Illithids, also known as mind flayers, are a race of horrifically evil creatures that live in the Underdark. Their origin is in the nightmarish Far Realm, but at some point long ago they came through to the Material Plane and created a massive underground empire. During the Illithid War they devastated the nation of Estorium, but were ultimately defeated and have been seen only rarely since then.
Few monsters inspire as much horror or terror as the illithids. They are rightly feared by everyone and everything. Few can match them in undiluted evil, cruelty, or cunning. Their origins are cloaked in mystery, their plans are enigmatic, and their culture and thought processes are utterly alien. Illithids are better known by their common name of mind flayers, and it is an apt title. A mind flayer’s attack strikes into a victim’s most vulnerable spot. It can psychically peel back a target's personality the way a surgeon draws back swollen flesh. Tentacles slick with slime writhe across ravaged edges of bone to caress gray, glittering coils of brain before drawing them from the conscious, screaming prey. Where the illithids came from, why they choose to live in eternal darkness beneath the earth, what they seek, why they take such delight in cruelty and horror... These questions have stymied sages for centuries. The answers are startling and horrible, yet provide little illumination. One can know the mind flayers without understanding them.
Illithids are humanoid monsters, roughly the size of humans. Their pale mauve flesh is covered in glistening slime, and in place of a human head they have a terrifying visage reminiscent of nothing so much as a four-limbed octopus. Their evil reputation is based on their horrid appetite for living brains. All mature illithids are psionically gifted, and some are wizards or (rarely) sorcerers as well. Mind flayers dwell underground and prefer to remain there. They have sometimes been encountered in deep-sided chasms and half-buried surface ruins, but the world beneath the surface is their realm. In that place, filled with fearsome races and monsters, mind flayers are among the most dangerous predators of the dark. A lone illithid hunting in its element is more than a match for a group of surface warriors, and seldom is an illithid alone. They travel with the protection of their minions, enslaved subterranean monsters, or worse yet, others of their own kind. Those who explore the deep underground paths agree that it is better to die almost any other death than to fall into the clutches of the mind flayers.
A mind flayer is roughly comparable to a thin human in height and build, but the external resemblance stops at that point. An illithid’s head is a monstrous sight, resembling a four-tentacled octopus sitting atop the creature's shoulders. The two eyes, uniformly pale white and without pupils, are sheltered beneath prominent brow ridges. The creature's soft, moist skin is mauve in color and glistens beneath a thin coating of mucus. Mind flayers have three long, slender fingers and an opposable thumb on each hand, and two webbed toes on each foot. Each finger and toe is capped with a wicked-looking nail, which aren’t as dangerous as they seem. In fact, the nails are composed of soft cartilage and present little danger to anyone struck or scratched by them.
Internally, illithids are similar to humans. They have hearts, lungs, livers, spleens, stomachs, and digestive systems that are recognizable to any anatomist. However, an illithid’s nervous system is more extensive and more advanced than any human's. Every part of an illithid's body is "wired" into the brain with direct connections unseen in any other creature. In effect, an illithid's entire body is an extended brain. One might question the cognitive power of a liver, but there is no doubting that an illithid has tremendous physical awareness of its own body and physical condition.
Because of the mind flayer's all-embracing nervous system, food does not pass through a simple gastrointestinal tract but through a cognitive, self-aware digestive system. That system absorbs more than just nourishment from food. It scavenges enzymes, hormones, and most important, psychic energy. Illithids are known for consuming brains, but they eat other food as well, most of which contains various amounts of these needed enzymes and hormones. Internal organs are good sources, and they rank high on illithid menus. Brains are ripe with all three and are the only external source of psychic energy.
An illithid’s tentacles can vary from 2 to 4 feet in length when fully extended. When the creature is at rest and not excited, the tentacles appear shorter. Even then, they are in almost constant motion, writhing absent-mindedly as the creature ponders. These limbs are extraordinarily dexterous and serve the mind flayer as an additional set of hands, even to the point of being used to punctuate or accentuate communication. The tentacles are also quite strong. The tentacle cluster surrounds a circular, jawless mouth ringed with rows of small, rasping teeth. The teeth serve primarily as tools for gripping and to prevent slippery gobs of brain matter from falling out of the mouth. An illithid does not bite through the skin and skull of a victim, instead dissolving it with a powerful enzyme transmitted through ducts in the tentacles. This enzyme acts so quickly that the tentacles appear to push right through the scalp and bone as if through soft clay. The enzyme is highly unstable and never survives more than brief contact with the air, making it impossible to harvest from slain illithids. No material other than illithid mucus is known to resist its corrosive effect.
Although their white, pupilless eyes suggest blindness, mind flayers see quite well; their acuity lies entirely in the realm of darkvision. Their hearing is a little less acute than a human's. They have relatively good auditory direction sense (they can tell where a sound is coming from), but they have poor discernment (ability to separate and recognize discrete components of a particular sound).