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One of the deadliest mortal creatures, the illithid (or "mind flayers" in the colloquial) are so prominent a threat, there likely isn't a berk in the entire Planes that doesn't know who these tentacle-mouthed beings are. Vicious and megalomaniacal, even most of the planes are wary around these beings. Thankfully, they're rarely encountered on the planes beyond the realms of their powers, Ilsensine's Caverns of Thought on the Outlands and Maanzecorian's Rictus on Gehenna; the illithid tend not to have much care for the Outer Planes, focusing their efforts on the mortal realm, the Prime Material Plane that they've nearly ruled before and desire one day to rule again. For many years, their ways were a mystery to most, but thanks to the recently-published Illithiad, a compilation of lore, legend, and interviews assembled by one S. Wakeman on the topic of the illithid, much of their nature has come to light, granting a greater comprehension of the species and why they are as they are.
The earliest origins of the illithid are an utter mystery. Despite the existence of a few references claiming to describe their origins — the Sargonne Prophecies, the Astromundi Chronicles, the Planetreader's Primer — nothing is truly known of where they first came from. Some theorize connections to the Far Realm, others the mutant products of experimentations, but these are merely supposition, and the illithid themselves have neither comment nor care for their true origins. The only that would know for certain are the deities or elder brains of the illithid, and neither is likely to speak on the matter to outsiders.
What is known is that the first appearances of the illithid date back to approximately 34,000 years ago upon the Prime, the great nautiloid ships of the illithid coming from the depths of Wildspace to move upon the various material worlds near the waning days of the Reptilian Age, when the earliest intelligent mammalian species were first emerging upon the many-varied worlds. Over the next thousands of years, they took planet after planet, holding hundreds upon hundreds at their peak. Beyond the Material Plane, their reach extended upon the Astral and Ethereal, and even to a few outposts on the Outlands. So great was the Illithid Empire that it drew the notice, and the worry, of even the various kinds of the planes, so far as a threat that the Blood War itself came to a halt for the first time since its commencement as the baatezu and tanar'ri came to a brief summit to discuss the illithid threat; a summit that ended up collapsing into threats and violence, but a summit nonetheless.
Even such a great empire cannot last forever, though; after almost 20,000 years of domination, the early humans most frequently used as subject thralls by the illithid, following generation after generation after generation of controlled breeding, began to grow psionic gifts and defenses of their own, entirely unbeknownst to their masters. Tempering and developing these talents in secret, the thralls snuck agents from world to world via the very supply lines of the Empire itself. Finally, near to 16,000 years ago, thralls across the entire empire rebelled under the leadership of two great warriors, Gith and Zerthimon. Within three decades the thralls had succeeded in shattering the Illithid empire, slaughtering millions of illithid and thousands of elder brains, and ending the threat to the Prime. From these thralls came eventually the githyanki and githzerai, set at odds with one another following the Pronouncement of Two Skies, but despite the ill will between the two factions, their acts together were an undeniable good for reality.
Shattered into any number of disparate communities across the planes, it was more than ten millennia before the illithid managed to re-establish itself as a single body, striking out again in the year Hashkar -2965 to try and regain their former glory by unseating the then-dominant power of the Prime, the Thri-Kreen Empire; and so the Great Illithid Wars began. They stretched on for the next 175 years, finally concluding with the destruction of both the Thri-Kreen Empire's capital world and the home world of the thri-kreen, both shattering the empire's ability to field a military and utterly demoralizing its dominant people. Once more, the illithid were the dominant power in the Prime, though it was a far different time by then, with many more forces standing against them. Though they managed to take a number of worlds, they had nowhere near the strength of the first Illithid Empire, and so it was that in Hashkar -2787, their greatest fleet was fought to a standstill and eventual defeat by a briefly-united beholder fleet. The illithid were unseated, and the forces that defeated them soon collapsed into the usual in-fighting and sniping of the beholders, leaving a power vacuum for the humanoids of the Prime to once more take.
Today, the illithid are a significant threat across many Prime worlds, but far more individualized. Having lost much in the way of knowledge, of resources, of communication, each community of illithid upon both the Prime and the planes is essentially its own independent force, with little-to-none in the way of communication with any other community. Still, the illithid have recovered from one near-defeat, and they are among the most tenacious of people; not simply hoping for success, they believe it to be truly inevitable, and that the illithid will one day again rule the Prime.
The illithid culture is simultaneously strictly regimented and highly individualistic. Every illithid chooses their role, and is expected to perform that role as tradition dictates and as decreed by the elder brain. Freedom of choice bound by the elder brain held by all as the pinnacle of illithid society. But below the elder brain, while some illithid may hold position over others as necessary for duty, no illithid truly stands above or below any other as an entity, all equal in their superiority above all other mortal life, all of which they see as thrall. And to thralls, they bring dominion, the joy of eventually serving the illithid as slave or food.
Indeed, dominion over all material reality is the overarching goal of all illithids, taking a chaotic, disordered universe and bringing it into perfect, simple order. The planar realms are an irrelevancy, nothing but hypernatural beings that would surely bend to illithid ways once the material is fully under their sway. To them, the multiverse would be a better place were they to hold rule over it, and any that disagree are only confused about the natural way of things, blind to the role of the illithid as the ultimate apex predator; even others of their kind that question this truth are quickly "corrected" in their ways. And though they may not hold position above all yet, they know it to be inevitable that they will, for it is simply what must be.
Part of this view is due to the odd illithid perspective on time. To them, the present is all that truly matters. The past is an irrelevancy, an existence in constant flux known only to the elder brain and relayed to the illithid as it is as necessary, while the future is merely the natural conclusion of the events of the present, the inevitable and unquestionable result of the actions of the now. They keep no histories of their own, and they do not plan for the future, but rather always proceed under the natural assumption of their own success.
Of course, this means that any examples of their failures bring about a special, intense ire amongst the usually-seemingly-staid creatures. The mere presence of those forces that most contributed to bringing them from their greatest heights — the githyanki, the githzerai, and the beholders especially — bring about a rage within the illithid drawn from the frustration of their dashed plans that's little-matched by any other influence. Indeed, frustration is the emotion most often underlying their thoughts, as any failure of a plan to work exactly as expected is seen as naught but a failure to plot properly.
The natural sense of superiority leads to an odd approach to religion and worship as compared to many people. The illithid view of Ilsensine, and to a lesser degree Maanzecorian, is a sense of deep reverence for their brilliance, and a jealousy for knowing so much on so many topics that the illithid that revere them wish for themselves. A sense of morality is of no concern for even the religious-minded, no guiding path that would bring them to their deity after death, for their ultimate fate is certain: merger with their elder brain. Nor is there any sense of respect for Ilsensine as a creator god (assuming that it indeed did create the species, which is still far from known), what with their total disregard of the past. No, those illithid that venerate Ilsensine or Maanzecorian do so only out of a desire to be granted their boons.
While the illithid may seem quiet and unemotive to most mortals, this is merely a manifestation of their psionic nature leading to a preference for means of communication hidden from the average mortal. Indeed, their psionic nature is so internal that it runs through near every aspect of their society. Empathy to them is merely another sense, with devices known as resonance stones emanating emotions in the homes of illithid in the same way other mortals might use scented candles. Even the "language" of the illithid is pure multilayered telepathic communication without reference to word. As a people of natural geniuses, such communication comes naturally to them, even extending to the written language of qualith developed so as to at least approximate the multiple levels of usual illithid communications. To them, the spoken word is laughably bereft of meaning and the most blatant sign of the weak minds of thralls, but for perhaps the languages of the baatezu or yugoloth, these tongues coming perhaps closest to the nature of illithid mindspeech.
This incorporation of psionics into their life is in fact so great that the very idea of arcanism is considered utterly vile, totally beneath the illithid. While there are always a slight few that come to the art via either birth or study, any such that have their predilections publicly revealed are shunned by the entire community, regardless of how much talent in the arcane arts they might have; greater skill, in fact, is merely a sign of greater unsuitability towards illithid culture. This exile that bars any such illithid from their ultimate reward of merger with an elder brain, coupled with the natural intellect of the illithid bringing its successful research far more easily than most, leads many arcane illithid to take the path of lichdom, resulting in a creature known as the alhoon; a deadly threat, combining the horrid nature of the illithid with the immense power of the lich.
The illithid life cycle is wholly dependent on their humanoid slaves, to a degree they hesitate to admit. Every few decades, an illithid will hermaphroditically spawn a clutch of a few hundred eggs, depositing it within the elder brain pool. These eggs eventually hatch as remora-like tadpoles, maturing over the next ten years as they're cared for by illithid caretakers, raised on a strict diet of brainmeal. Perhaps one in a thousand survives to maturity, the majority fed upon by the elder brain itself; those that survive are considered the best of the clutch, and allowed the act of ceremorphosis.
With ceremorphosis, the larval illithid is introduced to a proper humanoid host — any mammalian humanoid of between five and seven feet tall is a suitable candidate, and it is rare (but not unheard of) for any other variety of creature to do — where it burrows through the ear and begins consuming its new host's brain matter, leaving just enough for the body to survive and growing to fill the empty space as it melds with the tissue. The flesh and form of the body transforms, the skin more befitting the newly-amphibious form and growing a thin layer of mucous and the head and sensory organs are distorted horribly under the manipulations of the larva. The body's reproductive system and sexual characteristics are entirely lost, and the nervous system expands and multiplies throughout the body, giving total conscious control over the entire body; otherwise, the majority of the rest of the body's organs are left largely as is. Within an hour after implantation, the host's previous mind is entirely lost but for the occasional fragments of personality or memory drifting to awareness, with bodily transformation completed within a week's time; illithids do tell stories of a figure known as the Adversary, an illithid whose host's former mind managed to overpower the illithid's after ceremorphosis, and was then able to act against the illithids in the most perfect disguise imaginable, but near all dismiss this as mere legend, a total impossibility.
At this stage, an illithid appears full-grown, but it still has much growth to do to come into its full physical and mental capabilities. Such an illithid is referred to as "neophyte" within the culture, and doesn't reach their full potential until about another 20 years. Illithids tend to live up to 120 years before passing, regardless of what their host body once was, and once gone, their brains are brought to the pool of their city's elder brain, joining with the gestalt mind to live on forever.
Rarely (about 1 in every 25 tadpoles), a tadpole will be found that does not reach proper maturity for nearly 20 years. These tadpoles cannot be easily identified from their kin, and are fed upon by the elder brain just as much as any other. When implanted and allowed to ceremorph, however, these tadpoles grow not into illithids, but into the variant subspecies known as ulitharid; a towering creature with power and strength quite beyond that of the standard illithid, and able to live nearly twice as long as the standard variety.
When full-grown, the illithid needs surprisingly little nourishment to get themselves by. In truth, an adult illithid needs no more than one brain a month to survive, drawing the majority of its sustenance not even from the material itself (though it is necessary), but from the psychic essence of a sentient mind. Indeed, illithid brain digestion, psionically empowered via their enhanced nervous system, is wholly efficient, their mental abilities allowing them to transform anything they can't use directly into whatever their body might need. While this may be a minimum requirement for their kind, even in the best of cases a single brain a week is plenty for any illithid.
When full-grown, illithids stand around six feet tall, utterly hairless, with pale purple-to-mauve flesh glistening from the thin layer of psi-active mucous that, as with most amphibians, keeps moisture sealed in and keeps them from losing too much oxygen, while also boosting their natural psionic abilities. Their bodies tend to be lithe, with hardly any visible musculature to belie their great strength, and as hermaphroditic beings they bear no signs of either primary or secondary sexual characteristics. An illithid's elongated head is covered with thin ridges, with pure-white eyes above a set of four tentacles, each two to four feet long, surrounding a lamprey-like mouth where a normal humanoid's mouth would lie. Each tentacle is tipped with a small duct, where the flesh-and-bone-dissolving enzyme that allows them to so easily remove a brain from its skull is released.
Otherwise, they appear similar to many humanoid creatures, with four-fingered hands and two-toed feet with webbing that helps speed along their underwater movement, but that can be folded away for land maneuvering. Most illithid prefer to drape their bodies in elaborate robes, keeping an air of nobility in their appearance as befitting their natural sense of superiority.
For the most part the ulitharid variant appears largely the same. The ulitharid is far taller, though; whatever the host body was originally, after ceremorphosis it grows to over seven feet, slightly broader in build, and with two additional tentacles about its mouth.
- A Guide to the Astral Plane, pgs.44-45
- The Illithiad
- Lords of Madness, pp.61-82
- Temporum Sphaera