The Illithids are an aberrant race from the fictional universe of the Dungeons & Dragons pencil-and-paper roleplaying game.
The Illithids (aka, Mind Flayers) are cephalopod humanoids with two arms, two legs, and a bulbous head with a hideous mouth surrounded by fourth tentacles. Hermaphrodites whom two or three times in their lives spawn larvae. Which resemble tiny illithid heads or tentacled tadpoles. If they survive to reach 10 years of age, they are inserted into a sapient creature's brain. Hosts whom are usually humanoids ranging in size from 5'4" to 6'2" in height. Once inside the host, it grows and consumes it's , absorbs its body, and becomes sapint. There are some who end up taking on elements of their hosts former mind. Illithids feed on brains, using the tentacles which rips out the victim's brain.
Their cities are ruled by creatures known as Elder Brains, which reside in the center of their cities, in a pool of cerebral fluid. Upon their death, an Illithid's brain is extracted and taken to the Elder Brain. Illithids believe they will live on in the Elder Brain, their personalities being incorporated. In reality, only it's thoughts, memories, and experiences are incorporated into the Elder Brain. The God-Brains are incredibly ancient Elder Brains, called such due to their nearly limitless psionic powers. Elder Brains serve as vast troves of knowledge which can be telepathically called upon by Illihids, who in turn receive orders from the Elder Brains.
Illithids have formidable psionic powers, that ability being an important part of their identity. Powers which include levitation, telekinesis, mind control, and even teleporting themselves to other dimensions via plane shifting. They usually communicate with each other telepathically. Illithids for the most part look upon magic with disapproval, though they do tolerate the study of magic, but only to a limited degree and for the purpose of understanding their enemies. They see other races as being inferior and are forever plotting and scheming against them, seeking power and domination. Due to their psionic powers they are respected by the dominant races within the Underdark (e.g. Beholders and Drow) and are feared on numerous worlds across the multiverse.
They are an ancient race, one whose existence predates recorded history and which even the ancient Aboleths with their perfect racial memory, have no idea of their origins. Millennia in the past, they were the most powerful race in the Inner Planes. Controlling great empires from the Ethereal and Astral planes, that spanned numerous worlds. Commanding a massive spelljammer fleet consisting of countless nautiloids, conch-shaped flying ships. It was an empire of such power that at one point it even threatened the Outer Planes. The spacefaring Illithids operated differently from their land-bound cousins, operating more as traders than conquerors. Unfortunately for there was an uprising by the Gith, their most important slave race. Whom overthrew the Illithids and destroyed their interstellar empire, driving the Illithids to near-extinction. They are now forced to operate underground, hidden from the Gith who continue to hunt them down.