Elder brain

An elder brain is a collective of minds of past illithid (mind flayers) that resides in an illithid hive. It psychically connects all illithids of the hive to each other, sees anything that an individual illithid sees, and provides them with knowledge of past events. It is considered sacred by the species.

Description
The elder brain that Vox Machina encountered was a large collection of hundreds of illithid brains that had congealed into one brain structure in a 10- or 15-foot-wide pool of dull, bluish-purple liquid on a raised stone platform. The brains initially seemed like purple stones, and the collective entity had a pale, pulsing, purplish glow that started to grow brighter after it was freed from K'Varn's control. When attacked, it used psychic energy to resist impacts and poison, and it shrieked in a way audible in the minds of those nearby; when heavily damaged, a purplish-black liquid spurted from it.

An illithid with arcane abilities is typically disconnected from the elder brain and forever shunned from the colony. However, one such arcanist, Clarota, was rewarded for liberating his elder brain from K'Varn's domination by being allowed back into the colony.

Abilities
All of the following is taken from the Monster Manual, 5th Edition, though several of these abilities have been confirmed by Vox Machina's encounter with an elder brain.
 * Telepathic Hub: Can maintain telepathic conversations with up to 10 creatures at a time, and allow them to telepathically converse with each other.
 * Creature Sense: Aware of location and Intelligence score of all creatures with Intelligence of at least 4 who are within 5 miles.
 * Magic Resistance: Advantage on saving throws against spells and magical effects.
 * Legendary Resistance (3/day): Succeeds on a saving throw it would have failed.
 * Mind Blast: Creatures within 60 feet must succeed at an Intelligence saving throw or take psychic damage and be stunned for 1 minute. Stunned creatures can repeat the saving throw at the end of their turns.
 * Psychic Link: As an action, targets an incapacitated creature it can sense through its Creature Sense, and then use that creature's senses for perception. The target can sense this link after it's no longer incapacitated, and attempt to sever the link with a Charisma saving throw, though the target takes psychic damage on a success.
 * Sense Thoughts: Learns the emotional state and abiding thoughts of a creature with whom it has established a Psychic Link, and can use a Deception check contested against the target's Insight check to implant beliefs and emotions in that creature for 1 hour or until evidence is presented that reveals the lie.
 * Legendary Actions
 * Tentacle: psychic damage and grapples on a hit.
 * Psychic Pulse: Enemies within 10 feet of a nearby target creature with a psychic link take psychic damage.
 * Sever Psychic Link: Target creature with a psychic link within 120 feet loses the psychic link and has disadvantage on ability checks, attack rolls, and saving throws until the end of its next turn.
 * Break Concentration: Target creature with a psychic link within 120 feet loses concentration on a spell and takes psychic damage for each spell level of the spell it was concentrating on.
 * Innate Spellcasting (Psionics): Can cast the following without components.
 * At will: Detect Thoughts, Levitate
 * 1/day each: Dominate Monster, Plane Shift (self only)
 * Lair Actions
 * Regional Effects

History
K'Varn gained control of an entire city of illithids, Yug'Voril, by erecting a crystal-like structure above the elder brain which subverted its will and kept it in a trance-like state. Later, Vox Machina nearly destroyed the elder brain, but as the whole hive converged on the site, the party realized they weren't damaging it quickly enough and decided to escape with their lives. Allura Vysoren assured them that the illithids were unlikely to continue driving toward the surface after being freed from K'Varn's compulsion.