The article "The Sunset World" by Stephen Inniss in Dragon #150 (October 1989) presented a world that had been completely ravaged by mind flayers. The "Dragon's Bestiary" column, in the same issue and by the same author, described the Illithidae, the strange inhabitants of this world.
The ulitharid, or "noble illithid" was introduced in the Dungeon adventure Thunder Under Needlespire by James Jacobs in Dungeon 324 (July/August 1990), and later included in the Monstrous Compendium Annual One (1994).
The Complete Psionics Handbook (1991) presented ways on using mind flayers with psionic powers.
The Alhoon, also known as the illithilich or mind flayer lich, was introduced in the Menzoberranzan boxed set, in the booklet "Book One: The City" (1992).
The book "The Illithiad" (1998), and the Monstrous Arcana module series that accompanies it, greatly develops the mind flayer further. "The Illithiad" introduced the illithid elder brain and the illithid-roper crossbreed, the urophion. The module Dawn of the Overmind featured an origin story for the illithids.
The Expanded Psionics Handbook (2004) re-introduced the psionic mind flayer. Monster Manual V (2007) introduced the concept of "thoon", a commodity valuable to illithids.
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.
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 nervous habit and/or reaction, like nail-biting or tapping one's foot. An adult illithid has even been known to hum a tune that its host knew in life. Usually, when a mind flayer inherits a trait like this, it keeps it a closely guarded secret, because, were its peers to learn of it, the illithid in question would surely be killed. This is due to an illithid legend of a being called the Adversary. The legend holds that, eventually, an illithid larva that undergoes ceremorphosis will take on the host's personality and memory in its entirety. This "Adversary" would, mind and soul, still be the host, but with all the inherent abilities of an illithid.
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.
Wakeman, an enterprising trader and scholar of Underdark exotica, allowed himself to be captured by the illithids on one of his expeditions. Through the use of a non-magical mixture of various herbs that Wakeman named laethen, he was able to preserve his consciousness through ceremorphosis, and learned to use his psionic powers to keep from having to consume brains. Thus he worked against the illithid plots from within. The legend of the Adversary was born from his frequent sabotage, though the actual acts were never connected to him. The players' characters in the adventure become his agents in stopping the illithids' plans, as his own movements rely upon secrecy from his "fellow" illithids.
For another person to imitate Wakeman's deed would require at least one dose of laethen (the making of which Wakeman kept secret) and to be put under ceremorphosis within a week of consumption. The drug only has a 40% chance of success, and the new illithid must also never consume a brain, for the act destroys the host's personality and replaces it with the illithid personality.
Superior in nearly all ways to a regular mind flayer, ulitharids possess two extra tentacles, which are twice as long as the others, and an extreme arrogance, even by the standards of their own kind. Only the elder brain holds more sway within an illithid community.
After the shattering of the empire, an uneasy truce was struck, only broken by skirmishes and murder. The cha'asii went their separate ways; establishing villages like Ke-Cha-Yat where they could live in peace from the yaggol. This would all change with the coming of Gloomwing, a former orthlox Black Dragon that joined with the Brethren, the cult followers of Maladar an-Desh, Lord of Wizards.
Brainstealer Dragon: A mix of illithid and dragon, these powerful wyrms occasionally rule over illithid communities that lack an elder brain.
Illithidae: Illithidae are to mind flayers as less intelligent animals are to humans. Known types include the cessirid, embrac, kigrid, and saltor. Dragon magazine once published a template for use in creating an illithidae creature, for use with the 1st Edition of the Advanced Dungeons & Dragons game.
Illithocyte: Illithid tadpoles that survived the fall of a mind flayer empire, they evolved into a new life form and crawl about in groups seeking psychic radiation on which to feed.
Kezreth: A living troop transport and battle platform created from the severed head of a shamed illithid. They serve in this capacity in the hope of redeeming themselves and being allowed to return to the elder brain.
Mind Worm: Created by illithids to serve as assassins and bounty hunters, these powerful psionic creatures resemble smaller purple worms. They can attack from far distances with their probe worms.
Mindwitness: Inserting an illithid tadpole into a beholder results in these abominations, which are used as guards and sentries.
Mozgriken: An illithid tadpole inserted into a svirfneblin gnome while subjected to a dangerous psionic ritual creates a mozgriken. These three-tentacled ceremorphs are despised by all, but their aptitude for stealth and psionic powers of stealth and shape control make them useful spies for the illithids.
Neothelid: If an illithid tadpole survives but fails to undergo ceremorphosis, it will eventually grow into an incredibly powerful worm-like creature with illithid tentacles at the forefront of its body and immense mental powers.
Nerve Swimmers: Derived from immature illithid tadpoles, these entities are living instruments of torture and interrogation.
Nyraala Golem: A flailing, slimy, tentacled construct capable of launching surprise attacks. They often serve as guards, and are prized because their creation does not involve petitioning the elder brain to surrender part of its mass.
Octopin: A six-tentacled, purple-skinned monstrosity with a single eye created by mind flayers.
Oortlings: These docile humanoids with enlarged brains were bred by illithids as food.
Tzakandi: Illithid tadpoles inserted into lizardfolk create tzakandi, which the mind flayers use as slave labourers and personal guards.
Uchuulon: A chuul implanted with an illithid tadpole becomes an uchuulon. Also known as slime chuuls, illithids use them as hunters and guardians.
Urophion: Inserting an illithid tadpole into a roper results in these miserable creatures, which are used as guards and sentries.
Ustilagor: Mind flayers farm these larval intellect devourers for food and sentries.
Vampire Squid: Servitor creatures created by illithids to extend their reach below the surface of Underdark waters. They have a maw of sharp teeth which can be turned inside out and function as defensive spikes (found in Dragon #227).
Voidmind Creatures: A voidmind creature is an ordinary creature (such as a normal humanoid, animal, or whatever) whose mind has been nearly completely devoured by a mind flayer, but enough has been left intact for basic motor function. Further psionic rituals give these near dead creatures a semblance of life. The resulting creatures act as minions and spies for the Illithids.
Carapace symbionts are bred to serve as personal armor and boost other abilities. Known varieties include the backwatcher, fastbreak, hardy, silent, slippery, and strongarm carapaces.
The 3rd Edition D&D book Lords of Madness states that illithids are refugees from a far distant future. Facing extinction at the hands of some unknown adversary, they sacrificed a large number of elder brains to generate a temporal rift that transported the survivors aeons into the past, but little more than a mere two thousand years before the present (in this sense, they are aberrations because they exist before their natural time).
The 2nd Edition book The Illithiad suggests they may be from the Far Realm, an incomprehensible plane completely alien to the known multiverse. There is no mention of time travel in this theory. Instead, they emerged somewhere and somewhen countless thousands of years ago, beyond the histories of many mortal races, and spread from one world to another, and another, and so on. It is explicitly stated in this book that the illithids appear in some of the most ancient histories of the most ancient races, even those that have no mention of other races.
The 4th Edition preview Wizards Presents Worlds and Monsters once again has mind flayers originating from the Far Realm.
In these two differing versions of the story, much of the variance hinges upon a fictional text called The Sargonne Prophecies. The Illithiad described the Prophecies as misnamed, and that much of it sounds more like ancient myth than prophecy. Lords of Madness takes the name more literally, and states that The Sargonne Prophecies are in fact prophecy — or, perhaps more accurately, a history of the future.
Yet another version came from The Astromundi Cluster, a Spelljammer boxed set produced before The Illithiad. This version holds that the illithids are descended from the outcasts of an ancient human society that ruled the now-shattered world called Astromundi. The outcast humans eventually mutated, deep underground, into the mind flayers. (This boxed set also introduced the entity known as Lugribossk, who was depicted as a god of the Astromundi flayers then, but was later retconned into a proxy of the god Ilsensine.) In the retconned history of the illithids found in either The Illithiad or Lords of Madness, the emergence of illithids in Astromundi becomes a freak occurrence due to the intervention of Ilsensine through its proxy, since the illithids of Astromundi have their own histories as emerging solely upon that world.
However and whenever it occurred, when the illithids arrived in the Material Plane of the far past, they immediately began to build an empire by enslaving many sentient creatures. They were very successful, and soon their worlds-spanning empire became the largest one the multiverse had ever seen. They had the power — in terms of psychic potency and the manpower of countless slaves — to fashion artificial worlds. One such world was this empire's capital, called Penumbra, a diskworld built around a star, which was a thousand years in the making. Such was their might that the Blood War paused as the demons and devils considered a truce to deal with the illithid empire.
Eventually, the primary slave race of the illithids developed resistance to the mental powers of their masters, and revolted. Led by the warrior Gith, the rebellion spread to all the illithids' worlds, and the empire collapsed. The illithid race itself seemed doomed.
Fortunately for the illithids, Gith was betrayed by one of her own generals, Zerthimon, who believed she had grown tyrannical and over-aggressive. Civil war erupted, and the race factionalised into the githyanki and the githzerai. This disruption allowed the illithids to retreat to underground strongholds where they still dwell.
Illithids regularly conduct raids on all sentient settlements to acquire new thralls, because their existing stock of sentient thralls do not breed fast enough to satisfy their food and labor needs. Typically, a group of mind flayers will teleport to the settlement and swiftly incapacitate them with their psychic powers. The captives will then be marched all the way to the illithids' underground settlement by specially trained and conditioned thralls. Great care is taken to cover their tracks.
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.
Illithids typically communicate through psychic means. They project thoughts and feelings to each other in a way non-illithids can scarcely comprehend. When they do feel the need to write, they do so in "qualith." Instead of typical alphabet-based writing, illithids write in qualith by making marks consisting of four broken lines. They use each tentacle to feel the breaks in the lines, making it basically similar to braille. However, qualith is extremely complex, as each line modifies the preceding lines through explaining abstract concepts associated with the above words in ways no human can understand; only by understanding all four lines simultaneously can the meaning be understood properly.
Their archenemies are the githyanki and the githzerai, descendants of the rebellious slaves who destroyed their empire millennia ago. Hunting and slaying illithids whenever they can is an integral part of their cultures.
Illithids fear the undead because these creatures, even the sentient ones, are immune to telepathic detection and manipulation, and have no brains to consume. Confronting such mindless creatures can even be traumatizing to some of them.
According to the Lords of Madness history, Illithids are one of the only races respected by the aboleths. This is because the aboleths remember the origin of almost every other race, through their hereditary memory. However, illithids, as far the aboleths can remember, just appeared without preamble, which scares them.
Mind Flayers are one of the primary factions in the Spelljammer campaign setting. While less prominent than the neogi, illithids are in complete control of Glyth, a Realmspace planet, and have been for millennia.
Illithids' primary ship type is the nautiloid (shown at right), a 35-ton craft resembling a nautilus. Nautiloids are 125' , or 180' long including the tentacle-like piercing ram. The ships' coiled shell provides the comfort of enclosed space and protects the illithids from the rays of suns.
In the Spelljammer setting, the illithids are the creators of the oortlings, a humanoid race of high intelligence and enlarged size. Bred as food, the oortlings are completely docile and have little motivation and almost no instinct for self preservation.
In the MUD Lusternia: Age of Ascension, the Ilithoid are a race very similar to the Illithid. They are the only race to be born of a Soulless God, and are therefore looked upon as abominations by most of the other races. As a result, players who choose the Illithoid race are only allowed within the city of Magnagora and the commune of Glomdoring.
In World of Warcraft, Mind Flay is a channeled spell that can be acquired from the Shadow talent tree.
In the new series of Doctor Who, the Ood are bald aliens with a mass of tendrils in place of a nose and mouth. As introduced in The Impossible Planet, the Ood are a passive and peaceful slave race, telepathically linked; later, in The Satan Pit, a demonic entity dominates their hive mind to make them killers. In Planet of the Ood, we discover that humanity removes an auxiliary brain to make them servile, and isolates them from a single central brain to maintain control. The inspiration for the Ood may be Mind Flayers, or Cthulhu which indirectly inspired the Mind Flayers.