Ghatanothoa is a Great Old One and the first born of Cthulhu. It is a huge, amorphous monstrostity which is so hideous, that anyone who gazes upon it (or even a perfect replica) is petrified into a living mummy. The victim is permanently immobilized—the body taking on the consistency of leather and the internal organs and brain preserved indefinitely—yet remains fully aware. Only the destruction of the subject's brain can free it from its hellish prison, though the unfortunate is likely to be incurably insane long before the welcomed release.
Ghatanothoa is currently trapped underneath Mount Yaddith-Gho in sunken Mu. He was brought to Earth from the planet Yuggoth (Pluto in Lovecraft's fiction) by an ancient, alien race, possibly the Mi-go, who built a colossal fortress atop Yaddith-Gho and sealed Ghatanothoa inside the mountain beneath a large trapdoor. Ghatanothoa was worshipped by the ancient Muvians, who both feared and respected him because of his ability to turn any humans that beheld him into living, thinking mummies.
Many attempted in vain to defeat Ghatanothoa; most notably T'yog, the High Priest of Shub-Niggurath, whose story is recounted in Friedrich von Junzt's grimoire Unaussprechlichen Kulten or Nameless Cults (Robert E. Howard's answer to Lovecraft's Necronomicon). T'yog created a scroll that was supposed to protect him from the petrifying effect of gazing upon Ghatanothoa. But T'yog was defeated after Ghatanothoa's priests stole the scroll and replaced it with a fake one (however, this was not done out of praise for Ghatanothoa, but rather to protect the populace from its wrath had T'Yog failed). This occurred in the Year of the Red Moon, which is 173,148 B.C. according to von Juntz.
In Lin Carter's Xothic legend cycle, Ghatanothoa is said to be the first born of Cthulhu; his siblings, in order of birth, are Ythogtha, Zoth-Ommog, and Cthylla. Colin Wilson connected Ghatanothoa to the alien, reptilian race of energy beings—the Lloigor—as the deity's servants.