The term was coined by Lovecraft's associate August Derleth, and named after Cthulhu, a powerful fictional entity in Lovecraft's stories. Not a series per se, stories, novels and other works in the Cthulhu Mythos feature elements, characters, settings, and themes found in works by Lovecraft writers. Together, these works form the mythos that authors writing in the Lovecraftian milieu have used -- and continue to use -- in their ongoing expansion of the fictional universe, sometimes in ways far removed from Lovecraft's original conception.
During the latter part of Lovecraft's life, there was much borrowing of story elements among the authors of the "Lovecraft Circle", and many many others, a clique of writers with whom Lovecraft corresponded. This group included Clark Ashton Smith, Robert E. Howard, Robert Bloch, Frank Belknap Long, Henry Kuttner, and others.
Lovecraft recognized that each writer had his own story-cycle, and that an element from one cycle would not necessarily become part of another simply because a writer used it in one of his stories. For example, although Smith might mention "Kthulhut" (referring to Lovecraft's Cthulhu) or Iog-Sotôt (Yog-Sothoth) in one of his Hyperborean tales, this does not mean that Cthulhu is part of the Hyperborean cycle. A notable exception, however, is Smith's Tsathoggua, which Lovecraft appropriated for his revision of Zelia Bishop's "The Mound" (1940). Lovecraft effectively connected Smith's creation to his story-cycle by placing Tsathoggua alongside such entities as Cthulhu, Yig, Shub-Niggurath, and Nug and Yeb in subterranean K'n-yan.
Most of the elements of Lovecraft's Mythos were not a cross-pollination of the various story-cycles of the Lovecraft Circle, but were instead deliberately created by each writer to become part of the Mythos, the most notable example being the various arcane grimoires of forbidden lore. So, for example, Robert E. Howard has his character Friedrich Von Junzt reading Lovecraft's Necronomicon in "The Children of the Night" (1931), and Lovecraft in turn mentions Howard's Unaussprechlichen Kulten in both "Out of the Aeons" (1935) and "The Shadow Out of Time (1936). Howard frequently corresponded with H. P. Lovecraft, and the two would sometimes insert references or elements of each others' settings in their works. Later editors reworked many of the original Conan stories by Howard; thus, diluting this connection. Nevertheless, many of Howard's unedited Conan stories are arguably part of the Cthulhu Mythos.
Furthermore, Lovecraft may not have been serious when he spoke of developing a "myth-cycle" and probably would have had no need to give it a name anyway. Since he used his Mythos simply as background material, he probably had this in mind when he allowed other writers to use it in their own stories. It could be said that Lovecraft's Mythos was a kind of elaborate inside joke propagating among the writers of his circle. However, August Derleth's understanding of the Mythos appears to have been that Lovecraft wanted other authors to actively write about the myth-cycle rather than to simply allude to it in their stories.
Derleth combined Lovecraft's various cycles to create a large, singular story-cycle. For example, he appropriated Nodens from the Dunsanian cycle and leagued him with the Elder Gods against the Old Ones. He also introduced a good versus evil dichotomy into the Mythos contrary to the dark, nihilistic vision of Lovecraft and his immediate circle.
Derleth apparently treated any story mentioning a mythos element as part of the Mythos, and in consequence all other elements in the story also became part of the mythos. Hence, as Lovecraft made passing reference to Clark Ashton Smith's Book of Eibon, Derleth added Smith's Ubbo-Sathla to the mythos. Because of Derleth's broad canon the Mythos grew enormously.
Further removing the Cthulhu Mythos from its source were stories written by such authors as Lin Carter, Colin Wilson, and Brian Lumley. Carter was especially influential in setting out detailed lists of gods, their ancestry, and their servitors through his Mythos tales, attempting to codify the elements of the Mythos as much as possible. Through this process, more gods, books, and places were created and interlinked with each other.
Another influence has been the Call of Cthulhu RPG published by Chaosium in 1981. Largely developed by Sandy Petersen, this version of the Mythos broke Lovecraft's entities down into further sub-groupings: Outer Gods, Great Old Ones, servitor races and the nebulously-termed Other Gods. Material from these sources has slowly crept back into mainstream Mythos fiction, as Chaosium published fiction related to, or written by players of the game.
Many of the newer generation of Mythos authors (especially those published in Chaosium compendiums) take their cue from this more clinical, continuity-focused brand of the Mythos instead of Lovecraft's more mysterious version.
Despite his notoriety, Cthulhu is not the most powerful of the deities, nor is he the theological center of the mythos. Instead, this position is held by the demon-god Azathoth, an Outer God, ruling from his cosmically centered court. Nonetheless, Nyarlathotep, who fulfills Azathoth's random urges, has intervened more frequently and more directly in human affairs than any other Outer god. He has also displayed more blatant contempt for humanity, especially his own worshippers, than almost any other Lovecraftian deity.
Now and then, individuals can, by accident or carelessness, catch a glimpse of, or even confront the ancient extraterrestrial entities that the mythology centers around, usually with fatal consequences. Other times, they are represented by their non-human worshippers, whose existence shatters the worldview of those who stumble across them. Human followers exist as well. Because of the limitations of the human mind, these deities appear as so overwhelming that they can often drive a person insane. They are portrayed as neither good nor evil; within the Mythos these are concepts invented by our species as a way to explain intentions and actions which may otherwise seem inexplicable.
The Call of Cthulhu was the premiere story in which Lovecraft realized and made full use of these themes, which is why his mythology would later be named after the creature in this story, as it defined a new direction in both his authorship and in the horror fiction genre. This is also the only story by Lovecraft where humans and one of the cosmic entities called the Great Old Ones comes face to face.
In his final years, Lovecraft used fewer supernatural elements to represent the dangers which threaten humanity. Instead, he gradually replaced them with non-supernatural cosmic beings and phenomena, based on principles outside the laws of nature in our own space-time continuum. This sci-fi trend particularly becomes clear in works such as At the Mountains of Madness. Many of these later tales also humanize these aliens to some extent, and the degree to which they still retain the theme of nihilistic horror varies.
As Lovecraft conceived the deities or forces of his mythos, there were, initially, the Elder Gods... [T]hese Elder Gods were benign deities, representing the forces of good, and existed peacefully at or near Betelgeuze in the constellation Orion, very rarely stirring forth to intervene in the unceasing struggle between the powers of evil and the races of Earth. These powers of evil were variously known as the Great Old Ones or the Ancient Ones...
—August Derleth, "The Cthulhu Mythos
Lovecraft was an atheist, and claimed that Kant's ethical system "is a joke." Derleth's theories about the Cthulhu Mythos thus differ from Lovecraft's concept, which was not really a cohesive, singular entity, but rather a collection of ideas that could be used in separate works to provoke the same emotions.
The Elder Gods of Derleth's mythos never appear in Lovecraft's writings, except for one or two termed "Other Gods" such as Nodens in Lovecraft's "The Strange High House in the Mist" (though perhaps this is an example of how "very rarely [they stir] forth"; i.e., usually never). Furthermore, in Lovecraft the Great Old Ones, or Ancient Ones, have no unified pantheon. Indeed, the term "Ancient Ones" appears in only one Lovecraft story, "Through the Gates of the Silver Key," actually a collaboration between Lovecraft and his friend and correspondent E. Hoffmann Price.
Zhar and Lloigor*