An elder brain is a collective of minds of past illithid that resides in an illithid hive. It psychically connects all illithid to each other and provides knowledge of past events, anything that an individual illithid sees, and is considered sacred by the species.
K'Varn gained control of an entire city of illithid by erecting a crystal-like structure above the elder brain which subverted its will. Later, Vox Machina nearly destroyed the elder brain in an attempt to escape from the mind flayers within the city.
External Links Edit
Elder brain on the Forgotten Realms Wikia (Forgotten Realms lore is not canon to Critical Role)