The illithids, commonly known as mind flayers, are a sophisticated race of aberrations with formidable mental powers. Their horrific life cycle and alien technology have resulted in a host of illithid subspecies and related creatures, described below.

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True Illithids

Mind Flayer

Classification: Aberration (3E, 5E), Aberrant Humanoid (4E)
Challenge Rating: 8 (3E), 14 (4E), 7 (5E)
Playable: 3E
Alignment: Lawful Evil, Evil (4E)

Humanoid creatures with tentacled faces, psionic powers, and a rather unique dietary requirement. They dwell in alien colonies within the Underdark, but regularly raid the surface world for captives, or send their agents to manipulate other civilizations.

Aliens Are Bastards: The Illithids are the most iconic of D&D's evil aliens. They can only reproduce by turning humanoids into more Illithids and consume brains to live, and they are not particularly nice to each other either. Only the most evil of humanoids dare to trade or ally with them.
Amnesiac Resonance: Illithids are sometimes known to inherit minor tics from their body's original owner, something which they find both shameful and disturbing. In fact, there's a persistent superstition among illithids that one of them could be "The Adversary" - their name for a theoretical mind flayer who retains the entire personality of its host, becoming a Hunter of Their Own Kind who can infiltrate and destroy illithid society from within. All indications are that the birth of an Adversary isn't actually possible, and the illithids' fear of this event is entirely irrational. As such, any time the illithids discover one of their own has undergone a Heel–Face Turn, they will do everything they can do kill them to avert the possibility.
Anti-Magical Faction: In some of their depictions the Mind Flayers despise arcane magic, with deviant arcanists being shunned. The reason for this is twofold; firstly, Illithids already have innate psionics, and consider arcane magic, which requires long periods of study and practice, to be inferior. Secondly, elder brains discourage arcane magic because it works independently of the psionic network, and empowers individual illithids to strike out on their own. This trait is not consistent across editions, to the point where 3E illithids actually have wizard as their favoured class.
Bio-Augmentation: Illithids are known to "upgrade" their thralls with traits like heavy claws, eyes that can see in the dark, or the flayed skin of a humanoid (to grant their more monstrous servants a disguise). There's even a graft which allows thralls to act as relays for their master's mind blast. These modifications have the added bonus of reducing the thrall's mental strength and making them easier to control.
Bizarre Alien Psychology: A big factor behind illithids' Always Lawful Evil tendencies is how their minds work. Though they can sometimes seem dispassionate, illithids are in fact capable of intense emotions, but they internalize them almost completely. More significantly, their emotional range is entirely negative, limited to anger, fear, pride, shame, contempt, envy, etc. The closest a mind flayer comes to happiness is the joyous feeling experienced while consuming a brain, "but even this is mixed with such sadistic and hateful overtones that it can't be considered 'happiness' as most races would define it." Additionally mind flayers learn emotions from resonance stones (see below), not from each other, and fulfil their emotional needs with those objects rather than through friendship or love.
Bizarre Alien Reproduction: Illithid reproduction is two-part; first, an adult illithid vomits up or lays (it depends on the sourcebook) a mass of gelatinous eggs in the elder brain's pool, which hatch into tadpole-like illithid larvae. Assuming they aren't eaten by others in the pond or the elder brain itself before maturing, the tadpoles are then inserted into the ear of a helpless humanoid, whereupon they consume its brain and physically merge with its spinal column to become an adult illithid. Should a larva go for too long without implanting, which usually only happens if a mind flayer colony is destroyed and the larvae left to fend for themselves, it will grow into a massive, wormlike and largely mindless monster called a neothelid.
Blessed with Suck: Some illithid tadpoles are unable to fully transform their host body, leaving them with a human appearance and no brain-extracting tentacles but still requiring them to eat brains to survive. These failed illithids are generally employed up as spies and infiltrators in human cities, where they become Serial Killers with the Calling Card of carving their victims' heads open.
Brain Food: An illithid needs to eat at least one humanoid brain per month, and ideally prefers one per week. Lords of Madness explains this as being due to ceremorphosis leaving illithids without a true brain integrated into the body — instead, the skull is occupied by the original parasitic larva, which controls and is firmly rooted into the body but does not actually perform the metabolic functions of the organ it replaced. Thus, illithids cannot produce a number of important hormones, regulatory chemicals and psychic signals, and make do by consuming those of other beings. Note that illithids can't survive exclusively on brains, and require regular meals of "normal" nutrients as well, typically a stew of various proteins brewed in vats in illithid settlements.
The Chessmaster: Illithids work to undermine the civilizations of the surface world, not because the mind flayers view them as a threat, but as a sort of political experiment. Every empire that collapses due to the illithids' machinations is providing them with data they can use to avoid making the same mistakes when they (re)establish their own empire.
Coup de Grâce: In some rules, illithids can make an "extract brain" attack against a helpless or grappled foe, which for the vast majority of creatures is a One-Hit Kill. Combined with their mind blast's ability to stun targets for several rounds at a time, this makes mind flayers extremely dangerous even when they aren't using the rest of their psychic repertoire.
Cthulhumanoid: Humanoid shape; skin colored in shades of dark green, blue, or purple; and four tentacles emerging from their face, with a lamprey-like mouth in between them.
Emotion Bomb: Illithids know how to craft "resonance stones" out of crystal, precious gems, marble, quartz, etc., which can range in size from pea-sized to enormous spheres floating on a column of water in an illithid city's central plaza. Each resonance stone is keyed to a specific emotion and constantly "broadcasts" that feeling, growing more intense the closer one comes to the stone. Illithids stock their homes with resonance stones emanating sensations of pride, confidence and superiority, while resonance stones of loyalty and duty are placed around a mind flayer settlement. More malevolently, resonance stones of horror and surrender are used during interrogations, resonance stones of helplessness, resignation or satisfaction help keep order in the pits holding captives, and resonance stones emanating calm and contentment are placed around a creature undergoing the final stages of ceremorphosis, to help the "newborn" mind flayer adjust to its sudden sentience.
Even Evil Has Loved Ones: In a very twisted sense, but perhaps the only humanizing trait illithids have is their connection to their thralls, which is far more intimate than it would seem, as if their thrall dies their master will suffer in return, and having thralls around keeps them from going insane from loneliness. Each illithid also has a personal thrall whom they favor; going out of their way to not eat their brains in moments of hunger or anger, giving them toys and trinkets to entertain themselves with when not working, and even taking on pronounceable names for their benefit.
Fake Town: Almost every illithid settlement uses their thralls to help camouflage their real city, for example by setting up a modest duergar farming village around the hidden entrance to the illithid city below.
Fantasy Aliens: Probably the most cut-and-dried example of this in the game: they're Cthulhumanoids who not only come from outer space, but the future as well.
Formerly Sapient Species: Some depictions portray illithids as future humans who are in the middle of devolving into neothelids, and became so desperate to retain their humanoid forms that they travelled back in time to parasitise their own ancestors.
Hates Being Alone: Illithids' absolute desire to dominate is so ingrained into them that without minds to control they feel incomplete, and will put all other plans on hold to acquire more thralls lest they Go Mad from the Isolation. Mind flayers are also used to having the background psychic presence of their colony's elder brain, and so on missions outside its influence, they might bring along a "brainmate," a crystal globe containing a walnut-sized bud from the elder brain, which is nominally sentient and can respond to the mind flayer's telepathic questions... as well as perfectly record everything its bearer experiences, information the elder brain can acquire by simply re-absorbing the brainmate.
Heel–Face Turn: In exceptionally rare cases, individual illithids have been known to overcome their inherent desires and become morally neutral, and in extreme case even good, questioning the need to dominate others and consume brains. These individuals who think differently would quickly be rooted out and killed by other illithids if discovered, and as such most need to live in isolation from their kind, proving that they're Not Always Evil.
Magitek: Illithids are brilliant, and capable of examining and solving problems from scientific, psionic and arcane viewpoints. Artifacts like the "dampsuits" they wear in hot, arid climates are thus sophisticated hazardous environment suits that also carry a magical enchantment.
Mind Probe: In the rare cases a prisoner is immune to an illithid's psychic questioning, the mind flayers can make use of a device called a "thought extruder." Once the illithids use their tentacles' natural enzymes to make a fist-sized hole in the subject's skull, they stick the device's needles into the prisoner's brain to more directly read the subject's mind. It takes a minute to get an answer to a question asked in this manner, but the answer is always truthful, and unless the victim makes a successful Fortitude save, their Intelligence, Wisdom and Charisma are all permanently diminished with each question... and once the interrogation is complete, they have to make a Wisdom save to avoid going insane from the experience.
The Morlocks: The mind flayers of Krynn, known as "yaggol," are degenerate creatures who have forgotten much of their heritage, while their mental powers have similarly atrophied. Most of them can only manage one mind blast per hour, and the attack doesn't stun opponents, but leaves the yaggol dazed after unleashing it from the psychic "recoil" — as such, most yaggol prefer to fight hand-to-hand. The twist is that since the illithids are normally a subterranean species, these "morlocks" live on Krynn's surface, in dense, dark jungles.
One-Hit Kill: If a mind flayer manages to extract an enemy's brain, the creature dies instantly, unless it has multiple heads or is undead.
Outside-Context Problem: As per their Lords of Madness background, the illithids originate from the far future, when their empire of dead and dying suns was facing catastrophe at the hands of an unknown aggressor. Using a psionic ritual, the mind flayers cast themselves back through the aeons to a relatively short period of time before the D&D "present" time. This is why aboleths, who can remember things from before the dawn of time, are creeped out by the illithids, which as far as they can tell just came out of nowhere.
People Farms: Downplayed. Illithids can and do keep and breed humanoid slaves in order to have ready access to brains to consume, but there are a number of issues that make this scheme impractical. Firstly, illithids thrive best on humanoid brains, and humanoids breed and mature slowly, requiring disproportionately large farms to compensate — some illithids minimize this issue by farming quick-breeding and quick-growing species like goblins, orcs and grimlocks, but these still require a decade or more to grow to reproductive age and aren't actually as nutritious as slower-growing humans, dwarves or elves. Secondly, the "flavor" and nutrition of a brain are directly related to the complexity of its mind's experiences and emotions, which psychic thralldom strongly inhibits. Thus, while illithids keep some slaves as future food and even breed them, they chiefly rely on raiding independent settlements for food.
Picky People Eater: Besides just eating brains, illithids are extremely discriminating about their food. Firstly, they prefer complex minds, packed with knowledge and experiences and as intelligent and emotionally rich as can be achieved — typical peasants may just make do, experienced leaders and adventurers are relished, a sage or wizard is a rare treat. Secondly, they also have strong opinions about which species they most enjoy eating. Troglodytes are repulsive, and only eaten to avoid starvation. Goblinoids, orcs and ogres are acceptable, with surface-dwelling ones being preferred to their subterranean cousins, but are never a first choice. Humans, elves, drow, duergar and dwarves, being generally more intelligent and emotionally developed, are favored staples of the illithid diet. Grimlocks are a treat due to their lack of sight giving their brains a unique flavor. The long-lived, elusive and highly emotive fey are rare delicacies.
Psychic Powers: Illithids have many psionic powers, the most infamous of which is their mind blast, a cone-shaped psychic assault that stuns intelligent creatures long enough for the illithid to eat their brains.
Public Execution: The mind flayers have developed a gruesome variant to sate their hunger for brains. Since it's impractical for every illithid to gorge themselves on gray matter, they've come up with the spectacle of "performance eating." In a stadium-like part of a mind flayer colony, a choice victim is eaten by an illithid who telepathically shares every aspect of the experience with everyone in the audience, from the extraction of the brain to its consumption. Typically the victim is an adventurer or similar formerly-free person with a mind full of exploits and interesting experiences, and the consumption of their brain is enhanced by how the victim is fed and treated prior to the feast, as well as how they're restrained during it. Some "performance eaters" become something like star performers in their communities, earning esteemed living quarters for their efforts.
Retcon: Illithid lore underwent a number of changes during the 2nd-3rd Edition transition.
- In 2nd Edition, illithids reproduce asexually instead of requiring an elder brain to produce larvae. In addition, instead of converting an implanted victim into a new illithid, the larva feeds on the dead body it's planted into while maturing before bursting out as newborn illithid.
- Before and during 2nd Edition, illithids are atheist, as they believe themselves the only beings deserving of worship. Starting with 3rd, they worship Ilsensine.
- Pre-3rd Edition illithids are much more omnivorous than their later incarnations, and can subsist on a wide variety of foods. The eating of brains is primarily a symbolic gesture of dominance over other beings — illithids believe the mind to be all-important and superior to the physical body, and so enjoy humiliating defeated opponents by devouring the most important part of their bodies.
Series Continuity Error: One of the many complaints about the 3.5e splatbook Complete Psionic is its Illithid Heritage feat chain and Flayerspawn Psychic Prestige Class, representing the descendants of illithids mating with humanoids... something which should be impossible given how illithid reproduction works. Fans typically Hand Wave the flayerspawn as being people experimented on by illithid Mad Scientists instead, or even as the ancient predecessors of illithids who are evolving these traits for the first time. The existence of polymorph spells/psionic abilities may also help justify such things.
Slave Race: Mind flayers are fond of using these, but in a way qualify themselves, albeit a higher caste of one. They spend their entire lives under the influence of the elder brains, and any mind flayer that the elder brains can't control is made outcast and hunted down. And the elder brains are themselves made of the memories of the minds of the mind flayers they brainwashed. In Baldur's Gate III, you can get a Nonstandard Game Over where you turn into a mind flayer, and the elder brain addresses you as "thrall" in the cutscene after you transform.
The Unpronounceable: Illithids identify themselves to other illithids through strains of thoughts and images too complex to be spoken or expressed by other races, so any spoken names are actually rough translations that use descriptive words to get the general idea across. Occasionally an illithid will adopt a pronounceable name themselves, be it for the benefit of their thralls or to terrifiy enemies.
Weakened by the Light: Downplayed; illithids don't take damage or suffer any penalties from being in natural light, but they find the experience as repellent as a normal human would being immersed in blood. Some of the more ambitious elder brains thus scheme toward the goal of extinguishing the sun.
Why Did It Have to Be Snakes?:
- Purple worms are one of the only creatures that the mind flayers outright fear universally.
- Illithids also have trouble with the undead, since they have no minds to dominate, can evade psionic detection, and aren't inconvenienced by having their brains bitten out of their skulls.

Ulitharid

Classification: Aberration (3E, 5E)
Challenge Rating: 12 (3E), 9 (5E)
Alignment: Lawful Evil

Very rarely, an implanted tadpole transforms an individual into an ulitharid, a larger and more potent mind flayer that boasts six tentacles. In 5th Edition, they're revealed to be nascent elder brains.

Asskicking Leads to Leadership: The ulitharids' power over common illithids is rooted in two factors — their great rarity, and their much greater physical strength and psychic powers.
Large and in Charge: Ulitharids tower over common illithids, typically standing between seven and eight feet in height.
Metamorphosis: In 5th Edition, after an ulitharid establishes a new illithid colony, it ritually opens its own skull to expose its brain. Its illithid servants then plant its brain in its body, which rapidly dissolves into a pool of ichor to sustain the ulitharid's development into an embryonic elder brain.
Monster Lord: Ulitharids are large, strong and psychically powerful illithids who arise to be natural leaders over their lesser kin.
Mook Lieutenant: Ulitharids are typically seconds-in-command in their cities, obeying the elder brain but commanding the smaller and weaker illithids.
Staff of Authority: An ulitharid's status and authority are symbolized by the twisted black staff which it carries at all time.

Elder Brain

Classification: Aberration (3E, 5E), Aberrant Magical Beast (4E)
Challenge Rating: 25 (3E), 23 (4E), 14 (5E)
Alignment: Lawful Evil, Evil (4E)

The undisputed rulers of illithid colonies, the elder brains use their prodigious intellects to guide their subjects, and their formidable psychic powers to defend against any threats, whether external or internal.

Ambiguously Related: Prior to 5e it's not clear whether elder brains are truly a type of illithid as they appear, or are some kind of parasite or biological supercomputer that took over illithid society, possibly deceiving even the illithids themselves. Likewise it's stated that the elder brain of Thoon (see below), which is weaker and has different powers than a standard elder brain, might actually be just a creature that mutated to resemble one.
Big Brother Is Watching: An elder brain provides constant psychic surveillance of a mind flayer colony. In addition to making it very difficult for enemies to infiltrate it, this allows the elder brain to detect any dissent among its illithid subordinates.
Bizarre Alien Reproduction: In 5e, elder brains are the final stage of the mind flayer life cycle: once an ulitharid reaches the end of its life, it removes its brain, which grows into an elder brain.
Brain Monster: Illithid elder brains float in large brine pools in illithid cities. Each is made up of the combined brain matter of old illithids that sacrificed themselves to join it.
Hive Queen: Their role in illithid communities is comparable to that of a queen in an eusocial insect hive — that is, a vast, immobile specimen that sits in the center of the colony, producing endless swarms of larvae that grow into non-reproducing "workers", and periodically popping out a new "queen" in the form of an ulitharid.
Mind Hive: Illithids have no fear of death, as a dead mind flayer's brain can be removed from its body and placed into the elder brain's pool to be absorbed by the greater brain, thus allowing the mind flayer to join the mental gestalt of past illithids. At least, in theory — a secret that elder brains jealously guard is that they only keep the information within the brains, not the consciousnesses.
Scam Religion: While an elder brain does absorb information from illithid brains it absorbs, and feeds off their psionic energy, no part of the original mind flayers' consciousness survives the elder brain's consumption. The elder brains are careful to keep the truth about this secret, to better manipulate and ensure the loyalty of the mind flayers.
Stronger with Age: Elder brains are immortal unless killed, and never enfeeble or grow senile — they simply become wiser and stronger, their psychic power slowly but steadily increasing.

Ceremorphs

Generally speaking, only Medium-sized, mammalian humanoids produce "true" illithids through ceremorphosis, but the mind flayers are willing to experiment by implanting their tadpoles in other creatures. In most cases these efforts are unsuccessful, but in others, the result is a new Hybrid Monster that blends its original and illithid heritages to varying degrees, and typically inherits the mind flayers' brain-based diet and some portion of their psychic ability.

Brainstealer Dragon

Classification: Dragon (3E)
Challenge Rating: 4-25 (3E)
Alignment: Lawful Evil

Horrors produced from the thankfully rare implantation of an illithid tadpole inside a true dragon, introduced in Dragon Magazine #337. Their abilities are the same regardless of their draconic "parent", effectively functioning as a distinct species of true dragons in their own right.

Brain Food: If a brainstealer dragon can hold an enemy with its tentacles for five consecutive rounds, it extracts and eats the victim's brain.
The Chessmaster: Brainstealer dragons combine draconic hubris with illithid manipulativeness, and thus "view life as a game that they intend to win."
Combat Tentacles: Instead of a head, the brainstealer dragon has four tentacles which it uses to grab enemies and extract their brains.
Cooldown: Their mind blast functions as their equivalent of a normal True Dragon's Breath Weapon, making it exceptionally large and powerful but requiring a rest period between uses.
Long Neck: They don't have heads so much as long tentacle-tipped necks with eyes on the sides.
No-Sell: They're immune to acid attacks and other mind flayers' mind blast.
Psychic Powers: Brainstealer dragons possess various psionic powers, including a cone-shaped mind blast that serves as its equivalent of a Breath Weapon.
Stronger with Age: Their True Dragon biology allows their psionic abilities to keep progressing beyond the standard list for half-illithids and start unlocking powers normally exclusive to full mind flayers, eventually unlocking one ability (astral projection) which even they don't possess. Likewise their mind blast "Breath Weapon" starts out working like the nerfed version of mind blast in the half-illithid template, but eventually becomes larger and stronger than the original.
Turned Against Their Masters: The main reason for brainstealer dragons' rarity is that the elder brains can't control them, and some of their great wyrms have taken over mind flayer colonies.
Wrong Context Magic: Unlike other half-illithids in 3E, they retain their Dragon type rather than becoming Aberrations, and possessing a true dragon's ability to grow Stronger with Age means they can eventually surpass a full illithid in psionic power.

Ettin Ceremorph

Classification: Aberration (5E)
Challenge Rating: 8 (5E)
Alignment: Lawful Evil

Most giants' bodies are too large for an illithid tadpole to take over, but in the case of the two-headed ettins, the mind flayers found that two tadpoles can transform and operate the lesser giants in tandem.

Genius Bruiser: An ettin ceremorph's Intelligence score is 18, just one point shy of a normal mind flayer's, while their Strength score is just as high. This makes them excellent guardians of the Elder Brain or an illithid enclave's treasure hoard.
Multiple Head Case: After ceremorphosis, one of the ettin's heads sinks into its torso, with its brain focusing on controlling the giant body, while the other is free to focus on cognition and psionics.
No-Sell: They're immune to being charmed, frightened, stunned or KO'd.
Psi Blast: Instead of a conical mind blast, ettin ceremorphs can emit a "mind bolt" a few times per day that only affects a single target, damaging and potentially stunning them.

Gnome Ceremorph

Classification: Aberration (5E)
Challenge Rating: 5 (5E)
Alignment: Any

In the rare cases a ceremorphosis attempt on a gnome doesn't fail outright, the result can be a Small illithid that, even more unusually, retains much of its past life's personality.

Death of Personality: As mentioned, averted. Gnome ceremorphs retain at least partial memories of their previous lives, and maintain most of their original personality. They still need to eat brains, and gain an illithid's desire to control and experiment, but are otherwise free to follow their original character alignment.
Gadgeteer Genius: They retain their gnomish inclination towards tinkering, hence why the sample gnome ceremorph has a fantastic laser pistol.
Telepathy: While gnome ceremorphs are natural telepaths, they prefer to vocalize out loud, though their tentacles give them a "gooey" accent.

Gnome Squidling

Classification: Aberration (5E)
Challenge Rating: 1/2 (5E)
Alignment: True Neutral

Other attempts to implant an illithid tadpole within a gnome result in these "squidlings," deformed and mentally-stunted creatures with limited psychic power, but a true illithid's hunger for brains.

Bizarre Alien Locomotion: Gnome squidlings "walk" on two of their oversized face-tentacles, while using their psionics to levitate the rest of their bodies.
Clipped-Wing Angel: They're dumber than either of their parent species, having the intellectual and emotional capacity of toddlers, and their psychic powers are limited to levitation and a weak "mind tickle" attack. As such, most gnome squidlings are killed off by their illithid creators as failed experiments.
Telepathy: Gnome squidlings are telepathic, but limitedly so — at best they can communicate a single word at a time, accompanied by a burst of emotion.

Mindwitness

Classification: Aberration (3E, 5E)
Challenge Rating: 16 (3E), 5 (5E)
Alignment: Lawful Evil

Mindwitnesses are the result of a beholder being brought safely to the brine pool of the elder brain and converted through ceremorphosis.

Adaptational Wimp: In their 3rd Edition statblock, a mindwitness is a beholder with the "half-illithid" template applied to it, which means that it has every one of a beholder's supernatural abilities — an Anti-Magic cone and Eye Beams, in other words — as well as an illithid's tentacle attacks with an "extract brain" finisher, a once-per-day mind blast, and additional Psychic Powers. But in 5th Edition, a mindwitness is a Clipped-Wing Angel, a docile beast of burden that lacks a beholder's most dangerous spell attacks or anti-magic cone. Justified, since an immensely powerful, intelligent and insane Reality Warper is the last thing a mind flayer colony would want running around.
Happiness in Slavery: In 5E, they're actually fairly docile, and if the mind flayers are removed from the equation they will just drift around looking for new masters.
Oculothorax: A mindwitnesses resembles a fleshy, tentacled orb dominated by a single central eye.
Psychic Link: In 5E, a mindwitness is basically a living psychic relay, and they have the ability to transmit any psionic message they receive to a number of other entitites within the mindwitness' sight.

Tzakandi

Classification: Aberration (3E)
Challenge Rating: 4 (3E)
Alignment: Neutral Evil

Ceremorphosized lizardfolk, these creatures are prized servants and bodyguards for their mind flayer masters, being loyal, sturdy, and intelligent enough to use their combat abilities to their fullest potential.

Acid Attack: Some tzakandi are capable of firing a glob of acid from their tentacles once per day.
Art Evolution: Their 2nd Edition art depicts them as lizardfolk with a pair of tentacles extending out of the back of their skulls and down their necks, while 3rd Edition gives them a conventional illithid head upon a lizardfolk body. Though with the note that no two tzakandi look exactly alike, neither interpretation of them is inaccurate.
Blood Knight: While intelligent, tzakandi are also constantly on edge, looking forward to their next fight.
Imprinting: Newly "born" tzakandi attach themselves to the first illithid they see, obeying their orders without question.
Lizard Folk: They look like base lizardfolk to varying degrees, though their eyes are more intelligent.
Psychic Powers: Some tzakandi have access to psionics from the disciplines of psychokinesis and psychometabolism, while others can make a mind blast once per day.

Uchuulon

Classification: Aberration (3E)
Challenge Rating: 6 (3E)
Alignment: Lawful Evil

Also known as "slime chuuls," these creatures are technically failed ceremorphs since the illithid tadpole doesn't survive the implantation attempt, but the process reduces the chuul to a creature the illithids find a more tractable slave.

Clipped-Wing Angel: The failed ceremorphosis makes uchuulons more sluggish than chuuls, and renders their carapace translucent.
Covered in Gunge: The thick slime that uchuulons ooze gives them additional physical protection, and has even odds of negating a Critical Hit.
The Paralyzer: An uchuulon's tentacles exude a paralytic secretion that renders prey helpless, allowing the illithids to easily take them captive, or for the uchuulon to feed.

Urophion

Classification: Aberration (3E)
Challenge Rating: 13 (3E)
Alignment: Lawful Evil

A surprisingly viable roper ceremorph, these creatures are relegated to perimeter defense duty by their home colony.

And I Must Scream: Urophions are thoroughly miserable creatures, with the intelligence of a mind flayer trapped in a nearly-immobile roper body, and are described as living lives of "desperate loneliness and frustration."
Pet the Dog: About the only thing a urophion can look forward to after a life of Fantastic Racism and lonely servitude is the "honor" of joining the elder brain's pool upon death.
Psychic Powers: They possess a base illithid's mind blast ability, and can use detect thoughts and suggestion at will.
That's No Moon: Like a standard roper, urophions are hard to distinguish from a normal stalagmite, but their improved tentacles and psychic powers make them even more dangerous when the ruse is up.
You Will Not Evade Me: Urophions have the same tentacles and drag attack as a base roper, but can combine it with an "Extract Brain" attack once they reel prey in.

Related Creatures

Brain Golem

Classification: Construct (3E)
Challenge Rating: 10 (3E)
Alignment: Lawful Evil

8-foot-tall masses of brains in a roughly humanoid shape, created by the illithids as silent minions.

Brain Monster: They're burly masses of brain tissue harvested from a variety of intelligent species, capped by a bud from an elder brain.
Deflector Shields: In 3E, brain golems are surrounded by a force field similar to the mage armor spell, granting them added protection that can also block incorporeal attacks.
Enigmatic Minion: When they're not serving as added "muscle" to defend an illithid settlement, brain golems are created by the elder brain itself to carry out a task it doesn't trust to its illithid subjects. As such, mind flayers view the silent brain golems with some degree of awe as they go about their inscrutable work.
It Can Think: Brain golems are more intelligent than most golems, enough to for example prioritize striking a Squishy Wizard.
Psi Blast: They have a standard illithid's mind blast attack.

Cranium Rat

Origin: Planescape
Classification: Magical Beast (3E), Beast (5E)
Challenge Rating: 2 (small pack), 5 (medium pack), 11 (large pack) (3E); 0 (individual), 5 (swarm) (5E)
Alignment: Neutral Evil (4E), Lawful Evil (5E)

Rats imbued with psionic powers by the mind flayers. Individually they are no smarter than a normal rat, but swarms of cranium rats pool their powers to gain enhanced intelligence and dangerous abilities.

Animal Espionage: Illithids use cranium rats as spies, disseminating them in humanoid settlements and counting on the fact that humans don't generally pay much attention to rats to allow them to get anywhere and listen in to secret conversations.
Hive Mind: While an individual cranium rat is only as smart as a mundane rat, if enough cranium rats come together, they merge their minds into a single one with the accumulated memories of all constituents.
Psychic Powers: Cranium rats are implanted with psychic powers by their mind flayer creators.
Swarm of Rats: Cranium rats are at their most dangerous when in large swarms, as they can combine their intellects and coordinate very effectively with one another.

Eater of Knowledge

Origin: Planescape
Classification: Aberration (5E)
Challenge Rating: 6 (5E)
Alignment: Neutral Evil

Servitors of the illithid god-brain Ilsensine, these horrors can be found guarding its demesne in the Caverns of Thought beneath the Outlands, or traveling the planes, accumulating knowledge for their master.

Brain Food: Eaters of knowledge don't have a neat "extract brain" attack in 2E, instead they can grapple victims and, in an agonizing process, tunnel nerve tendrils into the victim's skull, dealing both physical as well as Intelligence, Wisdom and Dexterity damage each round. After one to four rounds of boring, the nerve tendrils are in position and begin extracting chunks of gray matter, leading to death in at most three rounds with No Saving Throw.
Eat Brain for Memories: The whole purpose behind their existence. Not only does this increase Ilsensine's knowledge, it also gives the eaters of knowledge access to the abilities of their victims, allowing them to potentially cast spells as a priest or wizard, sneak and backstab foes like a thief, or any combination of the above.
My Brain Is Big: Exaggerated to Body Horror levels. Eaters of knowledge are bloated, reeking things with leathery hides stretched over their distended skulls, trailing ganglia and leaking brain matter.
Psychic Powers: In 2E, they know abilities from the telepathy, psychoportation and psychokinesis disciplines, such as disintegrate, dimension door and invisibility. 5E gives them a list of spell-like abilities, ranging from plane shift to mass suggestion, but what powers they can use depend upon how many brains the monster has eaten recently.
Was Once a Man: One theory on eaters of knowledge's creation is that Ilsensine removes the burnt-out brains of some of its servants and replaces them with a piece of its own gray matter.

Elder Brain Dragon

Classification: Aberration (5E)
Challenge Rating: 22 (5E)
Alignment: Lawful Evil

A live dragon that has been captured by a mind flayer colony and parasitized by the local elder brain.

Body Horror: The dragon is mutated horrifically beyond its nature, with tentacles digging into its brain and elder brain slime running out of its mouth. The artwork further shows that the elder brain has burrowed under the skin of the dragon's back and replaced the dragon's tongue with an ovipositor.
Breath Weapon: It breathes out illithid tadpoles.
Dispel Magic: A variant. Elder brain dragons can use a legendary action to shatter an enemy spellcaster's concentration, instantly ending any spell the caster was concentrating on.
No Saving Throw: There is no way to prevent an elder brain dragon from breaking your concentration once it has gotten hold of you. If you can't stay out of its reach, then say goodbye to whatever spells you were maintaining and brace yourself for the imminent psychic damage.
Puppeteer Parasite: When a mind flayer colony manages to capture a dragon, the elder brain latches onto the dragon's back and digs its tentacles into the dragon's brain, creating an elder brain dragon.
Weaponized Offspring: The elder brain dragon can release a stream of briny liquid roiling with illithid tadpoles, which can swiftly transform foes into mind flayers, allowing the elder brain dragon to grow its own roving colony.

Encephalon

Classification: Aberration (5E)
Challenge Rating: 3 (gemmule), 10 (cluster) (5E)
Alignment: Unaligned

The influence of a Far Realm rift can mutate an illithid egg sac into an ambulatory, predatory horror that near-constantly spawns progeny.

Brain Monster: Encephalon clusters look like Large, slimy, brain-shaped masses creeping through the dark places of the world, though they're actually composed of eggs, not gray matter.
Explosive Breeder: An encephalon gemmule, after budding from its progenitor, grows into an encephalon cluster in the space of a month, after which it begins producing its own gemmules. Left unchecked, a single encephalon cluster can quickly overrun an entire mind flayer colony.
Living Polyhedron: Encephalon genmules look like pyramid-shaped nuggets of flesh skittering about on spindly legs.
Mutants: Encephalons are illithids mutated before they can even hatch as tadpoles, so that their egg cluster itself becomes a new monster.
Personal Space Invader: The Tiny genmules can latch onto larger creatures, dealing regular damage until they're yanked off.

Illithidae

A kigrid, embrac and saltor (3e)

Illithidae are an order of animal-minded aberrations related to illithids in the same manner in which common mammals are related to humans. They originate from the same alien future as the mind flayers and congregate around their cities, although the nature of their modern relationship remains unclear. Illithidae reproduce by depositing larvae within their bodies of slain victims, which incubate there until ready to metamorphose into adults.

Fantastic Fauna Counterpart: Illithidae often resemble, and occupy ecological niches largely equivalent to, more familiar animals. Cessirids look and act like much like wolves, kigrids fill niches equivalent to those of aggressive omnivores and scavengers such as boars, hyenas and bears, and saltors are similar to baboons. Embracs are the only exception, as the closest analogy to their niche would be an animal that acts like a carnivorous plant.
Psychic Powers: These creature are also latent psions, with powers ranging from blur and blink to crushing despair, depending on species.

Cessirid

Alignment: Lawful Neutral, sometimes Lawful Evil

Wolflike pack hunters with four stinging tentacles around their mouths.

Animal Jingoism: Cessirids and kaoulgrim, the hounds of the githyanki, despise each other and will always attempt to fight to the death when they meet.
Savage Wolves: Cessirids are essentially monstrous alien wolves, and are characterized as wily, treacherous and dangerous pack predators who, while capable of communication, typically only view other intelligent creatures as food and have distinct tendency towards the evil end of the alignment scale.

Embrac

Classification: Aberration (3E)
Challenge Rating: 7 (3E)
Alignment: Neutral Evil (2E), True Neutral (3E)

Hulking predators resembling bloated sacks mounted on eight armored legs, adorned with a saw-edged beaks, and topped with eight tentacles bristling with hooks. Embracs are ambush predators that prefer to attack their prey from hiding.

Art Evolution: The 2nd Edition embrac is a bearlike creature with a beak, slanted eyes, and four tentacles growing from around its mouth. The 3rd Edition version is a giant sac of flesh with segmented, arthropod-like legs, no visible eyes, and tentacles growing in a ring all around its upper body.
Combat Tentacles: An embrac's main weapons are its muscular, hook-lined tentacles.
Healing Factor: If an embrac's tentacles are severed, they grow back within a couple weeks.

Kigrid

Classification: Aberration (3E)
Challenge Rating: 5 (3E)
Alignment: True Neutral

Squat, feline predators that stalk prey in the tunnels of the Underdark.

It Can Think: Kigrids aren't just cunning enough to drive prey towards dead-end tunnels or cliffs, they can also understand most spoken languages and respond in Undercommon.
Poisonous Person: Their bite attacks deliver a Dexterity-damaging poison.

Saltor

Classification: Aberration (3E)
Challenge Rating: 3 (3E)
Alignment: True Neutral (2E), Neutral Evil (3E)

Baboonlike in shape and behavior, saltors are intelligent but very primitive and live as scavengers and opportunistic predators. Saltors are fairly close kin of illithids, and like them reproduce by planting their larvae in the skulls of prey to convert them into new members of their species.

It Can Think: They're intelligent enough to made crude tools to make their lives easier, and can speak Undercommon. This means parties who treat them well can trade metal tools for information about the region, though they would be wise to negotiate with the saltors from a position of strength.
Super-Scream: A saltor's main offensive tool is a loud screech capable of inflicting sonic damage.

Illithocyte

Classification: Aberration (3E)
Challenge Rating: 2 (3E)
Alignment: True Neutral

These four-foot-long, squirming, tentacled, sluglike creatures were mind flayer tadpoles who survived the fall of their civilization and were forced to evolve into a form capable of existing outside a host.

Hive Mind: Illithocytes share a communal consciousness, and any within 100 feet of one other are in constant psychic contact.
Supernatural Sensitivity: They have a "lifesense" ability that lets them pinpoint living creatures within 30 feet.
The Swarm: They're usually encountered in groups of three to 30 creatures.
Zerg Rush: Illithocytes spend most of their time in large family masses, and are adept at fighting side-by-side in close quarters and coordinating their attacks against a single target.

Intellect Devourer

Classification: Aberration (3E, 5E), Aberrant Magical Beast (4E)
Challenge Rating: 2 (ustilagor), 7 (intellect devourer) (3E); 2 (5E)
Alignment: Chaotic Evil (3E, 4E), Lawful Evil (5E)

Brain-like monsters created as guards by the illithids.

Brain Monster: An intellect devourer is basically a brain running around on four little legs. Its modus operandi is to crack a victim's skull open, remove the brain and take its place.
Puppeteer Parasite: Intellect devourers operate by killing victims, crawling inside their craniums and pupating their bodies to either exploit their size and strength or to impersonate them.
Supernatural Sensitivity: In 5E they can sense sentient beings (i.e. creatures with an Intelligence score of 3 or greater) out to a range of 300 feet.

Mind Worm

Classification: Aberration (3E)
Challenge Rating: 17 (3E)
Alignment: Lawful Evil

Dripping, 60-foot-long, worm-like monsters that the illithids use to assassinate specific foes.

Combat Tentacles: Mind worms can create up to four "probe worms," tentacles composed of crackling red energy they extend not from their bodies, but any pool of fluid or reflective surface the mind worm can see. These tentacles have a 15-foot reach, deal Constitution drain with their attacks, and on a critical hit subject a victim to the mind worm's "Far Swallow" ability. What's frightening is that the mind worm can use its psionic scrying ability to potentially make these tentacles form remotely, even across planar boundaries. Fortunately, these probe worms can be targeted and destroyed, forcing the mind worm to regenerate them over a week.
Geo Effects: When immersed in water, mind worms are totally aware of their surroundings, and so cannot be flanked or subject to critical hits.
Psychic Powers: They share the illithids' signature mind blast, on top of some other abilities.
Seers: Mind worms can use greater scrying at will, which combined with their probe worms lets them replicate the infamous "scry and die" strategy against the illithids' enemies.
Supernatural Fear Inducer: Living creatures struck by a mind worm's probe must succeed on a Will save or be shaken.
Swallowed Whole: While mind worms have a bite attack, their "Far Swallow" ability triggers when their probe worms score a Critical Hit, at which point their target is instantly teleported into the mind worm's gizzard and subjected to crushing and acid damage.

Neothelid

Classification: Aberration (3E, 5E)
Challenge Rating: 15 (3E), 13 (5E)
Alignment: Chaotic Evil

Extremely rare, dangerous creatures created when the mind flayer life cycle goes horribly wrong.

Brain Food: Neothelids first develop intelligence when they consume a thinking being's brain, and afterwards constantly hunger for more.
Horrifying the Horror: Illithids consider neothelids to be abhorrent abominations and a taboo subject.
Monstrous Cannibalism: Not only did each neothelid survive by feeding upon their fellow tadpoles, but they are oblivious of their mind flayer heritage and will happily feast upon illithid brains.
Sole Survivor: Neothelids never arise intentionally, instead they're the result of some enemy wiping out a mind flayer community, leaving only the pool of mind flayer tadpoles at its heart. The untended illithid tadpoles, free of the elder brain's predations, eventually turn on each other for lack of food until only a single tadpole remains. Desperation will force this survivor onto land where it hunts what it can, but should it ever devour an intelligent being, eating its brain triggers ceremorphosis, except without a host body to fuse with — the new neothelid becomes self-aware and its psionic powers awaken, but it remains a tentacled, worm-like monster, brilliant but savage, concerned only with consuming more brains.
Super Spit: Neothelids can spray tissue-dissolving enzymes from their tentacle ducts that reduces prey to a puddle of slime, but leaves the brain intact.

Nerve Swimmer

Classification: Aberration (3E)
Challenge Rating: 10 (3E)
Alignment: Lawful Evil

Creatures derived from immature illithid tadpoles, used to torture, interrogate or puppet other creatures. Whether the mind flayers created them is unknown, as they can be found serving various evil beings, or their own ambitions.

Cold-Blooded Torture: While inhabiting a host creature, a nerve swimmer swarm can, three times per day, inflict wracking pain, forcing the host to save or take Strength, Dexterity and Constitution damage.
Hive Mind: They're a swarm of bug-sized creatures that share a single consciousness, even as individuals die and are replaced.
Living Lie Detector: A nerve swimmer swarm can use discern lies while inside another creature, three times per day.
Puppeteer Parasite: Nerve swimmers burrow into the flesh and nerves of their victims, and once per day can force them to obey the swarm's commands similar to a dominate monster spell, except since the nerve swimmers are directly controlling the victim's body it isn't considered a mind-affecting ability. They can also use a "Nerve Strike" ability to block the victim's nerve impulses, replicating a hold monster effect that again isn't considered a mind-affecting ability.

Voidmind Creature

https://static.tvtropes.org/pmwiki/pub/images/d&d_voidmind_grimlock_3e.jpg

A voidmind grimlock (3e)

Classification: As base creature (3E)
Challenge Rating: +1 (3E)
Alignment: Lawful Evil while enslaved

Illithid servitors who have had most of their brains consumed and replaced with psionically-charged goo, leaving them intelligent minions able to infiltrate their home settlements, while their mind flayer masters spy through their eyes.

Acid Attack: Once per day, a voidmind creature can spray a cone of slime from their skulls, which deals acid damage, stuns victims, and causes them to take penalties to Will saves.
Combat Tentacles: They can extend a slimy tentacle from their skulls, which attacks of its own accord without interfering in the voidmind creature's actions
Glamour Failure: Voidmind creatures can often pass for normal members of their races, but close inspection will reveal the four wax plugs in its skull that occasionally leak greenish fluid, the sign of the creation process. Thus, voidmind creatures are prone to wearing hats, helmets or other head coverings.
Min Maxers Delight: The voidmind template is available to player characters at a Level Adjustment of +3; i.e. their Character Level must be 3 lower than the rest of the party. While LAs are often cripplingly inflated,^note the voidmind template is generally agreed to be a fair trade, and possibly the only template which pulls this off with a penalty as high as 3. Assuming your mind flayer masters are already dead, of course. Sadly, the mind flayer's LA of +7 (on a creature that's already level 8) means that having other player characters as masters only works at high levels and isn't a great idea even then.
No Ontological Inertia: The good news is that killing the three mind flayers who created the voidmind creature frees it from their control, the bad news is that this doesn't undo the physical changes made to the creature, which require magic like wish or miracle.
No-Sell: They're immune to ability damage or drain, as well as any mind-affecting spells or abilities (since their minds are already enslaved, thank you very much).
Psychic Link: A voidmind creature is bound to the three illithids who created it, and each of them can perceive the world through its senses, take control of their puppet as per dominate monster, and most dangerously, utilize their psionic powers through the voidmind creature.
Who Even Needs a Brain?: Voidmind creatures only have a tiny bit of their brains intact, the part necessary to preserve the motor functions of their bodies. The psionic goo that fills their skulls not only takes up the function of the missing gray matter, it leaves the creature smarter than it was originally.

Mind Flayers of Thoon

A renegade faction of illithids touched by the Far Realm, leaving them with very different, but no less sinister, objectives than their kin.

General Tropes

https://static.tvtropes.org/pmwiki/pub/images/d&d_mind_flayers_of_thoon_3e.png

Classification: Aberration (3E)
Challenge Rating: 8 (shadow flayer), 10 (Thoon disciple), 15 (elder brain) (3E)
Alignment: Neutral Evil

A century ago, an illithid nautiloid attempting to explore the outer reaches of the Astral Plane instead found itself in the Far Realm. The ship's crew returned with a fanatical devotion to something they call Thoon, and are obsessed with gathering "quintessence," a substance they use to create bizarre bio-mechanical constructs and power their own unnatural abilities.

Cast from Hit Points: "Shadow flayers" can draw upon their internal quintessence reserves to benefit from an improved invisibility effect, but doing so deals damage to the shadow flayer, suggesting that the breeding program that produced them hasn't been perfected yet.
MacGuffin: Quintessence has a physical existence, appearing under a modified detect magic effect as a black, coruscating aura, which can be refined and stored in lattice-like matrices as a green, glowing fluid. It can manifest in things as varied as rare ores or plants or even in individuals, and Thoon's followers draw upon it to create new monsters or trigger their special abilities. But a sidebar explains that there's no need to track these illithids' quintessence harvesting operations, and the most important thing about quintessence is that it "proceeds at the pace of the plot," driving Thoon's followers to take an interest in an iron mine, an NPC's daughter, or the PCs themselves, as an adventure dictates.
Magic Knight: As clerics, Thoon disciples are sturdier in combat than normal illithids, while possessing both a mind flayer's normal psionics as well as divine magic.
Magitek: The Thoon-worshiping illithids have an array of constructs powered by quintessence, giving them a greater semblance of life than others — they aren't quite classified as living constructs, but all Thoon constructs other than Thoon soldiers will very slowly heal damage if given time to stay inert.
Renegade Splinter Faction: They're such to illithids in general, who are thoroughly evil, yet rational, brain-eating monsters. The mind flayers of Thoon, in contrast, have been twisted by the Far Realm, and follow erratic "visions from Thoon" that don't demonstrate any long-term purpose. The fact that Thoon's followers are dedicated to something other than Ilsensine makes them heretics as well.
Riddle for the Ages: The nature of Thoon is unclear, whether it's a philosophy, powerful outsider, or full-fledged deity. Thoon's disciples respond to any questions about it with "Thoon is Thoon, and Thoon is all!"
Sneaky Spy Species: As mentioned, the shadow flayers are the product of a program to develop this. While they're sneakier than normal illithids, this has come at the cost of their psionic abilities beyond the signature mind blast. The Thoon disciples aren't troubled by this, as it makes the shadow flayers easier to manipulate with magic like suggestion and charm person.
Talkative Loon: The Thoon elder brain — the only one that currently exists, as it was in charge of the Astral expedition that went awry — is just as imperious and brilliant as a normal elder brain, but it occasionally broadcasts psychic gibberish, and even its cogent mental messages are slightly painful to receive and always end with an "All hail Thoon!" It can also weaponize this trait, scrambling the thoughts of nearby creatures as per a confusion effect, or hitting a target with an "overmind blast" that deals Wisdom damage that can leave them unconscious but muttering "Thoon... Thoon..."

Madcrafter of Thoon

Classification: Aberration (3E)
Challenge Rating: 10 (3E)
Alignment: Neutral Evil

Huge, slug-like creatures that consume quintessence to create other monsters.

Acid Attack: Any spawn launched by a madcrafter of Thoon are accompanied by a splash of caustic birthing fluid that deals a heavy hit of acid damage, and remains as a battlefield hazard for a minute afterward.
Brain Food: Madcrafters of Thoon only need to eat quintessence, but they still have a mind flayer's craving for brains, which send them into fits of ecstasy for several hours. Thoon elder brains and disciples thus use rewards of brains to keep madcrafters in line.
Mook Maker: Their primary purpose. In normal conditions a madcrafter of Thoon can create a stormcloud or scyther of Thoon over the course of a day, but if pressed in combat, they can quickly spit out such creations to a range of 60 feet. The catch is, a madcrafter only has enough quintessence to accelerate the birthing process twice per day — using it a third time shuts down its Healing Factor, while each use beyond that deals damage to it.
Was Once a Man: It's hard to believe, but yes, these things were once mind flayers, and still have a mind blast attack, though they can only use theirs three times per day.

Scyther of Thoon

Classification: Construct (3E)
Challenge Rating: 6 (3E)
Alignment: Unaligned

Warrior constructs that can slash foes with their weapons or sear them with their gaze, and are most dangerous in groups.

Dispel Magic: By taking some damage, a scyther of Thoon can make a dispel magic attempt as a touch attack. However, the constructs aren't smart enough to use this ability on all but the most obvious displays of magical defenses, so they work best with a supervisor with some ranks in Spellcraft.
Eye Beams: They can focus their glowing eyes to replicate a searing light spell, but one that deals more damage the more scythers of Thoon are around. However, once one scyther of Thoon has used this ability, the eye-lights of it and any others around it dim for the next round.
Glowing Mechanical Eyes: Scythers of Thoon have glowing eyes that grow more intense when multiple scythers are within 30 feet of each other — one construct provides shadowy illumination in a 5-foot radius, two provide 20 feet of bright light and 40 feet of shadowy illumination, while three or more scythers of Thoon will emit 60-foot cones of bright light from their eyes, and 120 feet of shadowy illumination beyond that.
Sinister Scythe: They get their names from the alien scythes they dual wield.

Stormcloud of Thoon

Classification: Construct (3E)
Challenge Rating: 5 (3E)
Alignment: Neutral Evil

Intelligent tentacled constructs used to hunt down both quintessence sources and the enemies of Thoon.

Glowing Mechanical Eyes: Theirs shed light in a 60-foot cone, with 120 feet of shadowy illumination beyond.
It Can Think: Unlike other Thoon constructs, stormclouds are sentient, but at Intelligence 5, they're "dullards in a community of evil geniuses" who defer to the mind flayers around them. This at least lets stormclouds direct the less intelligent soldiers and scythers of Thoon to greater effect. Amusingly, the stormclouds blame their low intelligence for their lack of understanding about Thoon's will, oblivious to the fact that the Thoon disciples and elder brain are just as in the dark as they are.
Not Quite Flight: Stormclouds don't fly so much as levitate. Their base land speed of 30 feet represents them floating five feet off the ground, walking on their tentacles. When they rise higher than that, they gain a perfect flight speed, but can only move 10 feet per round.
Shock and Awe: They can use lightning bolt at will, but take damage each time they do so.
Super-Senses: By burying a tentacle a few inches into the ground, a stormcloud of Thoon gains tremorsense out to 60 feet, pinpointing the location of anything in contact with the earth.

Thoon Hulk

Classification: Construct (3E), Aberrant Humanoid (4E)
Challenge Rating: 13 (3E), 22 (4E)
Alignment: Neutral Evil (3E), Evil (4E)

Brutal amalgamations of mind flayer and metal, serving as Thoon's shock troops.

Blade Below the Shoulder: Their two primary arms end in axelike blades.
Brain Food: Averted; as constructs, thoon hulks don't need to eat brains like normal illithids. They still have an "Extract" attack, but never consume the brain they tear out, instead carrying it back to base or giving it to a mind flayer if ordered.
Dumb Muscle: They're Large masses of muscle and metal, but in 3rd Edition are certifiably mindless, and as dim as ogres in 4th. They also lack any of a normal illithid's psionic power.
Reforged into a Minion: Thoon hulks are created from mind flayers who "the Thoon elder brain found unworthy," which are vivisected, augmented with metal, and placed in a cocoonlike structure filled with quintessence.
Stance System: By taking some damage, a Thoon hulk can engage a one-round attack or defensive overdrive, gaining a bonus on attack and damage rolls or saving throws, respectively.

Thoon Infiltrator

Classification: Monstrous Humanoid (3E)
Challenge Rating: 5 (3E)
Alignment: Chaotic Evil

Parasites from the Far Realm, able to take over humanoid bodies and serve as covert agents for the mind flayers of Thoon.

Human Shifting: Downplayed; a Thoon infiltrator can shape their facial features like clay, giving them a racial bonus on Disguise checks.
The Infiltration: Their primary purpose — blend into a humanoid population, and spot sources of quintessence for their mind flayer masters. That said, the Thoon infiltrators' Chaotic natures mean they chafe at the illithids' control and are impatient with long-term planning and careful quintessence acquisition.
Not Quite Dead: If its host body is "killed," a Thoon infiltrator merely goes dormant for a month, before waking with its full hit points (and typically using dimension door to escape its grave). It takes a high Search check to detect the faint vibration in the creature's neck tentacles to reveal that it's not quite dead.
Puppeteer Parasite: These creatures came to the mind flayers of Thoon's attention after they took over some humanoid captives in their nautiloid's prison deck. They look like a cat-sized bundle of wires that can burrow into the brain and vital organs of a host body, killing it, then the creature repairs the damage over the course of a day, after which point the body is wholly under the parasite's control. Normally the parasite is visible as a cluster of thin tentacles at the base of the host's skull, but it can hide itself by submerging beneath the stolen flesh — while this prevents the Thoon infiltrator from using most of its abilities, it does mean nothing short of a full dissection will reveal its presence. More dangerously, a Thoon infiltrator can, once per week, extend neck tendrils into a helpless victim's mouth, and if uninterrupted for a minute, will convert that victim into a Thoon thrall.

Thoon Soldier

Classification: Construct (3E)
Challenge Rating: 8 (3E)
Alignment: Unaligned

Combat servitors that can use their internal quintessence supply to reconfigure their bodies during battle.

Stance System: Their gimmick is that they can shift between different "aspects" in combat, gaining new abilities as the situation dictates. However, reconfiguring their bodies this way deals minor damage to Thoon soldiers. They also aren't smart enough to anticipate which form is best for a foe, instead the constructs will react to what happens in a fight, for example adopting the Aspect of the Bloody Slaughter or Fiery Sun if they get surrounded, or the Aspect of the Death Blossom if their destruction seems imminent. Finally, each change is accompanied by the Thoon soldier making an announcement in Undercommon, which can clue in opponents what to expect.
- Action Bomb: With the Aspect of the Death Blossom ("Death blooms in the name of Thoon! Thoon! Thoon!"), a Thoon soldier sets itself up to explode in a highly-damaging, 40-foot-radius fireball if reduced to 0 hit points.
- Defend Command: With the Aspect of the Impervious Tower ("Stand and fight! Thoon is Thoon!"), a Thoon soldier extrudes additional armor plates and magical wards, granting it a higher Armor Class and a bonus on saving throws.
- Hit-and-Run Tactics: With the Aspect of the Ravening Horde ("Walk with Thoon!"), a Thoon soldier reconfigures its legs, increasing its speed and letting it use the Spring Attack feat to strike foes as it passes.
- Playing with Fire: With the Aspect of the Fiery Sun ("All will burn for Thoon!"), a Thoon soldier ignites, dealing additional fire damage with its attacks and damaging foes who strike it in melee.
- Spin Attack: With the Aspect of the Bloody Slaughter ("Slaughter for Thoon!"), a Thoon soldier reconfigures its arm blades to enable it to make Whirlwind Attacks against multiple foes.

Thoon Thrall

Classification: Humanoid (3E)
Challenge Rating: As base creature +2 (3E)
Alignment: Lawful Evil

Slaves to the Thoon infiltrator who converted them, these unhappy creatures help undermine their communities, or are pressed into a fatal fight.

And I Must Scream: After their conversion, Thoon thralls retain their identities and free will, unless a Thoon infiltrator is around to give them orders, which they must obey as if dominated, with No Saving Throw. This horrific existence means most Thoon thralls welcome their deaths in battle.
Body Horror: Thoon thralls are normally "dormant," appearing indistinguishable from their original selves but slaves to a Thoon infiltrator's commands. But if given an order to end their dormancy, throbbing purple veins pop out of a Thoon thrall's skin, and its muscles grow at a freakish rate so that the creature gains 40 to 50 pounds, all while its skin blisters and swells. This is a one-way transformation, as explained below.
Explosive Overclocking: When a Thoon thrall ends its dormancy, it gains fast healing that can go beyond the thrall's normal hit point total as temporary HP. But should a Thoon thrall's temporary HP equal its starting HP, it has to start making Fortitude saves, or else it'll explode in a 10-foot-radius fireball. Worse, a Thoon infiltrator can command a Thoon thrall to fail this save.

Undead Illithids

Alhoon

Origin: Forgotten Realms
Classification: Undead (3E, 5E), Aberrant Humanoid Undead (4E)
Challenge Rating: 18 (3E), 10 (5E)
Alignment: Lawful Evil

Mind flayers that pursue arcane magic are exiled as deviants, and for them no eternal communion with an elder brain is possible. The road to lichdom offers a way to escape the permanency of death, but that path is long and solitary. Alhoons are mind flayers that use a shortcut.

And I Must Scream: When an alhoon's body is destroyed, its mind gets sucked into its periapt of mind trapping. It remains there, trapped alongside the souls of its previous victims, aware of its surroundings but powerless to do anything beyond telepathically ranting and raving at anyone who picks up the periapt.
Decomposite Character: In older editions, the terms illithilich and alhoon are synonyms. 5th edition draws a distinction between the two: an alhoon is a lesser form of lich with weaker spellcasting abilities and no ability to regenerate its body or suck out people's brains, whereas an illithilich has all the powers and abilities of a normal lich in addition to those of a mind flayer.
Horrifying the Horror: Like neothelids, they are regarded by illithids as unnatural abominations, and the process of becoming one is considered heretical, while the illithids who tend to pursue ascension are almost universally viewed with disgust and hatred by illithid society.
An Ice Person: In 5th edition, their basic attack is a touch spell that inflicts cold damage.
Non-Human Undead: They're the mind flayer equivalent of liches.
No-Sell: In 5E, an alhoon cannot be harmed by nonmagical weapons.
Soul Jar: Much like how a regular lich uses a phylactery to house its soul, an alhoon uses a periapt of mind trapping to store its mind if its body is destroyed.

Vampire Illithid

https://static.tvtropes.org/pmwiki/pub/images/vampiric_illithid.png

Origin: Ravenloft
Classification: Undead (3E, 5E)
Challenge Rating: 12 (3E), 9 (5E)
Alignment: Lawful Evil

Illithids who became vampires through an unknown process, which also destroyed their minds.

Feral Vampires: Vampiric illithids are mindless, predatory animals with no trace of their old genius — whatever process gave them their unlife also destroyed their rationality and capacity for higher thought.
Our Vampires Are Different: Pale grey, undead illithids who need to consume both blood and brain matter to survive. It's not known how they become vampires and they cannot produce spawn of their own, and the process of transformation leaves them feral beasts.
Took a Level in Dumbass: As stated above, the vampiric transformation scours away all rational thought from the illithid's mind, leaving behind nothing but a feral predator.

Characters / Dungeons & Dragons: Mind Flayers

True Illithids

Ceremorphs

Related Creatures

Cessirid

Embrac

Kigrid

Saltor

Mind Flayers of Thoon

Undead Illithids

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Characters / Dungeons & Dragons: Mind Flayers

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True Illithids

Ceremorphs

Related Creatures

Cessirid

Embrac

Kigrid

Saltor

Mind Flayers of Thoon

Undead Illithids

Previous

Index

Next