The Illithid is considered "Product Identity" by Wizards of the Coast and as such is not released under its Open Gaming License. However, "half-Cthulhu creeps" and "squidface brainsuckers" are totally okay, so go nuts.
Illithids consist of... purple, mostly. Just brains and purple.
Their alien physiology has stumped our crack (addict) biology team even after years of scientific scrutiny. It is thought that if we could crack the mystery of why these skinny creatures subsist on a diet of nothing but fat and calcium, we would be closer to... something.
Look, just look up "aberrant" in the dictionary. We can't offer you much for the time being.
In actuality, Illithid physiology is a moderately altered version of their host organism's body (see mating practices). The brain of the host is replaced by a tadpole thing, and the digestive tract is altered to aid in the consumption of brains and their psychic essence. And of course, they grow tentacles and Sarlaac mouths and lose all of their hair.
Explained somewhat in the tome of darkness 3.5. The resulting creature is entirely conscious of it's organs and body, as opposed to automatic. There is a tiny part of the creature's powerful mind constantly remembering to make sure the heart beats, and regulating digestion. Things like that. So they eat normally, and extract maximum nutrition from it. The method of creating an illithid destroys the brain, which includes all those hormone producing glands in the skull that keep a person healthy. This is why they need brains to keep their body from decaying. The memories and thoughts consumed are... well "flavor" to put it in the least disgusting way possible.
There is apparently a cult of illithid monks/clerics known as The THOON (yes all caps) who minmax their bodies as well as their minds. The elder brains hate them. They've been out of circulation since 4e landed. But if you need a mastermind who can roundhouse kick the fighter through a wall after the party blitzes through his mooks. Look no further.
For the morbidly curious about what'd happen if you ate an illithid's brain, according to 5e's Volo's Guide to Monsters, nothing good. Apparently, each bite into an illithid's brain releases an intense flood of memories that the illithid developed/consumed in its life, so strong it'll probably drive the typical consumer insane. As to how Elminster knows this, he refuses to explain.
Illithid Mating Practices
This morbid fascination of yours is going to get you killed one of these days, you know that?
Anyway, if you've ever seen that Bruce Campbell movie Mindwarp, with those burrowing leech things, you've got a pretty good grasp on what to expect. Consider this your warning.
Twice in an illithid's life, it will go to the pool containing the Mind Flayer community's Elder Brain (though any briny pool will do), and literally vomit a huge amount of tadpoles out of their mouth (they're asexual like that.) These tadpoles will be nurtured (though the elder brain will eat some of the tadpoles, who themselves engage in cannibalism) over the course of ten years (20 for ulitharids) and once they are ready, they are taken from their pool to undergo Ceremorphosis.
An unlucky creature will be restrained as the Tadpole is brought to it, the Mind flayer carrying the tadpole will place it within the most direct route to the brain (typically the ear or nose) and the tadpole will quickly burrow its way through the skull and into the brain where it will devour the host organism's brain over the course of a week as it alters the host with mind flayer traits. Very, very rarely this process partially fails and no external changes are brought about (though it will still internally be a mind flayer and need to eat brains) in these cases the new mind flayer is used as an infiltrator for obvious reasons. Even rarer, to the point where it's practically considered an urban legend is the opposite, where the host undergoes the full transformation, but their original mind remains intact and in control of their newfound powers. Mind flayers are scared shitless of these creatures, referred to collectively as The Adversary. Not only are they perfectly capable of blending in flawlessly with mind flayer society, they are almost certainly in the vengeance seeking mind set, and in the perfect position to bring it all crashing down on their tormentor's heads.
When the process is complete, no form of magic or psionics, excluding wish and reincarnate and true ressurection, they can bring back the victim, but the mind flayer will still be alive. Aside from that, nothing can reverse the effects; there is no way to bring back the original host, so he/she/it is for all intents and purposes, dead forever. Though the newborn mind flayer will be physically mature, it will take some time to mentally mature (though its community will be more than willing to assist it in this manner).
If the host is any species besides a human, the result is a half-mindflayer, whose template can be found in the Fiend Folio. Special cases are present though; placing a mind flayer tadpole into a beholder results in a mind witness (essentially replace four of the eyestalks with large thick tentacles and the rays of doom with psionics), a docile creature whose main role is to facilitate telepathic communications that will look for another telepathic being to serve should the illithids it works for be slain. Placing it within a true dragon results in the unspeakably powerful gribbly horror that is the Brainstealer Dragon, whose psychic might is only surpassed by the Elder Brain. Put it in a Roper and you get the Urophion, a creature with a genius level intellect whose talents are wasted on sentry duty due to racism (but with their slow, slow, slow move speed they can't do much of anything else). Implanting one in a Svirfneblin usually kills both the tadpole and its host, but if it survives the result is a mozgriken, a small shapechanging creature with no mouth and only three tentacles, a trait that forces them to feed only on brain fluids and earns them no small amount of abuse from true illithids. The tadpoles typically placed within lizardfolk are more bellicose than their kin, and form the Tzakand- creatures similar to lizardfolk save for the two acid-spitting tentacles emerging from their heads that become fanatically loyal to the first illithid they see. If a tadpole is placed into a Chuul, the tadpole dies, but the Chuul is changed into a creature called an uchuulon, or a slime chuul, which is like a chuul, but slimier.
If a tadpole is not placed into a host and is placed in the wild (which normally happens only in spawning pools whose Elder Brain is dead, as it eats all tadpoles that do not undergo ceremorphosis), it will eventually eat and grow into a gigantic fucking psychic worm of doom called the Neothelid (think the dune sand worm, only quite a bit smaller and with tentacles and head asploding psychic powers) with only minimal sentience- that is, unless it eats a sentient creature. Regular Mind Flayers consider it highly taboo to speak about Neothelids (probably because Neothelids remind the Illithids too much of their "natural" state which their progeny would devolve into, were it not for the availability of suitable hosts their tadpoles could Ceremorph), and to actually create one is outright forbidden. Oddly, Neothelids are not considered Product Identity by Wizards of the Coast, possibly because "giant slimy worm that spits acid and shoots mind bullets" would not have been copyrightable.
One out of a hundred or so tadpoles will take twice as long to become ready for Ceremorphosis, these tadpoles will create Ulitharids when implanted into a host. Ulitharids are large sized and have two extra long tentacles and much more powerful psionic abilities, they also live for a lot longer than normal mind flayers. Sadly, the only statistics for Ulitharids are for Ulitharid Tadpole-Human combinations, which is a disappointment since an Ulitharid Brainstealer Dragon would have been fuck-awesome.
Elder brains are created via a process that varies from edition to edition (in those editions that elaborate on the process at all, that is). In some editions all ulitharids that survive long enough eventually morph into one. In others it happens when a bunch of deceased mind flayer brains are dumped into a single pool filled with a briny solution; these brains will merge and eventually become alive and turn into a CR 25 TPKing horror. There is also an idea that elder brains were created by the Illithid god Ilsensine, who performed ceremorphosis on some lesser gods or demigods. One of them, called the "Illithid God-Brain," is even trapped on Ravenloft and rules over an Illithid colony named Bluetspur, with the official backstory left open as to why one of these alien monstrousities would ever earn the place of a Darklord on that benighted plane, save that it must have done something atrocious even by Illithid standards.
A rare few Illithids who end up mastering actual arcane magic instead of psionics opt to become Liches, named Alhoons or Illithiliches, since Elder Brains can't absorb magical abilities from the brain of an Illithid mage and magical talent (unless it's that of a Cleric to Ilsenine, see below) is looked down on in Illithid society. These squid-faced liches are extremely dangerous examples of psionic undead and can still rip your brain from your skull despite having no need to consume brains after the transition to undeath, with no explanation being given for how they still continue to secrete the biocorrosive slime needed to soften the flesh and bone of the skull to allow for easy extraction of living brains while undead (and thus having no metabolism to speak of).
Illithids function as a hive mind, serving the Elder Brain. Other than maintaining the Elder Brain, the day-to-day duties of the average illithid consist of evil science and making cheeky wood burnings. The purpose of these wood burnings is an inextricable mystery, as they seem to disappear shortly after their completion. They can make Brain Golems out of buds from said queen-like Elder Brain, which are exactly what they sound like and exactly as retarded as you would think. Even the Illithids and the Elder Brain itself know how impractical this can be, and would prefer to use any other type of golem instead. The Illithids live for the fact that after they die, their brains, and thus their minds, will be merged with the Elder Brain after death. This is, of course, complete and utter bullshit, as the Elder Brain just eats their knowledge and discards everything else. Elder Brains discourage studying magic too much because Elder Brains can't eat a mind flayer's magical abilities, though they can eat a mind flayer's knowledge of psionics (if it ever took levels in Psion, Erudite, Wilder, or what have you.) Below the Elder Brain on the mind flayer hierarchy are the Brainstealer Dragons (very rare because it's notoriously difficult to restrain a true dragon) and the Ulitharids.
The mind flayers have one deity they pay respects to in Illsensine, who did not create them. Perhaps to make up for the fact that Illsensine is the only Mind Flayer God, Illsensine is a very powerful rank 20 greater deity who omnoms the knowledge of everyone the instant they die. The Mind Flayers don't exactly worship Illsensine per se, they just respect it for its great knowledge, and Illsensine returns the favor by considering Mind Flayers to be the only worthy species in the multiverse. This is not to say that there are no Mind Flayer clerics to Illsensine, because said clerics of Illsensine do indeed exist, but their numbers are rather small relative to say the number of Dwarven clerics of Moradin, however; Mind Flayer communities have an exception to their rule of "don't study magic too much" for the Clerics of Illsensine, and in fact greatly respect these clerics and leave them to their business.
The mind flayers used to have another god called Maanzecorian, but Tenebrous (actually Orcus, it's a long story) used the last word on him, so his head asploded.
Some illithids spend their time finding ways to put out the sun, because they hate that bright light shit. Unfortunately for them, putting out a 1,392,000+ kilometer ball of ungodly hot hydrogen and helium is nigh on impossible (unless your name is Father Llymic.) Most other Illithids think that these illithids are wasting their time when it's common knowledge that all they have to do is wait many, many billions of years for the sun to eventually puff up and die. And it goes without saying putting out the sun would indirectly eliminate their food source as well.
Some mind flayers went to the Far Realm and apparently encountered something called Thoon. However, the exact nature of Thoon is unknown even to the mind flayers themselves, so whether Thoon is a god, an aberration, a philosophy, or something else entirely is open to interpretation. In any case, the mind flayers' encounter with Thoon changed them completely, so they're a lot more irrational and unpredictable (NE instead of LE), they make use of a lot of really weird constructs and altered humanoids (and a variant of the mind flayer that trades the ability to control minds for ninja powers), and for some fucking insane reason a CR 25 Elder brain came back as a CR 15 entity that's totally batshit insane. Oh, and they also seek to gather quintessence for some reason.
|This article or section is about Monstergirls (or a monster that is frequently depicted as a Monstergirl), something that /tg/ widely considers to be the purest form of awesome. Expect PROMOTIONS! and /d/elight in equal measure, often with drawfaggotry or writefaggotry to match.|
Now, you'd see the face on these things and the how of their reproduction and figure that this has to be one species that can't get the monstergirls treatment? ...But then you realize that they have tentacles, and you suddenly remember that Japan exists. "Feminized" illithids, which are basically mind-flayers with boobs and often skimpily clad, occasionally pop up in monstergirl-related threads on /tg/, usually under the same joking context as the thri-kreen/xixchil.
More importantly than that, Mindflayers are one of the "Aberrant" monstergirls that show up in the Monster Girl Encyclopedia. Formally considered a relative of the scylla, they take the form of tall, curvy, slime-dripping women with tentacles for hair and a Mortica Addams-like dress implied to actually be tentacle-legs. They are ultra-perverted hedonists, using their psionics to brainwash victims into playmates of their obscene lusts, and are reputated to eventually transform men they take as their lovers mentally and physically into grotesque, squid-like tentacle-beasts, which spend the rest of their lives fastened to the mindflayer's hips and shoving her genitals full of tentacle-penises.
Later sources clarified that the man in question can freely transform back and forth between squid-form and human form, but that the mindflayers are uncharacteristically shy about public displays of affection when their lovers aren't tentacled, so this is only done in private. And they usually use their mind-powers to help friends engage in emotionally-resonant roleplay when in societies with other monsters.