Which author wrote stories in the universe of the Cthulhu Mythos?

The Cthulhu Mythos is a mythopoeia and a shared fictional universe, originating in the works of American horror writer H. P. Lovecraft. The term was coined by August Derleth, a contemporary correspondent and protégé of Lovecraft, to identify the settings, tropes, and lore that were employed by Lovecraft and his literary successors. The name "Cthulhu" derives from the central creature in Lovecraft's seminal short story, "The Call of Cthulhu", first published in the pulp magazine Weird Tales in 1928.

August William Derleth (February 24, 1909 – July 4, 1971) was an American writer and anthologist. Though best remembered as the first book publisher of the writings of H. P. Lovecraft, and for his own contributions to the Cthulhu Mythos and the cosmic horror genre, as well as his founding of the publisher Arkham House (which did much to bring supernatural fiction into print in hardcover in the US that had only been readily available in the UK), Derleth was a leading American regional writer of his day.

Cthulhu is a fictional cosmic entity created by writer H. P. Lovecraft. Considered a "Great Old One" within the pantheon of Lovecraftian cosmic entities, this creature has since been featured in numerous popular culture references. Lovecraft depicts it as a gigantic entity worshipped by cultists, in the shape of a green octopus, dragon, and a caricature of human form.