Illithid are a race from the Far Realm
Illithid are tall and frail-looking monstrous humanoids. They possess octopus-like heads and are bald - incapable of growing hair of any form. Their mouths, full of razor sharp teeth, are surrounded by anywhere between four and eight prehensile tentacles protruding from their face. Illithid have skin colors ranging between pale blue and dark purple. Besides their otherworldly head, Illithid have similar physiology to humans, the only other notable distinctions being their hands, which have three fingers and one thumb, each possessing one talon-like nail, and their feet, which have no toes. Illithid eyes are deep-set underneath heavy brows and they lack any visible nose or ears.
No one really knows when the first Illithid came to be only that they are abominations and are created rather than born. The Illithid, or the Mind Flayers, are a direct result of mortals being influenced, and subsequently transformed, by the Pool of the Oldmind.
It is said that the Illithid used to rule the universe conquering world after world with their immeasurable intellect and unspeakable cruelty. Through the use of slaves, the most numerous of which being the present day Githyanki/Githzerai, the Illithid were able to create a massive empire spanning across the multiverse. Unfortunately for the Illithid the Gith eventually developed a resistance to the mental powers of their masters and a rebellion began and which spread all throughout the empire. The empire collapsed, the Illithid were crippled and doomed.
The only reason the Illithid still remain to this day is because of a civil war breaking out amongst the collective Gith subsequently leading to the race being factionalised into the Githyanki and the Githzerai. The distruption allowed the Illithids to retreat and continue out their existence in the Underdark.
Even though people consider Illithid and Mind Flayers to be the same it is theorized that the Mind Flayers are servants of the Oldmind and the like of which existed while the race still ruled the multiverse. The Illithid, on the other hand, are Mind Flayers that have developed independence and only became present after the near destruction of the Mind Flayers and the eradication of nearly all Elder Brains.
The Illithid are a unified people and as such are all generally after the same goal: the complete domination and rule of all existence. Before the near destruction of their existence at the hands of the Gith it was accepted that everything that an Illithid did was for the advancement of the people as a whole but in present times it is much more likely to find Illithid geared more towards personal gain.
An Illithid city is ruled by a creature called an Elder Brain which is believed to be the final stage of an Illithid's life cycle. The Elder Brain lives at the center of an Illithid city and is ruthlessly protected by the city's inhabitants as well as secured within a well protected cave. Whenever an Illithid passes on it's brain is extracted and taken to an Elder Brain where the memories, throughts, and experiences are consumed and become one with the Elder Brain. As such Elder Brains are said to have near god-like psionic abilities and are often regarded as advisers and de-facto leaders. With its immense psionic power an Elder Brain can communicate with anyone in its community and uses this power to issue orders and ensure conformity.
Since the uprising of the Gith the Illithid have proven to be a more independent people and have even proven to be hostile toward one another often killing one another for nothing more than sport.
Probably the most disturbing aspect of the Illithid is their diet: they use their tentacles to capture and subdue creatures so that they can extract the creatures brain. Each devoured brain adds to an Illithids pool of intelligence and makes (EDITING)
Since Illithid are xenomorphic they often take on heavily bastardized titles derived from their "old" names.