Mind flayer

Mind flayers, also known as illithids, are creatures found in the Underdark.

Creature Information
Illithids are characterized by their exceptional height, purplish skin, facial tentacles surrounding a toothy maw, and levitation ability, which is often used for movement. Each illithid community is connected by a central intelligence, known as an elder brain, that telepathically projects its desires to the illithids within several miles. Mind flayers are suspicious of the arcane, and practice of such can lead to expulsion from their society. They are considered evil.

Clarota described illithid society as well-structured, with groups of mind flayers being broken into creeds. Each individual mind flayer supports the whole.

Intellect devourers seem to act as a sort of pet or extension of an illithid's will. Whether or not they are created by the illithids or naturally reproduce in Critical Role is unknown.

Illithids that have been banished from their colony seem to go insane and long to return to their hive.

Vox Machina encountered several mind flayers during their travels in the Underdark. They also infiltrated a mind flayer city called Yug'Voril and nearly destroyed the elder brain residing within.

Known Illithid

 * Clarota: a magic-wielding illithid and former ally and companion of Vox Machina, killed after betraying the group.

Abilities
While illithids do not typically possess arcane abilities, they do have powerful psionic capabilities. Most forms of attack from a mind flayer target the mind of a creature. While they know where a creature is, they can invade that creature's mind. If they are blinded and do not already have a firm mental link with a creature, they cannot use their abilities.
 * Levitation
 * Mind Blast
 * Mind Control
 * Telepathy
 * Teleportation

Trivia

 * Due to Mind Flayers being considered Product Identity of Wizards of the Coast under the Open Game Licence, they are referred to simply as "Flayers" in the Tal'Dorei Campaign Setting Guide.