Neothelid

Neothelids were extremely rare creatures in mind flayer society.

To create a neothelid, not only must an illithid tadpole not undergo ceremorphosis, but the elder brain of the colony must have died, as the elder brain would normally eat all tadpoles that failed to undergo ceremorphosis. With the elder brain dead, chaos would erupt in the spawning pools, as the society around them collapsed as well. If one tadpole survived the resulting chaos, it could grow into the form of a gigantic slug-like creature.

One of these creatures took up residence in the old illithid colony of Phanlinksal.