Inhabitants of Yekub
It was a gigantic, pale-grey worm or centipede, as large around as a man and twice as long, with a disc-like, apparently eyeless, cilia-fringed head bearing a purple central orifice. It glided on its rear pairs of legs, with its fore part raised vertically—the legs, or at least two pairs of them, serving as arms. Along its spinal ridge was a curious purple comb, and a fan-shaped tail of some grey membrane ended its grotesque bulk. There was a ring of flexible red spikes around its neck, and from the twistings of these came clicking, twanging sounds in measured, deliberate rhythms.
—C. L. Moore, A. Merritt, H. P. Lovecraft, Robert E. Howard and Frank Belknap Long, The Challenge from Beyond
The Inhabitants of Yekub first appeared in "The Challenge from Beyond" (1935), a "round robin" commissioned for Fantasy Magazine, written by C. L. Moore, A. Merritt, H. P. Lovecraft, Robert E. Howard and Frank Belknap Long.
The Yekubians destroyed all intelligent life in the galaxy where they dwelt and sought to extend their influence throughout the universe. As part of their grand scheme, they sent out cube-shaped probes that could effect a mind-swap with any intelligent creature who found one. In this way, Yekubian agents could infiltrate the finder's world. One such cube landed on the Earth during the reign of the Great Race of Yith. When several of its members were taken over, the Yithians realized the danger of the cube and sequestered it under heavy guard. Eventually, however, the cube was lost.