Illithid

In the role-playing game Dungeons & Dragons, illithids (also known as mind flayers) are semi-humanoid beings with an octopus-like head with psychic powers that often live in moist caverns and underground cities. They capture cave dwellers and other creatures living underground such as drow and dwarves, using them as slaves and as a food source, much like cattle. Illithids believe themselves to be the dominant species of their world, and thus they have no problem in treating any other creatures or humanoids cruelly.

Mind flayers have four octopus-like tentacles around their beaklike mouths, and require the brains of sapient creatures as part of their diet. A mind flayer who snares a living creature in all four of its tentacles can extract and devour its living brain. Mind flayers possess magical abilities in addition to their psychic powers, though their culture forbids them from developing their arcane abilities, as psionic powers are considered superior, and are often able to penetrate illusions. One of the most feared powers is the dreaded Mind Blast, where the Illithid emits a cone-shaped psionic shockwave to incapacitate any creature with its mind for a short amount of time.

Mind flayers also appear in some roguelike games, including Angband and Nethack. These games are mostly based on D&D, but use slightly different names and rules to avoid copyright problems. The mind flayers themselves were originally inspired by the Spawn of Cthulhu, creatures from H. P. Lovecraft's Cthulhu Mythos.

Illithids are ruled by creatures called Elder Brains - when an Illithid dies its brain is extracted and taken to a special pool in the center of the city, where the fused brain tissue of every Illithid who has ever died floats. Due to their psionic nature, the brains are still "alive" and have in fact amalgamated their intellects together to form a gestalt consciousness. This Elder Brain is the city's leader, and the pool where it floats is also where the Illithids deposit their spawn/tadpoles- despite the fact that the Elder Brain can and often does devour the larvae. An extremely ancient Elder Brain is called a God-Brain, because its psionic powers are almost limitless.

Illithids are hermaphroditic creatures who each spawn a mass of tadpole-like larva twice in their life. The larva are left to develop in the pool of the Elder Brain, awaiting to undergo the process of "Ceremorphosis", where the larva will be inserted into the brain of a sentient creature, usually a human. Upon doing so, the larva will consume the host's brain and absorb the host's physical form in order to metamorphose into an adult Illithid. When a non-human creature is used, the Illithid will be created as a hybrid, with some of the powers and physcial traits of their hosts (an Illithid hybrid Ceremorphed from a Troll would have regeneraton and greater physical strength).

Illithids rule an Island of Terror in Ravenloft called Bluetspur, where their God-Brain is actually directed by the mind of a human psionicist. These Illithids have mastered a biology-related magic, and so are capable of creating all manner of monstrous minions. The spawn of these Illithids are far more likely to create Illithid hybrid beasts when Ceremorphed.

The seat of the ancient empire was an artificial world called Penumbra, an immense disc of matter that encircled a sun. After the fall of their empire, the mind flayers largely abandoned this world, which is now inhabited by savage tribes descended from thralls of various races, including forerunners and grimlocks.

Though intensly ordered and lawful in nature, every society has its black sheep and illithids are no exception. In Illithid society, these black sheep are those illithids who choose to ignore their psychic abilities and instead develop their innate magical talent, quickly being banished from society. These illithid-mages quickly transform themselves into undead creatures called Illithiliches (a kind of lich more commonly known as an Alhoon).