Keepers of the Cerulean Sign
Among the strongest taboos in illithid society is the idea of not implanting a mature tadpole into a donor brain. While some tadpoles are implanted experimentally, with death the almost certain outcome, none are intentionally kept back from implantation.
On rare occasions, an illithid community collapses, typically from an external assault, and the elder brain is killed. When that happens, the tadpoles are suddenly freed from predation by the always-hungry elder brain. They, in turn, are no longer fed by their caretakers, and the tadpoles grow increasingly hungry. Driven by hunger, they turn to cannibalism. Only one tadpole survives from the thousands in the pool.
The psychic energy that this one survivor absorbs from its fellow tadpoles (and possibly from the decaying corpse of the elder brain, as well) allows it to grow much larger than any normal tadpole would—to the size of a small dog. Eventually, hunger again drives it to a desperate act: crawling out of the briny pool into the greater subterranean world in search of food. Once free of the pool, the creature lives by preying on rats and other vermin, a diet that allows it to grow even larger.
Eventually, such an abomination catches and devours a sentient creature, whether a duergar, a drow, or a luckless human far from home. Consuming that first thinking brain triggers the same reaction in the tadpole that would have occurred if the tadpole had been implanted normally. It becomes self-aware, and its latent psionic potential awakens.
At that point, the creature becomes a neothelid. Untrained, savage, feral, and brilliant, it knows nothing beyond the squalid, predatory existence it has lived so far. Neothelids prowl subterranean passages in search of more brains to sate their constant hunger, growing ever larger and more vicious. Truly leviathan specimens have been spotted prowling the deepest underground.
In addition to having the usual psionic abilities of an illithid, these creatures can spray tissue-dissolving enzymes from their tentacle ducts, reducing victims to a puddle of slime and leaving only the pulsing brain unharmed. They have no knowledge of their links to illithids, so they’re just as likely to prey on mind flayers as on anything else.