(From 5th Edition Monster Manual - 2014):

Mind flayers, also called illithids, are the scourge of sentient creatures across countless worlds. Psionic tyrants, slavers, and interdimensional voyagers, they are insidious masterminds that harvest entire races for their own twisted ends. Four tentacles snake from their octopus-like heads, flexing in hungry anticipation when sentient creatures come near.

In eons past, illithids controlled empires that spanned many worlds. They subjugated and consequently warped whole races of humanoid slaves, including the githyanki and githzerai, the grimlocks, and the kuo-toa. Conjoined by a collective consciousness, the illithids hatch plots as far-reaching and evil as their fathomless minds can conceive.

Since the fall of their empires, illithid collectives on the Material Plane have resided in the Underdark.

Psionic Commanders. Mind flayers possess psionic powers that enable them to control the minds of creatures such as troglodytes, grimlocks, quaggoths, and ogres. Illithids prefer to communicate via telepathy and use their telepathy when issuing commands to their thralls.

When an illithid meets strong resistance, it avoids initial combat as it orders its thralls to attack. Like physical extensions of the illithid’s thoughts, these thralls interpose themselves between the mind flayer and its foes, sacrificing their lives so that their master can escape.

Hive Mind Colonies. Solitary mind flayers are likely rogues and outcasts. Most illithids belong to a colony of sibling mind flayers devoted to an elder brain — a massive brain-like being that resides in a briny pool near the center of a mind flayer community. From its pool, an elder brain telepathically dictates its desires to each individual mind flayer within 5 miles of it, for it is able to hold multiple mental conversations at once.

Hunger of the Mind. Illithids subsist on the brains of humanoids. The brains provide enzymes, hormones, and psychic energy necessary for their survival. An illithid healthy from a brain-rich diet secretes a thin glaze of mucus that coats its mauve skin.

An illithid experiences euphoria as it devours the brain of a humanoid, along with its memories, personality, and innermost fears. Mind flayers will sometimes harvest a brain rather than devour it, using it as part of some alien experiment or transforming it into an intellect devourer.

Qualith:

On the rare occasion that mind flayers need to write something down, they do so in Qualith. This system of tactile writing (similar to braille) is read by an illithid’s tentacles. Qualith is written in four-line stanzas and is so alien in construction that non-illithids must resort to magic to discern its meaning. Though Qualith can be used to keep records, illithids most often use it to mark portals or other surfaces with warnings or instructions.

Variant: Mind Flayer Arcanist

A few mind flayers supplement their psionic power with arcane spells. However, they are regarded as deviants by their illithid peers and usually shunned. A mind flayer arcanist has a challenge rating of 8 (3,900 XP) and the following trait.

Spellcasting. The mind flayer is a 10th-level spellcaster. Its spellcasting ability is Intelligence (save DC 15, +7 to hit with spell attacks). The mind flayer has the following wizard spells prepared:

Cantrips (at will): blade ward, dancing lights, mage hand, shocking grasp

1st level (4 slots): detect magic, disguise self, shield, sleep

2nd level (3 slots): blur, invisibility, ray of enfeeblement

3rd level (3 slots): clairvoyance, lightning bolt, sending

4th level (3 slots): confusion, hallucinatory terrain

5th level (2 slots): telekinesis, wall of force

(From Volo's Guide to Monsters - 2016):

Variant: Mind Flayer Psion

Mind flayers sometimes devote themselves to deeper study of psionic power, and many excel at using their innate psionic energy to duplicate the casting of spells. A mind flayer psion has the same statistics as a mind flayer (as presented in the Monster Manual), except that its challenge rating is 8 (3,900 XP) and it has the following trait:

Innate Spellcasting (Psionics). The mind flayer is a 10th-level spellcaster. Its innate spellcasting ability is Intelligence (spell save DC 15; +7 to hit with spell attacks). It can innately cast the following spells, requiring no components:

At will: guidance, mage hand, vicious mockery, true strike

1st level (4 slots): charm person, command, comprehend languages, sanctuary

2nd level (3 slots): crown of madness, phantasmal force, see invisibility

3rd level (3 slots): clairvoyance, fear, meld into stone

4th level (3 slots): confusion, stone shape

5th level (2 slots): scrying, telekinesis

(From 2nd Edition Monstrous Manual - 1993):

The illithid, or mind flayer, is an evil and feared creature of the Underdark; its powers are formidable and it feeds on the brains of any creature it encounters. Using arcane powers, it enslaves or destroys its foes, which include such powerful creatures as drow and kuo-toa.

Mind Flayers stand about 6 feet tall and have hideous mauve skin that glistens with slime. The head resembles an octopus, with white eyes (no pupils are evident) and four tentacles around its mouth, a round, many-toothed orifice like that of a lamprey. The creature has three reddish fingers and a thumb on each hand.

Illithids have infravision. They can communicate with any creatures via innate telepathy; they have no spoken language, although they often accompany their thoughts with hissing, and the eager lashing of their tentacles. Mind flayers dress in flowing robes, often with high, stiff collars, adorned with symbols of death and despair.

Combat: A mind flayer’s preferred method of attack is the mind blast, projected in a cone 60 feet long, 5 feet wide at the mind flayer, and 20 feet wide at the opposite end. All within the cone must make a saving throw vs. wands or be stunned and unable to act for 3d4 rounds. The illithid tries to grab one or two stunned victims (requiring normal attack rolls if others try to prevent this) and escape with them.

The illithid keeps some victims as slaves and feeds on the brains of the others. When devouring the brain of a stunned victim, it inserts its tentacles into the victim’s skull and draws out its brain, killing the victim in one round. A mind flayer can also use its tentacles in combat; it does so only when surprised or when attacking a single, unarmed victim. A tentacle which hits causes 2 hp damage and holds the victim. A tentacle does no damage while holding, and can be removed with a successful bend bars/lift gates roll. Once all four tentacles have attached to the victim, the mind flayer has found a path to the brain and kills the victim in one round. If preferred, the DM can simply roll 1d4 for the number of rounds required to kill a struggling victim.

A mind flayer can also use the following arcane powers, one per round, as a 7th-level mage: suggestion, charm person, charm monster, ESP, levitate, astral projection, and plane shift. All saving throws against these powers are made at a -4, due to the creature’s mental prowess.

If an encounter is going against a mind flayer, it will immediately flee, seeking to save itself regardless of its treasure or its fellows.

Habitat/Society: Mind flayers hate sunlight and avoid it when possible. They live in underground cities of 200 to 2,000 illithids, plus at least two slaves per illithid. All the slaves are under the effects of a charm person or charm monster, and obey their illithid masters without question.

The center of a community is its elder brain, a pool of briny fluid that contains the brains of the city’s dead mind flayers. Due to the mental powers of illithids, the elder brain is still sentient, and the telepathic union of its brains rules the community. The elder brain has a telepathic range of 2 to 5 miles, depending on its age and size. It does not attack, but telepathically warns the mind flayers of the presence of thinking creatures, so a mind flayer within its telepathic radius can be surprised only by non-intelligent creatures. The range of the elder brain determines the territory claimed and defended by the community, though raiding parties are sent far beyond this limit.

Mind flayers have no family structure. Their social activities include eating, communicating with the elder brain, and debating on the best tactics to conquer the Underdark. For amusement, they inflict pain on their captives and force slaves to fight in gladiatorial games.

Mind flayers are arrogant, viewing all other species only as cattle to be fed upon. They prefer to eat the brains of thinking creatures.

Ecology: Mind flayers live about 125 years. They are warm-blooded amphibians, and spend the first 10 years of life as tadpoles, swimming in the elder-brain pool until they either die (which most do) or grow into adult illithids. On an irregular basis, adult illithids feed brains to the tadpoles, which do not molest the elder brain. Illithids are hermaphroditic; each can produce one tadpole twice in its life.;

Mind flayer ichor is an effective ingredient in a potion of ESP.

Psionic Illithids: Psionic flayers, considered the only true illithids by some (including themselves), have most of the same statistics and abilities as other mind flayers. Instead of magic-based abilities, however, theirs are purely psionic. Psionic mind flayers have a beak-like mouth and disdain the stiff-collared robes preferred by their cousins.

Illithids use psionics for attack, mind control, and travel.

Psionics Summary

Level Dis/Sci/Dev Attack/Defense Score PSPs

10 4/5/15        EW,II/All       =Int       1d100+250

Psychokinesis — Sciences: nil; Devotions: control body, levitation.

Psychometabolism — Science: body equilibrium; Devotions: nil.

Psychoportation — Sciences: probability travel, teleport; Devotion: astral projection.

Telepathy — Sciences: domination, mind link; Devotions: awe, contact, ESP, ego whip, id insinuation, post-hypnotic suggestion.