Who was Lovecraft's friend and literary executor who expanded the Cthulhu Mythos?

Lovecraft’s friend and literary executor who expanded the Cthulhu Mythos was August Derleth. After Lovecraft’s death in 1937, Derleth helped preserve, publish, and popularize his work while developing the broader Mythos tradition.

Derleth had corresponded with Lovecraft and was one of the writers associated with the circle later known as the Lovecraft Circle. With Donald Wandrei, he founded Arkham House in 1939 to publish Lovecraft’s fiction in book form. This was crucial at a time when much of Lovecraft’s writing was scattered across pulp magazines.

Derleth also wrote new stories using Lovecraft’s fictional entities and coined or popularized the term “Cthulhu Mythos.” His interpretation often organized the supernatural beings into elemental categories and framed their conflicts in terms of opposing good and evil forces. Those ideas influenced later readers and writers, although scholars debate how closely they reflect Lovecraft’s own deliberately ambiguous cosmic horror.

Brian Lumley and other authors expanded the Mythos later, but Derleth was the key early editor, publisher, and posthumous collaborator who shaped its public afterlife.