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

Mind flayers that pursue arcane magic are exiled as deviants, and for them no eternal communion with an elder brain is possible. The road to lichdom offers a way to escape the permanency of death, but that path is long and solitary. Alhoons are mind flayers that use a shortcut.

Arcane Temptation. Elder brains forbid mind flayers from pursuing magic power aside from psionics, but it isn’t an interdiction they must often enforce. Illithids brook no masters but members of their own kind, so it isn’t in their nature to bow to any god or otherworldly patron. However, wizardry remains a rare temptation.

In the pages of a spellbook, an illithid sees a system to acquire authority. Through the writings of the wizard who penned it, the illithid perceives the workings of a highly intelligent mind. Most mind flayers who find a spellbook react with abhorrence or indifference, but for some a spellbook is a gateway to a new way of thinking.

For a time, the study of such forbidden texts can be hidden from other illithids and even from an elder brain. Understanding of wizardry eludes the mind like a living thing. Yet eventually, understanding comes, and a mind flayer arcanist must accept itself as deviant and flee the colony if it is to live.

Existential Fear. Arcanist deviants that taste freedom from the colony react in a variety of ways. Some prize their privacy, others seek to commune with similar minds, and still others seek to dominate a colony, elevating themselves to the position of leadership normally held by an elder brain. Regardless of the arcanist’s personal inclinations, it faces the same stark fact: When it dies, it will not join the host of minds in the elder brain. Deviant minds are never accepted as part of the collective. For it, death means oblivion.

Dreadful Deliverance. Lichdom offers salvation and the prospect of being able to pursue knowledge indefinitely. Having feasted on the brains of people when alive, a mind flayer has no compunction about feeding souls to a phylactery. The only hindrance to a mind flayer becoming a lich is the means, which is a secret some mind flayer arcanists stop at nothing to discover. Yet lichdom requires an arcane spellcaster to be at the apex of power, something many mind flayers find is far from their grasps.

Confronting this awful reality, a group of nine mind flayer deviants used their arcane magic and psionics to weave a new truth. These nine called themselves the alhoon, and ever afterward, all those who follow in their footsteps have been referred to by the same name.

A Psionic Secret. Alhoons can cooperate in the creation of a periapt of mind trapping, a fist-sized container made of silver, emerald, and amethyst. The process requires at least three mind flayer arcanists and the sacrifice of an equal number of souls from living victims in a three-day-long ritual of spellcasting and psionic communion. Upon its completion, free-willed undeath is conferred on the mind flayers, turning them into alhoons.

Initially, an alhoon can be difficult to distinguish from a normal mind flayer. The most obvious difference is the lack of the mind flayer’s ever-present mucus coating. Without that protection, an alhoon’s skin becomes dry and cracked. Its eyes might appear shriveled and sunken. Both of these clues are easily missed by someone who hasn’t seen a mind flayer. However, in short order, an alhoon’s flesh withers away and its empty eye sockets gleam with cold pinpricks of light like other liches.

Precarious Immortality. Unlike with true lichdom, the periapt of mind trapping doesn’t restore the alhoons to undeath if they are destroyed. Instead, a destroyed alhoon’s mind is transferred to the periapt where it remains in communion with any other trapped alhoon minds, as well as the souls of those sacrificed.

The undeath conferred by a periapt of mind trapping lasts only so long as the life of the living victim selected. Thus an alhoon who brought a 200-year-old elf to be sacrificed looks forward to a much longer existence than one that sacrifices a 35-year-old person. Alhoons can extend their existence by repeating the ritual with new victims, effectively resetting the clocks for themselves.

Destruction of a periapt of mind trapping consigns those trapped within it to oblivion, and thus alhoons often work together to create elaborate protections about the periapt and their preferred ritual site. Sometimes a single alhoon is entrusted with the periapt of mind trapping, but this is a dangerous proposition. Anyone who holds the periapt of mind trapping gains advantage on attacks, saves, and check against the alhoons associated with its creation, and those alhoons in turn suffer disadvantage on attacks, saves, and check against the holder. In addition, the holder of the periapt can telepathically communicate with any sacrificed soul trapped within, and alhoons within the periapt can speak telepathically with the holder. A creature carrying the periapt can’t prevent communication from alhoons but can silence trapped souls.

Variant: Mind Flayer Lich (Illithilich):

The path to true lichdom is something only the most powerful mind flayer mages can pursue, since it requires the ability to craft a phylactery and cast the imprisonment spell. A mind flayer lich uses the lich stat block (see the Monster Manual), with the following changes:

• It has a challenge rating of 22 (41,000 XP).

• It speaks Deep Speech and Undercommon, and has telepathy out to a range of 120 feet.

• It has the Magic Resistance and Innate Spellcasting (Psionics) traits, as well as the Tentacles, Extract Brain, and Mind Blast action options (all described below). So long as a mind flayer lich feeds captured souls to its phylactery, it maintains the muscular power of its tentacles and the ability to extract brains.

• Its suite of legendary actions (described below) is different from that of the normal lich.

Magic Resistance. The lich has advantage on saving throws against spells and other magical effects.

Innate Spellcasting (Psionics). The lich’s innate spellcasting ability is Intelligence (spell save DC 20). It can innately cast the following spells, requiring no components. At will: detect thoughts, levitate 1/day each: dominate monster, plane shift (self only)

Actions:

Tentacles. Melee Weapon Attack: +12 to hit, reach 5 ft., one creature. Hit: 21 ((1d20+12)) psychic damage. If the target is Large or smaller, it is grappled (escape DC 15) and must succeed on a DC 20 Intelligence saving throw or be stunned until this grapple ends.

Extract Brain. Melee Weapon Attack: +12 to hit, reach 5 ft., one incapacitated humanoid grappled by the lich. Hit: 55 ((3d10+5)) piercing damage. If this damage reduces the target to 0 hit points, the lich kills the target by extracting and devouring its brain.

Mind Blast (Recharge 5–6). The lich magically emits psychic energy in a 60-foot cone. Each creature in that area must succeed on a DC 18 Intelligence saving throw or take 27 ((3d10+5)) psychic damage and be stunned for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Legendary Actions:

The lich gains the following legendary action options, which replace all of the lich’s legendary actions.

Tentacles. The lich makes one attack with its tentacles.

Extract Brain (Costs 2 Actions). The lich uses Extract Brain.

Mind Blast (Costs 3 Actions). The lich recharges its Mind Blast and uses it.

Cast Spell (Costs 1–3 Actions). The lich uses a spell slot to cast a 1st-, 2nd-, or 3rd-level spell that it has prepared. Doing so costs 1 legendary action per level of the spell.

(From 2nd Edition Monstrous Manual - 1993):

Alhoon (also called illithiliches) look like living mind flayers (mauve-skinned, octopusheaded beings with four mouth-tentacles, and three-fingered hands). The only visible difference between illithid and illithilich is that an alhoon’s skin is dry and often wrinkled, never glistening with slime.

Combat: Alhoon attack with four tentacles, as living mind flayers do. Once a tentacle hits, it does 1d4 points of damage per round automatically, as it bores on into the victim’s body. Attacks on a tentacle (consider it AC 7) doing it 5 points or more of damage in a single round cause it to recoil, drawing out of the victim’s body. It will then always strike at a new spot; an attack roll is required, and boring time to the victim’s brain remains 1-4 rounds. Tentacles striking a victim elsewhere than its head do damage for 4 rounds and then withdraw; they are not long enough to reach the brain.

Illithiliches have the mind blast it had in life (cone 60 feet long, opening from 5 to 20 feet; save vs. wands or stunned 3d4 rounds) and psionic abilities common to all true mind flayers (the equivalent of a 7th-level psionicist – 4 disciplines, 5 sciences, and 14 devotions). Alhoon additionally attack with mind thrust, and individual abilities es are possessed as well (consult PHBR5 The Complete Psionics Handbook).

In addition to their tentacles and psionic abilities, illithiliches can cast spells as 9th level mages (spells: 4, 3, 3, 2, 1). Typically, they use a wide variety of spells seized from human mages, spellbooks found in tombs, and the like.and always avidly seek more, driven by their hunger for power. An alhoon can use a spell (as well as its tentacle attacks) in any round in which it does not use psionics.

Alhoon gain no special undead attacks (such as a human lich’s chill touch), but do have standard undead immunities to sleep, hold, and charm-related magics. They cannot be turned or dispelled by priests, and are not harmed or impeded by holy water, cold iron, protection from evil, sunlight, or silver weapons – but are subject to spells specifically affecting undead.

Habitat/Society: Alhoon spurn illithid societies ruled by elder brains, and do not hesitate to take living mind flayers as thought controlled slaves (just as they took all other creatures as slaves, when alive – a process continued in lich state). They usually live alone in the surface world, often slaying a human wizard and taking over his remote tower, but in the Underdark cooperate for mutual survival, sharing spells and aid freely to overcome drow, duergar, cloakers, aboleth, and living mind flayers alike. Alhoon are capable of diplomacy and of loyally adhering to alliances when they see an ultimate benefit – but they consider all other beings cattle, and promises to them merely empty conveniences. Alhoon regard true liches and beholders as their greatest rivals, and accordingly destroy them whenever prudently possible.

Ecology: Alhoon have no need for sustenance, but their bodies adapt only imperfectly to lich state; many magical steps of most lichdom processes used by others fail on a strongly-magic resistant mind flayer body. Alhoon are plagued by ongoing skin wrinkling and tissue desiccation, which they counteract by bathing, or by drinking water, soup, alcohol, and other liquids. Nutrients need not be ingested, and poisons absorbed in this way will harm an alhoon (lowering its bit points no further than a minimum of 6 hp and not killing it). The illithilich state neutralizes most poisons (restoring all damage done by them) in 2d4 turns.

Illithiliches enjoy devouring brains just as they did in life, but do not need to do so. Sometimes (3 in 12 chance), devouring a brain gives an alhoon mental glimpses of 1d12 thoughts that the brain held, either at random, or (if the alhoon concentrates on a topic, such as magical items, written spells, or treasure locations), thoughts most closely related to a chosen topic.

Essence of alhoon brain is a general ingredient in spell-writing inks, and can be employed with great advantage in the crafting of any magical item concerned with effects of the minds of creatures.

Psionics Summary:

Level  Dis/Sci/Dev Attack/Defense Score PSPs

9  4/5/14        EW,II,MT/All = Int        250 + d100

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

Psychometabolism — Science: body equilibrium; Devotions: nil.

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

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

Other Powers: Various; two sciences, five devotions.