Illithids (commonly known as mind flayers) are monstrous, humanoid aberrations with psionic powers. They live in the moist caverns and cities of the enormous Underdark. Illithids believe themselves to be the dominant species of the multiverse and use other intelligent creatures as thralls, slaves, and chattel.
Illithids have humanoid bodies, with octopoid heads. They have four tentacles around a lamprey-like mouth, and require the brains of sentient creatures as part of their diet. An illithid who snares a living creature in all four of its tentacles can extract and devour its living brain. Illithid eyes are pale white, and though their vision is limited to roughly 60 feet, they can see perfectly well in both darkness and light. Their sense of hearing is slightly poorer than a human's; they have difficulty distinguishing between several sounds mixed together. Their skin is mauve and covered in mucus, and is very sensitive to sunlight; illithids who make extended forays to the surface must wear protective clothing to maintain their skin moisture.
Illithids spend much of their time in intense study and experimentation, gathering knowledge of all sorts that will enable them to eventually reconquer the universe and hold it for good. They frequently meddle in the politics of other races through subtle psychic manipulation of key figures, not to cause chaos but so as to better understand the dynamics of civilization. They regularly probe the minds of surface dwellers so as to gather intelligence and learn about new advances in magic and technology. Illithids also do a good deal of research themselves, mainly focused on developing new psychic powers.
Illithids regularly conduct raids on all sentient settlements to acquire new thralls, because their existing stock of sentient thralls do not breed fast enough to satisfy their food and labor needs. Typically, a group of mind flayers will teleport to the settlement and swiftly incapacitate them with their psychic powers. The captives will then be marched all the way to the illithids' underground settlement by specially trained and conditioned thralls. Great care is taken to cover their tracks.
Mind flayer communities range in size from two hundred to two thousand, and that is counting only the illithids. Each mind flayer in the community likely has at least two slaves to do its bidding. In these communities, the number of slaves often far outstrips the number of mind flayers.
Mind flayer religion was once non-existent. In the time when they had invaded the mountain dwarf kingdom of Shanatar and captured many shield dwarves, there were no gods to the mind flayers. Originally, their plan was to create a superior race of dwarves, skilled in both mechanics and psionics that would be loyal solely to the mind flayers. However, when the dwarves began to stage uprisings and rebellions, the city of Oryndoll, where the experimentation was occurring, was plunged into chaos. The only reason the city did not fall in the duergar rebellions was because of the sudden appearance of the mind flayer god Ilsensine. Since Ilsensine's appearance, the mind flayers have become deeply religious and begun to develop the formidable psionic powers that the race now has. llsensine’s favored proxy is Lugribossk.
Illithids typically communicate through psychic means. They project thoughts and feelings to each other in a way non-illithids can scarcely comprehend. When they do feel the need to write, illithids do so in qualith. Instead of typical alphabet-based writing, illithids write in qualith by making marks consisting of four broken lines. They use each tentacle to feel the breaks in the lines, making it basically similar to braille. However, qualith is extremely complex, as each line modifies the preceding lines through explaining abstract concepts associated with the above words in ways no human can understand; only by understanding all four lines simultaneously can the meaning be understood properly.