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.
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. Cthylla is destined to birth Cthulhu again should he ever be destroyed in the distant future.
Before her subsequent imprisonment, Cthylla was said to have mated with the primordial beast of the seas, Leviathan. Their union produced the Kraken, monstrous marine creatures resembling octopi but titanic in size and far more dreadful to behold. According to ancient Jewish rabbis, Leviathan originally had a female counterpart with which to mate, but when the female was destroyed, Leviathan took to Cthylla instead.
Cthylla was among the invasive foes that attempted to seize the kingdom of Hyperborea for herself after the supposed destruction of the Ogdru Hem at the hands of Earth's deities, forcing them to hide, only to be thwarted by Rahab, the Angel of the Deep, each time and often at times would even be repelled by the Hyperboreans themselves with their advancements in science and mysticism.
As Hyperborea was ravaged by "undersea quakes", Cthylla would take the opportunity to have claim the drifting splintered mass of the continent for herself all while her Kraken children feasted upon the wayward corpses of the Hyperboreans. Cthylla only grew enraged when she discovered that the Kraken accompanying her were slain and turned into binds for Rahab as punishment for his actions against Hyperborea. This opted Cthylla to exact vengeance against the Heavens.
Cthylla opted to raid the Heavens alongside her children by breaking through the gates of the South Wind, a gate which led to Heaven itself, undeterred seeing as Rahab was imprisoned for his crimes. When God knew of this, He made the Kraken fight amongst each other in tumultuous battles that rocked the oceans before destroying them with arrows made of "fire that could not be quelled by water". Cthylla herself was defeated by Uriel, imprisoning her within a gigantic holding cell, later a palace, known as Cthygonnaar.
Untold centuries pass as her prison of Cthygonnaar would be find land mass forming atop of it and in the modern days a town by the name of Oakmont, Massachusetts would be built atop of it. However, while sleeping, she is still able to communicate with the upper world, and uses her mental powers to call those 'elected' to Cthygonnaar so that they may free her. However, according to a cyclical event, Cthygonnaar periodically erupts upon Oakmont, causing a Flood that spread madness, violence, and monstrosities that prevent people from leaving the city until one of the elected (called "The Seed") either re-seals her for several more centuries or releases her, ending the cycle. Every Seed has kept Cthylla imprisoned, to prevent the destruction of the city and humanity itself.
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.
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.