Illithid

Better known as "mind-flayers", illithids are the scourge of the Inner Planes. They once held dominion over several planes, and enslaved millions of intelligent beings as labor and livestock. But their empire has been in decline for the last few millennia. Rebellions, infighting, and intercessions by gods (both good and evil) are slowly tearing them apart.
Illithids are known for their extraordinary psionic abilities, but their greatest weakness is that they are essentially parasites. They use sentient creatures as both sustenance and a means of reproduction. This forces them to stay close enough to civilization to risk discovery.

Basic Information

Anatomy

Illithids resemble a nightmarish combination of humanoid and cephalopod. They have tall, thin, bony bodies, with long limbs in proportion to the torso, ending in hands and feet with four webbed fingers and toes. Their heads feature a ring of four tentacles in place of a nose and mouth, which surround a round orifice with multiple rings of radulae, and the cranium is noticeably oversized.
Internally, one of their more interesting features is that their nervous system is extremely developed. In a way, the entire body is an extension of the brain.

Genetics and Reproduction

Rather than reproducing the usual way, illithids have a bizarre and generally horrifying life cycle. Each adult asexually produces two to three clutches of a thousand or so eggs during their life, which are laid in the same tank of brine that holds the Elder Brain. These hatch tadpole-like larvae, which spend about ten years feeding on brain matter and their siblings until only the strongest remain. Mature tadpoles are then given a host body. A captive humanoid is rendered docile with a psionic blast, and the tadpole burrows into its cranium through the nose or ear, where it proceeds to latch onto the brain stem and devour the brain. This triggers the process of ceremorphosis, a week-long transformation into an illithid.

Dietary Needs and Habits

Though they are known mainly for eating brains, illithids actually subsist on psychic energy. The gray matter provides some sustenance, but what they really need is the psychic energy, enzymes, and hormones. The latter are what keep the host body alive; since ceremorphosis involves replacing the entire brain with a parasitic entity, the substances normally produced by the brain have to be acquired from outside sources. Illithids also eat various glands and organs to gain these. Brains are simply the main course.

Additional Information

Social Structure

Illithid colonies consist of less than a hundred individuals, plus several hundred thralls and an Elder Brain. (Their population is limited by necessity, because otherwise the supply of fresh brains is harder to maintain.)
The Elder Brain is an enigmatic entity. Each one was formerly an ulitharid, a rare, exceptionally powerful kind of illithid that is occasionally born by chance into a colony. But the process of metamorphosis causes them to transcend far beyond the comprehension of even an illithid's mind. It absorbs the brains of dead members of the colony, forming into a mass of gray matter filled with the memories of hundreds of illithids. Mainly, however, the Elder Brain is the psychic hub of the colony. Its mental presence is inescapable within the colony's territory, and it can contact its subjects at any time. Its rule is absolute. The punishment for disobedience ranges from a stern warning to death by psionic overload, or worst of all, exile. To die without being absorbed by the Elder Brain is abhorrent to mind-flayers.
Normal illithids form the next level of the colony, with the eldest at the top of the hierarchy. Newly ceremorphosized mind-flayers take about twenty years to mature, until which point they are kept close to the colony. Elders have considerably more autonomy, as much as an illithid can have in the presence of an Elder Brain.
Directly below the regular illithids in the hierarchy are non-humanoid illithidae. Mind-flayers occasionally experiment with non-humanoid subjects for ceremorphosis, resulting in beings that are less intelligent than normal illithids but more powerful than the base species. The most common are urophions (created from ropers), tzikandi (from lizardfolk), uchuulons (from chuul), and mozgriken (deep gnomes). These beings are used as guards and occasional shock troops. More rarely, a tadpole may be implanted in a beholder to create a mindwitness, or rarest of all, in a dragon to create a brainstealer dragon.
Naturally, thralls are the bottom of the social pyramid and the food chain. These are creatures that have been psychically dominated by mind-flayers to become their loyal servants and eventual meals. They include not only humanoids, but various other creatures such as quaggoths and grell. On roughly the same level are pets and constructs, like intellect devourers.

Perception and Sensory Capabilities

Illithids are prodigiously powerful psionics, some of the most powerful in Erden. They use psionics as naturally as other creatures use their arms and legs.

Civilization and Culture

History

The origins of the mind-flayers are one of the great mysteries of the world. Even the gods don't know where they came from; they were not in this universe, and then they were. It is entirely possible that they crossed over from another reality, just as the gods are said to have done.
In any case, this multiverse was unprepared for them when they first arrived, about a thousand years after the Dawn War. Constrained by the Veil, the gods were unable to stop the illithids from conquering large swaths of the Inner Planes, and they could barely be prevented from establishing footholds on the Material Plane. They enslaved several races: kuo-toa, duergar, grimlocks, smaller populations of many others. A few races have been subjugated for so long that they don't even remember what they once called themselves.
But things eventually took a sharp turn for the worse. At some point, one of their oldest slave races managed to shake off the illithids' psionic control and staged a massive revolt. Whether this happened because they discovered some kind of weakness, or simply because the illithids became complacent, is a mystery. But it resulted in the destruction of millions of their kind as these people mounted a campaign to purge every single mind-flayer and every last trace of their existence from the universe. This fragmentation created a state of chaos. The duergar escaped, the kuo-toa were abandoned (as they had gone mad anyway), many colonies were abandoned. The remaining illithids went deep into hiding.
Now, all of mind-flayer life is devoted partly to survival, and partly to the "Grand Design", the plan to restore their empire to its former greatness. Every single member of the species spends their time either collecting captives, repelling attacks, or engaging in some kind of research to advance the Grand Design. But despite their efforts, they remain on the brink of extinction.

Origin/Ancestry

Extraplanar

Lifespan

125 years

Average Height

6'