Neothelid

Neothelids were extremely rare creatures in the realms. Not only must the illithid tadpole not undergo ceremorphosis, the elder brain of the colony must have died. The elder brain would have eaten all tadpoles that failed to undergo ceremorphosis. With the elder brain dead, chaos would have erupted in the spawning pools, as the society around them collapsed as well. Only one tadpole would have survived the resulting chaos, to grow to form into a leviathan slug.

One of these creatures had taken up residence in the old Illithid colony of Phanlinksal.