The Netflix series Stranger Things uses the name “Mind Flayer” for the unknown enemy faced during its plot. The protagonists are all D&D players, so they picked the common name given to illithids, monstrous humanoid aberrations with psionic powers. and that believe themselves to be the dominant species of the multiversem using other creatures as thralls, slaves, and chattels. Illithids are well known for feasting on the brains of intelligent beings.
Mind flayers were created by Gary Gygax, who has said that one of his inspirations for them was the cover painting of the Titus Crow book The Burrowers Beneath by Brian Lumley. The illithid is considered “Product Identity” by Wizards of the Coast and as such is not released under its Open Game License.
Illithids have a humanoid body with an octopus-like head. 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, yet they are good at discerning from what direction sounds came from. Their skin is purplish blue to gray-green and covered in mucus, and is very sensitive to sunlight. They loathe sunlight, though it does not actually harm them.
Illithids are hermaphroditic creatures who each spawn a mass of larvae two or three times in their life. The larvae resemble miniature illithid heads or four-tentacled tadpoles. Larvae are left to develop in the pool of an Elder Brain. The ones that survive after 10 years are inserted into the brain of a sapient creature. The most desirable of races for hosts are humans, drows, elves and giths. Upon being implanted (through the ear canal), the larva then grows and consumes the host’s brain, absorbing the host’s physical form entirely and becoming sapient itself, a physically mature (but mentally 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. The transformation between the host (almost always a human or similar humanoid, such as an elf or dwarf) takes about a week, unless detected and removed within about thirty minutes of injection into the incapacitated host.
When an Illithid undergoes ceremorphosis, it can occasionally take on some elements of the absorbed host creature’s former mind, such as mannerisms. This typically manifests as a minor personality feature, and some adult Illithids have even been known to hum a tune that its host knew in life.
An illithid city is ruled by a creature called an Elder Brain which lives in a pool of cerebral fluid in the city’s center. When an illithid dies its brain is extracted and taken to the pool. Illithids believe that when they die their personality is incorporated into the Elder Brainbut in fact it’s only the memories, thoughts and experiences which are consumed and added to the sum of the whole. This fact is a closely guarded secret of the Elder Brains, since all illithid aspire to a form of immortality through this merging process. An extremely ancient Elder Brain is called a God-Brain because its psionic powers are almost limitless.
Since the Elder Brain contains the essence of every illithid that died in its community, it functions in part as a vast library of knowledge that a mind flayer can call upon with a simple telepathic call. The Elder Brain in turn can communicate telepathically with anyone in its community, issuing orders and ensuring everyone conforms.
Illithids generally frown upon magic, preferring their natural psionic ability. Psionic potential is an integral part of the illithid identity, and the Elder Brain cannot absorb the magical powers of an illithid mage when it dies. They tolerate a limited study of wizardry, if only to better understand the powers employed by their enemies. However, an illithid who goes too far and neglects his psionic development in favor of wizardry risks becoming an outcast. Denied the possibility of ever merging with the Elder Brain, such outcasts often seek their own immortality through undeath, becoming alhoons.
Illithid society also maintains a long-standing taboo related to deviations to or failures of the ceremorphosis process and hunt and destroy such exceptions. Occasionally mind flayer communities are attacked (often by vengeful Githyanki and Githzerai) and their 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 (brains). This unmorphed larvae are known as Neothelids. The existence of these beasts is a guarded secret among Illithids, and it is considered impolite to speak of them.
In Eberron, the illithid come from Xoriat, the plane of Madness. They were created by the Daelkyr in their invasion plans. It is not known if they have elder brains, but their continued existence implies that they can breed on their own. The mind flayers of Eberron are resistant to damage from all weapons except those made out of byeshk, a hard and dense metal with a purple sheen mined by hobgoblins around the Graywall Mountains in Droaam.
According to Keith Baker’s “Eberron Expanded” article about the Lords of Madness sourcebook, “The mind flayers are the chief servants of the daelkyr. The elder brains may be daelkyr creations, or they may actually be spiritual and physical extensions of the daelkyr. In either case, the elder brains form the backbone of the telepathic network that links the daelkyr together and allows them to monitor their servants. Eberron is not the first world that the daelkyr have attacked. It is possible that the mind flayers were created when the daelkyr destroyed the native world of the gith races. If so, the githzerai and githyanki are descendants of those few survivors who fled to Kythri and the Astral Plane, while the mind flayers are descended from survivors of the progenitor race who were twisted in the same manner as the dolgaunts and dolgrims.” Keith Baker has an entire article about the Daelkyr and their cults you can also read.
In your campaign
Mind flayers use stealth when possible to creep toward its enemies unseen, and then unleashes its mind blast. From its dazed foes, it chooses one creature to attack with its tentacles until the victim dies or manages to free itself. Mastermind mind flayers, though, may remain adjacent to at least one of its thralls, using it as a meat shield to block attacks while it targets enemies with its psionic powers. It might devour a brain in combat if it needs to bring horror to the other heroes involved in the combat, but it risks enraging them instead.