Cthylla is a Great Old One and the daughter of Cthulhu. She is the the youngest spawn of Cthulhu and his androgynous mate. She is said to have been born upon and arrived from the star Xoth, but now dwells on Earth in an unknown location where she is guarded by Cthulhu's minions. Cthylla is destined to birth Cthulhu again should he ever be destroyed in the distant future. She is essential for Cthulhu's plans, and is thus vigilantly guarded by countless monstrosities that are affiliated with the Slumbering God.
While her true form has not been physically seen by mortal eyes, she is described as having the appearance of a gigantic, red-bodied, black-ringed, and six-eyed octopus with small wings. Like her father, she is able to shape-shift her body into whatever form she desires as well as alter her body proportions at will, such as by enlarging her wings to enable her to fly.
Despite being the youngest of Cthulhu's children, she is said to be one of, if not the most cunning and dangerous of all his spawn. Unlike the vast majority of her race, Cthylla is not imprisoned, instead residing in a remote location or possibly realm where she is guarded, and manipulating certain events behind the scenes in order to hasten the awakening of her father once more while also preparing for the rise of the other Old Ones. Having spent many centuries on Earth, Cthylla has acquired a human form and is able to walk among and interact with the mortals, completely undetected and with many unaware of her true nature.
Cthylla is also the leading child of Cthulhu's children, which are comprised of four with her included. However, she is the only child who is female while her three other elder siblings are male. While her brothers are very powerful in their own right, Cthylla's intelligence more than makes up for how much of a threat she can be. This is evident to where her brothers also have no issue with following her commands, even if they must begrudgingly do it.