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

The ultimate expression of illithid domination, an elder brain sprawls within a vat of viscous brine, touching the thoughts of creatures near and far. It scrawls upon the canvas of their minds, rewriting their thoughts and authoring their dreams.

Psychic Infiltrators. When an elder brain infiltrates a mind, it alters the creature’s perception and deceives its senses, causing it to see, hear, touch, taste, or feel reality according to the elder brain’s intent. From across great distances, it implants subconscious suggestions or subtly influences dreams to compel creatures toward a course of action that benefits its grand plan.

When its insidious suggestions fail to take hold, an elder brain asserts its dominance more directly. It seizes control of a resistant mind and controls the creature’s body as it would a puppet. Against the rare, strong-willed stalwart that defies it or attacks it, an elder brain sends a blast of overwhelming psychic force to crush the upstart’s mind, rendering the creature a thoughtless, drooling shell.

Devourer of Thoughts. An elder brain sustains itself by consuming the brains of other creatures. When the mind flayer servants that guard and tend to an elder brain don’t bring its meals directly to it, the elder brain reaches out with tendrils of thought, mentally compelling creatures to come to it so that it may feed upon them.

When a mind flayer perishes, the elder brain’s servants feed the contents of its skull to their master, which absorbs the illithid’s brain and all the knowledge and experience contained therein. In this way the elder brain continually increases its knowledge, uniting the thoughts and experiences of the illithid colony into a unified whole. Mind flayers conceive of this “oneness” as a sacred state in the same way that a worshiper of a human deity might view an eternal afterlife in the heavens — for an elder brain can evoke the persona of any illithid it has ever absorbed.

Hive Mind. Non-illithids call this creature an elder brain because it acts as the central communication hub for an entire mind flayer colony just as a brain does for a living body. Linked to the elder brain, the colony acts like a single organism, acting in concert as if each illithid were the digit of a hand.

Ego Unhindered. Each elder brain considers itself and its desires the most important things in the multiverse, the mind flayers in its colony nothing more than extensions of its will. But no two elder brains are alike, and each presides over its colony according to its own unique personality and storehouse of collected knowledge and experience. Some elder brains reign as domineering tyrants, while others serve more benignly as sages, counselors, and repositories of information and lore for the mind flayers that protect and nourish them.

The ambitions of an elder brain are always tempered by its relative immobility. Although its telepathic senses can reach for miles, moving anywhere is always a dangerous proposition. If forced outside its brine pool, an elder brain will swiftly expire, and transporting an elder brain in its pool through confining and tortuous subterranean tunnels frequently proves difficult or impossible.

An Elder Brain’s Lair

The lair of an elder brain always lies deep in the heart of a mind flayer colony. The creature dwells in a dimly glowing brine pool, filled with foul and brackish water infused with the elder brain’s vital fluids and with psionic energy.

 

Lair Actions

When fighting inside its lair, an elder brain can use lair actions. On initiative count 20 (losing initiative ties), an elder brain can take one lair action to cause one of the following effects; the elder brain can’t use the same lair action two rounds in a row:

- The elder brain casts wall of force.

- The elder brain targets one friendly creature it can sense within 120 feet of it. The target has a flash of inspiration and gains advantage on one attack roll, ability check, or saving throw it makes before the end of its next turn. If the target doesn’t or can’t use this benefit in that time, the inspiration is lost.

- The elder brain targets one creature it can sense within 120 feet of it and anchors it by sheer force of will. The target must succeed on a DC 18 Charisma saving throw or be unable to leave its current space. It can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

 

Regional Effects:

The territory within 5 miles of an elder brain is altered by the creature’s psionic presence, which creates one or more of the following effects:

- Creatures within 5 miles of an elder brain feel as if they are being followed, even when they are not.

- The elder brain can overhear any telepathic conversation happening within 5 miles of it. The creature that initiated the telepathic conversation makes a DC 18 Wisdom (Insight) check when telepathic contact is first established. If the check succeeds, the creature is aware that something is eavesdropping on the conversation. The nature of the eavesdropper isn’t revealed, and the elder brain can’t participate in the telepathic conversation unless it has formed a psychic link with the creature that initiated it.

- Any creature with which the elder brain has formed a psychic link hears faint, incomprehensible whispers in the deepest recesses of its mind. This psychic detritus consists of the elder brain’s stray thoughts commingled with those of other creatures to which it is linked.

If the elder brain dies, these effects immediately end.

(From 2nd Edition Monstrous Manual - 1993):

The elder brain is a huge, fibrous mass of cognizant brain tissue covered with writhing feelers. A single elder brain floats within the depths of a briny pool found at the center of its illithid community. An elder brain’s prodigious psychic abilities cause it to pulsate and glow like a windblown ember, charging its gray matter with vitality and purpose. This vitality allows it to remain active long after the bodily deaths of the individual mind flayers whose brains were harvested to engender it.

Elder brains sense the world via an innate telepathy with a radius of up to 5 miles (in the oldest specimens). Within this radius, an elder brain detects all non-psionically shielded sentient beings — even through solid rock. Within this same range, an elder brain can communicate with any creature through the use of its innate telepathy. It can also scry through the eyes of any willing (or psionically dominated) individual within its telepathic radius, although its worldview is biased towards the mental plane.

Combat: If given sufficient warning, an elder brain can bud and grow a brain golem; this golem functions as a physical extension of the elder brain itself. An elder brain can but up to three brain golems, requiring one full hour of undivided attention to fully form each avatar. During the budding process (which costs 1d10 PSPs per HD of the brain golem formed), the elder brain cannot exert any external psychic influence or ability beyond communication.

An elder brain only resorts to budding when its psionic abilities prove inadequate — an infrequent occurence at best — as its psionic arsenals contain the combined psionic knowledge of possibly hundreds of contributing illithid minds. The elder brain can use its psionic abilities at twice normal range.

An aggressor able to withstand the psionic fury of an elder brain must still overcome a physical obstacle in order to engage in melee with the mature. Since an elder brain is approximately 10 feet in diameter and floats 10 feet below the surface of its pool (a 3Q-foot-wide by 30-foot-deep basin), attackers must first enter the creature’s watery domain (incurring underwater combat modifiers) before engaging in melee. Normal missile attacks (fired from the surface into the water) will not reach the submerged elder brain — though most spell attacks function normally, provided the brain is within the caster’s line of sight and the spell in question does not change effects when cast into water.

If death is imminent, an elder brain relinquishes its hold on the Prime Material Plane and withdraws completely into the Astral Plane, where the bulk of its mass resides. Once it transports itself in this way, an elder brain loses its anchor to the prime and becomes trapped on the Astral Plane — a rogue creature without ties to its community. It is uncertain what becomes of a rogue elder brain; however, illithid communities that lose their elder brain swiftly fall apart.

Habitat/Society: The elder brain is the center of its illithid community, serving as an advisor and, most importantly, the living repository of the community�s technology, history, and psionic expertise. It is the right and obligation of every illithid to merge with the elder brain after death — living in exalted mentality while guiding and shepherding its erstwhile community. While most illithids believe that their personality will survive the transition, individual egos are suborned to the gestalt consciousness suffusing the tissue mass.

Ecology: An elder brain preys upon the thousands of tadpoles that share its pool; it extracts the pre-sentient psionic complexus from each tadpole to fuel its own existence. Despite the gradual addition of tadpole life force and the mass of new illithid brains, the size of an elder brain never swells beyond a 10-foot-diameter. It shunts any excess mass directly into a psionically maintained node on the Astral Plane.

Psionics Summary:

Level 20

Dis/Sci/Dev = 6/21/31

Attack/Defense = EW, II, PB/All

Score = Int

PSPS = 1d200+450

Clairsentience — Sciences: clairaudience, clairvoyance; Devotions: all-round vision, know location.

Psychokinesis — Sciences: create object, molecular rearrangement, telekinesis; Devotions: ballistic attack, control body, control lights, create sound, inertial barrier, levitation.

Psychometabolism — Sciences: complete healing, energy containment, metamorphosis; Devotions: , body equilibrium, suspend animation.

Psychoportation — Sciences: banishment, probability travel, teleport, teleport other; Devotions: astral projection, time shift, time/space anchor.

Telepathy — Sciences: domination, ejection, mass domination, mind wipe, probe, psionic blast; Devotions: awe, ESP, false sensory input, id insinuation, mind thrust, post-hypnotic suggestion, psionic crush, taste link.

Metapsionics — Sciences: empower, psychic surgery, ultrablast; Devotions: cannibalize, magnify, prolong, psionic drain, psionic inflation.

This psionic summary indicates the minimum psionic ability of an elder brain.