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

If the beholder can be stunned and brought safely to the brine pool of the elder brain, it can be converted through ceremorphosis into a mindwitness. The process of ceremorphosis transforms four of the beholder’s eyestalks into tentacles similar to those of a mind flayer, and alters some of the beholder’s eye rays.

Less intelligent than beholders and less liable to endanger the colony, mindwitnesses are psionically imprinted with devotion to the elder brain and submission to illithid commands, making them almost as obedient as intellect devourers.

Telepathic Hub. The primary function of a mindwitness is to improve telepathic communication in a mind flayer colony. A creature in telepathic communication with a mindwitness can converse telepathically through it to as many as seven other creatures the mindwitness can see, allowing the rapid spread of commands and other information.

Solitary Seekers. If separated from its illithid masters, a mindwitness seeks out other telepathic creatures to tell it what to do. Mindwitnesses have been known to ally with flumphs and telepathic planar beings such as demons, shifting their worldview and changing their alignment to match that of their new masters.

(From Underdark - 2003):

A mindwitness is a 6-foot-diameter orb dominated by a central eye and a small, lampreylike maw. Ten smaller eyes on stalks sprout from the top of the orb, as do four waving tentacles.

Mindwitnesses speak their own language and the Common tongue, ‘plus they can communicate telepathically with any creature within 100 feet that has a language.

Combat: Mindwitnesses begin combat by using mind blast on their foes, then follow up with their devastating eye rays. They usually refrain from attempting to extract the brains of creatures that have not been stunned or otherwise befuddled.

Extract (Ex): A mindwitness that begins its turn with all four tentacles attached and wins a grapple check automatically extracts the opponent’s brain, instantly killing that creature. This power is useless against constructs, elementals, oozes, plants, and undead. Extraction is not instantly fatal to foes with multiple heads, such as ettins and hydras.

Eye Rays (Su): Each of a mindwitness’s ten small eyes can produce a magical ray once per round as a free action. During a single round, the creature can aim only three eye rays at targets in any one 90degree arc (up, forward, backward, left, right, or down). The remaining eyes must aim at targets in other arcs or not at all. A mindwitness can tilt and pan its body each round to change which rays it can bring to bear in any given arc.

Each eye’s effect resembles a spell (caster level 13th), but follows the rules for a ray (see Aiming a Spell in the Players Handbook). Each ray has a range of 150 feet and a save DC of 19. The save DC is Charisma-based. The ten rays are charm monster, charm person, disintegrate, fear, Singer of death, flesh to stone, inflict moderate wounds, sleep, slow, and telekinesis. See the Beholder entry in the Monster Manual for more information.

Improved Grab (Ex): To use this ability, a mindwitness must hit a creature of its size category or smaller with its tentacle attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and attaches the tentacle to the opponent’s head. A mindwitness can grab a Huge or larger creature, but only if it can somehow reach the foe’s head. If a mindwitness begins its turn with at least one tentacle attached, it can try to attach its remaining tentacles with a single grapple check. The opponent can escape with a single successful grapple check or an Escape Artist check, but the mindwitness gets a +2 circumstance bonus for every tentacle that was attached at the beginning of the opponent’s turn.

Mind Blast (Sp): Once per day, a mindwitness can produce a mind blast in a cone 40 feet long. Anyone caught in this area must make a DC 19 Will save or be stunned for 1d4 rounds. The save DC is Charisma-based. This ability is the equivalent of a 4th level spell.

Psionics (Sp): 3/day—charm monster (DC 18), detect thoughts, levitate, suggestion (DC 17). Caster level 8th. The save DCs are Charisma-based.

All-Around Vision (Ex); A mindwitness’s many eyes give it a +4 racial bonus on Search and Spot checks, and it can’t be flanked. :

Antimagic Cone (Su); A mindwitness’s central eye continually produces a 150-foot cone of antimagic. This functions just like antimagic field (caster level 13th). All magical and supernatural powers and effects within the cone are suppressed—even the beholder’s own eye rays. Once each round, during its turn, the mindwitness decides whether the antimagic cone is active or not (the mindwitness deactivates the cone by shutting its central eye). 

Flight (Ex): A mindwitness’s body is naturally buoyant. This buoyancy allows it to fly at a speed of 20 feet and grants it a permanent feather fall effect (as the spell) with personal range.

Telepathy (Su): A mindwitness can communicate telepathically with any creature within 100 feet that has a language.