The mind flayer, also called the illithid, 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. Some theorized mind flayers to be aliens from the future, who had come back in time to save their species from extinction. Another tale is that they are cursed inbred mutant offspring of humans from a distant world. However, more convincing evidence points to a more sinister origin as being a creation of the daelkyr. 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, duerger and the other dominant races of the Underdark.
The mind flayers 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, a mind flayer 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 mind flayer 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 mind flayer uses its other spells mainly to enslave its minions and keep them under total control, as well as on the battlefield.
- An undead mind flayer that has achieved lichdom
- A superior verson of a illithid
- Vampiric illithid
- a feral illithid with vampiric powers
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.
- When problems arise or the mind flayers wish to discover some secret, they form inquisitions. These inquisitions are made up of a team of mind flayers, not unlike an adventuring party – each mind flayer 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 mind flayer community puts together a cult. A cult is much larger than an inquisition and is spearheaded by two mind flayers who constantly vie for the greater power within the cult.
Some mind flayers undergo process called laceremorphosis, which makes them bigger, stronger and more powerful than regular mind flayers. They become Ulitharid or illithid-nobles, and posses six face tentacles instead of the regular four.
Although mind flayers 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 mind flayers in the community.
A Urophion is a sub species mind flayer created as guardians for a mind flayer community. A Urophion is a Roper that has survived the tadpole implantation process. They are usually treated as second class citizens but may join with the elder brain upon death. 
Mind flayer religion was once non-existent. Since Ilsensine's appearance as a mind flayer god, 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.