Mind flayer

The illithid, also called the mind flayer, is an evil and sadistic being, humanoid in appearance, but with a four-tentacled octopus-like head. These beings are feared throughout the Underdark for their telepathic abilities and usually are not without two or more slaves, mentally bound, to each individual mind flayer. Although they cooperate to achieve a goal, they will back away at the first sign that it is not something that will be self-profitable. They are capable of speaking Undercommon, but prefer telepathic communication and will attempt to mentally dominate any non-slave, non-illithid they meet. Theorized to be aliens from the future, they come back in time to save their species from extinction. They feed on the brains of sentient creatures and are thus feared. Their natural psionic abilities also make them respected in the eyes of the drow, beholders, and duergar, the other dominant races of the Underdark.

Illithid psionics
The illithids are feared by all beings in the Underdark because of their great mental prowess. Every illithid has a small array of mind-affecting spells to take control of their prey as well as a powerful mind blast to affect a multitude of foes. The mind flayer's mind blast is a 60-foot (20m) cone that stuns anyone caught within it. The mind flayer can also innately cast three mind-affecting spells: charm monster, detect thoughts, and suggestion. Mind flayers are also capable of casting levitate and plane shift.

Normally, an illithid will use its mind blast ability to stun a few foes and then drag them away to feed. Once it has its victims, it will attach all of its tentacles to the head of its victim. Then, the illithid uses its extract ability to suck out the brains of its victims, instantly killing the victim, as long as it only has one head. The illithid uses its other spells mainly to enslave its minions and keep them under total control, as well as on the battlefield.

Illithid sorcerers
Although most illithid casters are wizards, few are also born with gift of sorcery. Because a mind flayer sorcerer is naturally more charismatic than other mind flayers, it is better able to resist psionics. For the most part, an illithid with the gift of sorcery will use defensive spells such as greater invisiblity and resist energy as well as spells to further hinder enemies, such as ray of exhaustion and touch of idiocy. An illithid who decides to follow the path of wizardry can become an undead creature known as an alhoon or an illithilich.

Society
Illithid communities range in size from two hundred to two thousand, and that is counting only the illithids. Each illithid 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 illithids. For example, the illithid city of Oryndoll has a total population of 26,000, but illithids only make up for 3,450 of that number. Any illithid knows better than to probe the mind of the slave of another of their kind.

When problems arise or the illithids wish to discover some secret, they form inquisitions. These inquisitions are made up of a team of illithids, not unlike an adventuring party – each illithid gives their own talents and abilities to achieve the inquisition's goal. If a situation is too large for just an inquisition to handle, the illithid community puts together a cult. A cult is much larger than an inquisition and is spearheaded by two illithids who constantly vie for the greater power within the cult.

Some illithid undergo process called laceremorphosis, which makes them bigger, stronger and more powerful than regular illithid. They become Ulitharid or illithid-nobles, and posses six face tentacles instead of the regular four.

Although illithids willingly come together to achieve an end, they are always vying for more control in the community, but even then they are always beneath the elder brain. The elder brain is the heart of the community. Held in a pool of briny fluids, the elder brain consists of all the brains of the dead illithids in the community.

Religion
Mind flayer religion was once non-existent. In the time when they had invaded the shield 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.