Chains Of The Underdark
illithids (commonly known as mind flayers) are monstrous humanoid aberrations with psionic powers, who live in the moist caverns and cities of the enormous Underdark. Illithids believe themselves to be the dominant species of the multiverse and use other intelligent creatures as thralls, slaves, and chattel.
Typical physical characteristics
Illithids have a basic humanoid body, but their head is octopus-like. They have four tentacles around a lamprey-like mouth, and require the brains of sentient creatures as part of their diet. An illithid who snares a living creature in all four of its tentacles can extract and devour its living brain. Their eyes are pale white, and they can see perfectly well in both darkness and light. Their sense of hearing is slightly poorer than a human’s; they have difficulty distinguishing between several sounds mixed together. Their skin is purplish blue to gray-green and covered in mucus, and is very sensitive to sunlight. Though sunlight does not actually harm them, they loathe it with incredible ferocity.
One of the most feared powers is the dreaded Mind Blast, where the illithid emits a cone-shaped psionic shockwave with its mind in order to incapacitate any creature for a short amount of time. Illithids also have other psionic powers, generally telepathic in nature, although their exact effects have varied over editions. Other powers include a defensive psionic shield and powers of psionic domination for controlling the minds of others.
Illithids are hermaphroditic creatures who each spawn a mass of larvae twice in their life. The larvae resemble miniature illithid heads or four-tentacled tadpoles. Larvae are left to develop in the pool of the Elder Brain. The ones that survive after 10 years are inserted into the brain of a sapient creature. Hosts are determined in a very specific manner. Hosts generally are humanoid creatures that are between 5 feet 4 inches and 6 feet 2 inches. The most desirable of races for hosts are Human, Drow, Elves, Githzerai, Githyanki, Grimlock, Gnoll, Goblinoid, and Orc. Upon being implanted, the larva then grows and consumes the host’s brain, absorbing the host’s physical form entirely and becoming sapient itself, a mature (but still young) Illithid. This process is called ceremorphosis. Illithids often experiment with non-humanoid hosts, but ceremorphosis involving other creatures usually fails, killing both host and larva.
Occasionally, ceremorphosis can partially fail. Sometimes the larva does not contain enough chemicals to complete the mutation, sometimes there is psionic interference. Whatever the reason, it has happened that ceremorphosis has ended after the internal restructuring, resulting in a human body with an illithid’s brain, personality and digestive tract. These unfortunates must still consume brains, typically by cutting open heads (as they lack the requisite tentacles). These beings are often used as spies, where they easily blend in with their respective host types.
The illithid society also maintains a long-standing taboo. Every so often, a mind flayer community is attacked (often by vengeful githyanki and githzerai) and its inhabitants must flee. This leaves the larvae unattended. Bereft of exterior nourishment, they begin to consume one another. The survivor will eventually leave the pool in search of food (i.e., brains). This unmorphed larvae is known as a Neothelid. If the neothelid consumes an intelligent creature it will awaken to sapience and psionic abilities, while retaining its memories of savage survival. In Complete Psionic, it was revealed that illithids have a step between larva and neothelid called a Larval Flayer, which looks like an overgrown tadpole. The existence of these beasts is a guarded secret among illithids, and it is considered impolite to speak of them.