(From Volo's Guide to Monsters - 2016):

Very rarely, a tadpole from an elder brain’s brine pool transforms an individual into an ulitharid, a larger and more potent mind flayer that boasts six tentacles.

Master Minds. Illithids innately recognize that an ulitharid’s survival is more important than their own. An elder brain’s reaction to the rise of an ulitharid varies. In most colonies, the ulitharid becomes an elder brain’s most favored servant, invested with power and authority. In others, the elder brain perceives an ulitharid as a potential rival for power, and it manipulates or quashes the ulitharid’s ambitions accordingly.

Birth of a Colony. When an ulitharid finds sharing leadership with an elder brain to be insufferable, it breaks off from the colony, taking a group of mind flayers with it, and moves to another location to form a new colony. After the death of the ulitharid’s body, mind flayers take its brain and place it in a brine pool, where it grows into an elder brain over a few days. This process doesn’t work on the brain of an ulitharid that dies a natural death, as a brain that succumbs to old age is too decrepit to be used in the creation of an elder brain.

Extractor Staff. Each ulitharid carries a psionically enhanced staff made of black metal. When the ulitharid is ready to give up its life, it attaches the staff to the back of its head, and the staff cracks open its skull and peels it apart, enabling its brain to be extracted.

 The brain and the staff are then planted in the ulitharid’s corpse, causing it to dissolve into ichor. This psionically potent slime helps to fuel the transformation of the area into a brine pool that surrounds an embryonic elder brain.

(From the Illithiad - 1998):

Approximately 1 out of every 25 illithid tadpoles require twice as long (20 years) to mature sufficiently for ceremorphosis; these tadpoles ceremorph into ulitharids if given the chance. However, prior to ceremorphosis, nothing distinguishes them from other tadpoles in the brine pool; that is, ulitharid tadpoles do not stand a better chance of surviving predation by the elder brain than do normal tadpoles. Normally, there are about 40 ulitharid tadpoles per clutch, and only 2 out of 1,000 tadpoles in a clutch survive to ceremorphosis. Only 1 ulitharid in 20,000 tadpoles (20 clutches) ever reaches full maturity. Ulitharids tower over their lesser kin, standing approximately 7% feet tall. Besides their size, ulitharids possess two additional tentacles, granting them a total of six intrusive tentacles and the reverence of most four-tentacled mind flayers. Finally, an ulitharid can expect to live for at least 250 years, and some have survived far longer before joining the elder brain.

Ulitharids are, in a sense, “super” mind flayers; they quickly rise to the top of the community which engendered them, exercising their will upon normal illithids for the greater glory of the race. An ulitharid’s “favored” status, arising from its two extra tentacles, often places it in the office of Creed-master, expedition leader, godly liaison, or all three simultaneously. However, the infrequency of an ulitharid’s birth means that only 1 out of every 10 illithid colonies can boast an ulitharid elder.

(From 2nd Edition Monstrous Compendium Annal Volume One - 1994):

Ulitharids, or Noble Illithid, are the elite of mind flayers, favored by their god the elder brain and free to exercise their will upon lower illithids and all other humanoids that fall under their power.

Ulitharids tower over their lesser kin by more than a foot, standing 7� feet tall. They are colored similarly to other illithids, but they are darker than normal mind flayers. They have six writhing tentacles surrounding their mouths, not four like their common underlings, which are filled with sawlike teeth. Like most mind flayers, ulitharids dress in robes that cover their grotesque bodies from the ground to the neck, and high-crowned headresses are not uncommon.

With their incredible intellect, ulitharids can understand the spoken language of many races, but their mouths are illsuited for speech. Of course, they can communicate freely with any creature through the use of their innate telepathy.

Combat: The ulitharid’s six faceted tentacles are much stronger than those of normal illithids, so the monster requires only 1d3 rounds to reach a victim’s brain, once the tentacles have taken hold of the victim’s head. Each tentacle inflicts 1d4 points of damage upon a successful hit (at which point the tentacle has seized the victim’s head unless the DM rules otherwise, and only thee tentacles are needed to establish a grip that allows the creature to begin boring into the opponent’s head in quest of his brains.

A ulitharid’s mind blast is also much than more deadly than its common counterpart. It has the same area of effect as a normal mind blast – a cone 60 feet long, 5 feet wide at its point of origin, and 20 feet wide at its terminus – but those who fail to save vs. spell with a -4 penalty become feebleminded (as per the 5th-level wizard spell).

Ulitharids also have the following spell-like powers which they can employ, one at a time, once per round, as a 10th-level wizard: astral projection, charm person, charm monster, ESP, eyebite, forget, legend lore, levitate, plane shift, suggestion, telekinesis, and true seeing. All saving throws vs. these powers are rolled with -4 penalties. Ulitharids can also heal up to 25 points of personal damage per day. This process requires a full round to occur, during which the ulitharid must pause and concentrate fully upon healing.

Habitat/Society: Ulitharids are the noble folk of illithid society. About one in every 25 illithid tadpoles matures into a ulitharid. The ulitharids become caretakers for the community's elder brain, ambassadors to other illithid cities, and leaders of small illithid villages or outposts. A few sages believe that they answer to even more powerful illithids, although none of these beings have ever been seen by surface dwellers.

Ecology: Ulitharids live twice as long as normal Illithids, or about 250 years. They also spend twice as much time in the tadpole state. Ulitharids are among the most feared creatures of the subterranean world, and few creatures will challenge them.