Mind flayer

Mind flayers, also known as illithids, are aberrations with potent psionic abilities. On Critical Role they have so far only been encountered in the Underdark.

Description
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, as mind flayer arcanists are, seem to go insane and long to return to their hive.

Abilities
The following is drawn from the Monster Manual, 5th Edition, pp. 221–222, but it has been demonstrated onstream. 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.
 * Magic Resistance: Advantage on saving throws against spells and magical effects.
 * Innate Spellcasting (Psionics): Can cast the following spells without using components.
 * At will: Detect Thoughts, Levitate
 * 1/day each: Dominate Monster, Plane Shift (self only)
 * Mind Blast: In a cone, all creatures must succeed on an Intelligence saving throw or take psychic damage and be stunned for one minute; they can repeat the saving throw at the end of each of their turns.
 * Tentacles: Melee attack, deals psychic damage and grapples; creature must succeed at Intelligence saving throw or be stunned until the grapple ends.
 * Extract Brain: Targeting an incapacitated creature in its tentacle grapple, deals piercing damage; if it reduces the creature's hit points to zero, the creature instantly dies as its brain is removed and eaten.
 * Telepathy (120 ft. range)

History
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.

Trivia

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