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 illithid 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 it 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, called an alhoon, is typically disconnected from the elder brain and forever shunned from the colony. However, one alhoon, Clarota, was rewarded for liberating his elder brain from K'Varn's domination by being allowed back into the colony.

History
K'Varn gained control of an entire city of illithid, 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. Lady Kima of Vord assured them that the illithid were unlikely to continue driving toward the surface after being freed from K'Varn's compulsion.