Mind flayer

An illithid, also called a mind flayer, was an evil and sadistic being, humanoid in appearance, but with a four-tentacled octopus-like head. These beings were feared throughout the Underdark for their telepathic abilities and usually were not without two or more slaves, mentally bound to each individual mind flayer. Although they cooperated to achieve a goal, they would back away at the first sign that something was not profitable to themselves. They were capable of speaking Undercommon, but preferred telepathic communication and would attempt to mentally dominate any non-slave, non-illithid they met. They fed on the brains of sentient creatures and were thus feared. Their natural psionic abilities also made them respected in the eyes of the drow, beholders, duergar, and the other dominant races of the Underdark.

History
Some theorized mind flayers to be aliens from the future (as stated in The Sargmne Prophecies), who had come back in time to prevent the end of the world. Facing extinction, they sent the greatest spelljamming fleet in history back in time. Due to their futuristic technology and ships, many believe that they were a cursed in-bred mutant offspring of humans from a distant world. Some even believe that signs point to a more sinister origin in the Far Realm.

Powers
The mind flayers were feared by all beings in the Underdark because of their great mental prowess. Every illithid had 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 was a 60-foot (20-m) cone that stunned anyone caught within it. The mind flayer could also innately cast three mind-affecting spells: charm monster, detect thoughts, and suggestion. Mind flayers were also capable of casting levitate and plane shift.

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

They also have the ability to craft psionic seals.

Society
Mind flayer communities ranged in size from two hundred to two thousand, and that was counting only the illithids. Each mind flayer in the community likely had at least two slaves to do its bidding. In these communities, the number of slaves often far outstripped the number of mind flayers. For example, the illithid city of Oryndoll had a total population of 26,000, but mind flayers only account for 3,450 of that number.

When problems arose or the mind flayers wish to discover some secret, they formed inquisitions. These inquisitions were made up of a team of mind flayers, not unlike an adventuring party—each mind flayer used their own talents and abilities to achieve the inquisition's goal. If a situation was too large for just an inquisition to handle, the mind flayer community put together a cult. A cult was much larger than an inquisition and was spearheaded by two mind flayers who constantly vied for greater power within the cult.

Although mind flayers willingly came together to achieve an end, they were always vying for more control in the community, but even then they were always beneath the elder brain. The elder brain was the heart of the community. Held in a pool of briny fluids, the elder brain consisted of all the brains of the dead mind flayers in the community.

Some mind flayers underwent a process called laceremorphosis, which made them bigger, stronger, and more powerful than regular mind flayers. They became ulitharids or illithid-nobles, and possesed six face tentacles instead of the regular four.

A urophion was a subspecies of mind flayer created as guardians for a mind flayer community. An urophion was a roper that had survived the tadpole implantation process. They were usually treated as second-class citizens but were permitted to join with the elder brain upon death.

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, the mind flayers had no gods. 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 to the duergar rebellions was because of the sudden appearance of the mind flayer god Ilsensine. Since Ilsensine's appearance, the mind flayers became deeply religious and began to develop the formidable psionic powers that the race was known for. Ilsensine's favored proxy was Lugribossk.

Mind flayers once had an additional god called Maanzecorian, who was killed by Tenebrous, the undead shadow of the demon lord Orcus, though this fact was unknown to most.

Illithid magic
Although most mind flayer arcanists were wizards, a few were also born with the gift of sorcery. Because a mind flayer sorcerer was naturally more intelligent than other mind flayers, it was better able to resist psionics. For the most part, an illithid with the gift of sorcery would use defensive spells such as greater invisibility and resist energy as well as spells to further hinder enemies, such as ray of exhaustion and touch of idiocy.

A mind flayer who decided to follow the path of wizardry could become an undead creature known as an alhoon or an illithilich.

Subraces

 * Alhoon: An undead mind flayer that had achieved lichdom.
 * Ulitharid: A superior version of a illithid.
 * Vampiric illithid: A feral illithid with vampiric powers.

Notable mind flayers

 * Galuum
 * Methil El-Viddenvelp
 * Yharaskrik
 * Xetzirbor