Origin: Clark Ashton Smith, "The Treader of the Dust" (1935)
It was a figure no larger than a young child, but sere and shriveled as some millennial mummy. Its hairless head, its unfeatured face, borne on a neck of skeleton thinness, were lined with a thousand reticulated wrinkles. The body was like that of some monstrous, withered abortion that had never drawn breath. The pipy arms, ending in bony claws were outthrust as if ankylosed in the posture of an eternal dreadful groping. The legs, with feet like those of a pigmy Death, were drawn tightly together as though confined by the swathings of the tomb; nor was there any movement or striding or pacing. Upright and rigid, the horror floated swiftly down the wan, deathly gray beam toward Sebastian.
— Clark Ashton Smith, "The Treader of the Dust"
The Great Old One Quachil Uttaus is rarely encountered, and apparently referenced only in one tome of the Mythos, the Testament of Carnamagos. He takes the form of a diminutive mummy with outstretched claws. He is the concentrated essence of corruption and decay; everything in his vicinity suffers the ravages of time, and when he is about to appear, objects begin to age inexplicably. Those who are visited by Quachil Uttaus are reduced instantly to dust, as though by the passage of long ages, and the Great Old One commonly leaves his paired footprints in the dust of his victims. This is the source of his usual title, "The Treader of the Dust."
An incantation in the Testament of Carnamagos can be used to summon Quachil Uttaus. It is dangerous even to read this incantation, though, because if the reader has even the slightest longing for death, the Great Old One may suddenly appear and annihilate him.
Clark Ashton Smith, "The Treader of the Dust" (1935)