Illithids are insidious, vile creatures, that live in the Underdark, the underground of Gaera. If they're capable of good deeds, nobody knows. Those who have encountered one never forget the experience, no matter how they try to.
Illithids are called "Mind Flayers" by most other races, because of their abilities. An illithid looks much like a humanoid, though the limbs are thin and the hands and feet end in claws. Their heads look like a squid, having tentacles draping over a beak that constantly drips ichor. Two eyes, burning with hatred for all sentinent beings, are in the head.
In addition to their indimidating physique, Mind Flayers are also extrememly potent psychics. They use their abilities to subdue their victims and charm them. Illithids are not adverse to slave labor; in fact, they depend on it.
The diet of an illithid consists of brains. Brains of sentient creatures are the best, and thus preferred. Anyone who has had rich experiences or profound insights have the best brains, so an illithid would prefer the brain of a seasoned adventurer over that of, say, a child. This does not mean they are merciful, however. A Mind Flayer would gladly eat the brain of a child if he or she knows that it would cause pain and distress in someone else.
Mind Flayers are not born so much as spawned. When they hatch from their eggs, they look like tadpoles. When they reach maturity, they are implanted into a slave, and over the next few weeks, transforms the host into an illithid. Nothing is left of the host when this is completed, and by all intents and purposes, the larvae is now a fully grown Mind Flayer. All illithid are asexual, though they adopt a personal pronoun of their choice.
Illithids live in cities underground. Since all Mind Flayers are telepaths, they can float and levitate, so construction goes vertically as well as horizontally. In the center of each Illithid city is the Elder Brain. Nobody knows where they came from, but the elder brain contains the memories of everything that has ever happened to any Illithid that has died, as well as anything the brain itself learned. It's sentinent, it's pure evil, and it is the heart of any hive. If an illithid dies, their brain is removed and thrown into the pool containing the elder brain, which then absorbs the memories and personality. It is considered a great dishonor to die without becoming one with the Elder Brain. It is also considered the greatest crime to consume the brain of another Mind Flayer. There is no repentance for this.
Illithids also keep slaves for breeding, so to have a fresh supply of brains. Each city is small, consisting of roughly 20-50 flayers. There are frequent raids to towns above the ground, to obtain fresh slaves and better tasting brains. However, these happen at night. Daylight dries the illithid's skin, and causes severe pain. As such, you are safe in daylight.
Nobody knows what the flayers are up to. They're considered one of the Great Threats to those who know about them. Some have met an illithid, but never realized there are more of them.
Xzar'il - An illithid (later illithilich or alhoon) that terrorized Gaera for several years.
Flayer - the only known illithid to abandon his kind, and joined the Mithril Order. Consumes the brains of the purest evil creatures, such as other illithids. Considered "most wanted" among other Mind Flayers.