An enormous, octopus-like being with six eyes, vestigial wings and razor sharp claws, Cthylla is coloured red with black rings. She has the ability to alter her form to a certain degree, retracting some or all of her tentacles or extruding additional ones, and enlarging her wings to enable flight.
The youngest of Cthulhu and Idh-yaa's progeny, Cthylla was spawned on the planet Xoth along with her siblings Ghatanothoa, Ythogtha, and Zoth-Ommog, but later travelled to Earth with her family where she took up residence, heavily guarded, in Yhe.
In the Mythos, Great Cthulhu is aware that he will be destroyed at some point in the distant future, and as part of his plans has destined Cthylla to be the creature to rebirth him. She is therefore essential to his grand scheme, and so Cthulhu has assigned countless Deep Ones and Yuggya to protect her. The Wilmarth Foundation launched an operation known as Project X sometime during the latter half of the 20th century in an attempt to destroy Cthylla, along with Father Dagon and Mother Hydra, through the use of an atomic weapon, but the beast was only injured and escaped. Great Cthulhu then exacted a terrible revenge on those who would meddle in his plans.
Another account tells of a group of researchers who captured Cthylla, mistakenly believing her to be a species of previously-undiscovered octopus. These scientists, in an effort to preserve her race, attempted to impregnate Cthylla using her own DNA.
Cthylla has also been featured as a more human-like creature (although this was possibly merely one of her avatars) who was presented as a bridal candidate to Hastur, Cthulhu's half-brother.
- The Transition of Titus Crow (1975), by Brian Lumley
- In His Daughter's Darkling Womb (1997), by Tina L. Jens
- In the Hall of the Yellow King (2011), by Peter Rawlik