adventurers, the lich Avaereene, or even just retired! Even the ones that were slain by their successors may not even have had corpses to fill the tanks, seeing as though the disintigration ray eyestalk is a thing.
That being said, not all tanks need to be Xanathar's! Some could be would-be usurpers and rivals. Or perhaps there were a handful more Xanathars than the tanks would hint at? That being the history of the Xanathar, I'd say, Zushaxx should probably be at least the sixteenth of his name.
Here's the question though: what if Izulktur was so insulted by this young punk of a beholder tarnishing the shared great name of Xanathar with his weaknesses and embarrassments (undying love for a fish for example) and came out of retirement? Monocle on his eye, brandishing a rolled up newspaper, somehow having grown a bushy white beard. "Young whippersnapper! I'll show ya how a real tyrant fights! Back in my day, we ain't need no fish to whup-butt in Waterdeep! Oop, my back's gone"
According to Volo's it would appear many many many. It is a title and not a person and the title passes from one to another, presumably upon death.
Cum catapultae proscriptae erunt tum soli proscript catapultas habebunt
It's probably impossible to know, what with how secretive they are. Have it their way, no one would even know he was a beholder, let alone the latest in a long line of beholder crime lords.
The official lore is convoluted, more rumours and glints of truth in tales of conflicting information. Years are pretty much undecided about what happened when.
Another piece of information is that apparently, he had other beholders in his service in Undermountain, though it doesn't specify how many or why. This is before later lore begins to claim all beholders hate one another's guts I guess, but maybe they are lesser beholders, and therefore acceptable? Anyway, the main point is that he had plenty of underlings and biological children to replace him.
After this, things get vague.
Though the wiki says Zushaxx is the fourteenth, and Dragon Heist may hint at it, this is of course, unlikely, seeing as though not all the previous beholders may have been around to have been put in tanks, if they've been slain by adventurers, the lich Avaereene, or even just retired! Even the ones that were slain by their successors may not even have had corpses to fill the tanks, seeing as though the disintigration ray eyestalk is a thing.
That being said, not all tanks need to be Xanathar's! Some could be would-be usurpers and rivals. Or perhaps there were a handful more Xanathars than the tanks would hint at? That being the history of the Xanathar, I'd say, Zushaxx should probably be at least the sixteenth of his name.
Here's the question though: what if Izulktur was so insulted by this young punk of a beholder tarnishing the shared great name of Xanathar with his weaknesses and embarrassments (undying love for a fish for example) and came out of retirement? Monocle on his eye, brandishing a rolled up newspaper, somehow having grown a bushy white beard.
"Young whippersnapper! I'll show ya how a real tyrant fights! Back in my day, we ain't need no fish to whup-butt in Waterdeep! Oop, my back's gone"