Cthylla appears in expanded Mythos fiction, most notably in the work of Brian Lumley. She is described as the secret daughter of Cthulhu and serves as the ultimate guarantee of his immortality: should Cthulhu be destroyed, Cthylla can give birth to him anew.
For this reason she is guarded with extreme secrecy and bears the epithet 'The Secret One.' Her form resembles a vast, winged, octopoid creature.