Neothelid

Neothelid
A slime-covered worm of immense size, a neothelid is the result of the mind flayer reproductive cycle gone horribly wrong. On rare occasions, an illithid colony collapses, typically after an external assault, and the elder brain is killed. When that happens, the colony's tadpoles are suddenly freed from their fate. They no longer serve as food, and in turn are no longer fed by their caretakers. Driven by hunger, they turn to devouring one another. Only one tadpole survives out of the thousands in the colony's pool, and it emerges as a neothelid.
Abhorrent to Illithids
Among the strongest taboos in illithid society is the idea of allowing a mature tadpole to survive without implanting it into a donor brain. Under normal circumstances, any tadpole that grows larger than a few inches in length is killed by the elder brain to be food for it or for less mature tadpoles. Any tadpole that survives beyond that state is perceived as a threat to the colony, and the mind flayers organize hunting parties to exterminate the abomination. Lacking enough intelligence to be detected by an elder brain's power to sense thoughts, neothelids warrant such precautions.
Savage Behemoth
As a feral thing, a neothelid knows nothing beyond the predatory existence it has lived so far and struggles to comprehend its new psionic abilities. Neothelids prowl subterranean passages in search of more brains to sate their constant hunger, growing ever more vicious. 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 link to illithids, so they're just as likely to prey on mind flayers as on anything else.