The Cthulhu Mythos is a fictional universe created by famed American horror author H.P. Lovecraft in the 1920's. Although it was originally created by Lovecraft, the Cthulhu Mythos has been sampled from and expanded upon by numerous other authors. Generally, stories set in the Cthulhu Mythos feature the ideas and themes originally created by Lovecraft, and many authors who write in the mythos attempt to expand upon or clarify Lovecraft's original ideas. The mythos received its name from Cthulhu, an aquatic demigod that is referenced in many stories, and is considered to be the most popular of Lovecraft's creations.
Stories set in the Cthulhu Mythos generally involve mankind's theological ignorance. The universe is ruled by deities called the Great Old Ones or simply Old Ones, and their counterparts, the Elder Gods. Although in most stories, these Outer Gods once ruled the Earth and are now in hibernation, it is promised that they will someday awaken and dominate the world once again.
A common theme of stories set in the Cthulhu Mythos is a person being exposed to the fact that mankind is just a pawn in the greater scheme of things, with no divine spark or special spiritual meaning. This usually occurs by either willing or unwilling contact with the Great Old Ones or their human followers. Often, after learning the truth, these individuals go insane.
During Lovecraft's life, his mythos was sampled by other famous writers of the period, such as Robert E. Howard and Henry Kuttner. Often, Lovecraft and his correspondences would borrow materials from each others work, such as locations or demi-gods. After Lovecraft's death, other writers, such as August Derleth, worked to continue the Cthulhu Mythos.
August Derleth was the next primary author in the Cthulhu Mythos. Since Lovecraft was an atheist, many of his stories depicted the Old Ones and Elder Gods as generally neutral regarding the fate of mankind. In Derleth's continuation of the Cthulhu mythos, he attempted to portray the Elder Gods as benevolent beings who had imprisoned the Old Ones for cosmic transgressions.
Other less well known writers who sampled the Cthulhu Mythos again expanded Lovecraft's mythology. The stories that they produced introduced a larger pantheon of deities and demi-gods, as well as additional sources that fictional characters used to build their knowledge of the hidden occult world of the Old Ones. By building upon Lovecraft and Derleth's stories, these writers helped the Cthulhu Mythos obtain a small degree of notoriety.
The Cthulhu Mythos has expanded into several other forms of media. References to the Old Ones or locations and characters of Lovecraft's stories are frequently referenced by rock bands. Several table top role playing games have been created over the years by gaming companies looking to expand, and profit from, the Cthulhu Mythos. Also, a large number of independent films have been created that are based on some of the more popular stories.