Cthulhu Mythos
[According to August Derleth]

In August Derleth stories, the Cthulhu Mythos is a cycle of legends about evil alien gods called the Great Old Ones (3) and their rebellion against the benevolent Elder Gods (1).

Derleth used varying and sometimes contradictory terms for these groups of alien gods. For further information on the names he used in various stories, see Elder Gods and Great Old Ones: God Terminology in Derleth's Mythos Stories.

Some of Derleth's key concepts are not to be found in Lovecraft; notably, Derleth created

The term "Cthulhu Mythos".
The benevolent Elder Gods (1).
A single group of evil gods called the Great Old Ones (3), which combined some of Lovecraft's god-groups: the Great Old Ones (1), Old Ones (2), and Other Gods.
The failed rebellion of the Great Old Ones (3) against the Elder Gods (1), resulting in the Great Old Ones (3) imprisonment. Derleth frequently drew parallels between this rebellion and the fall of Satan.
Correspondences between the Great Old Ones and the various elements of Earth, Air, Fire, and Water.
The star stones of Mnar, which help to imprison the Great Old Ones and provide protection against their minions.

For an overview of Lovecraft's mythos (or "Yog-Sothothery") as he originally developed it, see The Myth Patterns of H. P. Lovecraft. It is not completely clear to me how Derleth came to project the concept of the Elder Gods (1) onto Lovecraft's work. Certainly the two authors had differences of world view and temperament, but the very vagueness and evasiveness of Lovecraft's concepts must have made it easier for Derleth to "fill in the gaps" with his own ideas. Derleth was probably also encouraged by Lovecraft's enthusiastic appreciation for Derleth's early Mythos tales. Apparently Lovecraft was either too kindly to complain about Derleth's theological changes, or else he really didn't care. Lovecraft even referred to Derleth's Tcho-Tcho people in his own stories [HPL Museum (online text); Time (online text)], though he never referred to Derleth's benevolent Elder Gods.

The following pages describe the various aspects of Derleth's Cthulhu Mythos:

The Importance of Cthulhu

The Great Old Ones Rebel

The Great Battle

Legendary Catastrophes

The Great Old Ones' Broad Domain

Invaders of Earth

Banishing of the Great Old Ones

The Second Punishment of Cthulhu

Star Stones

The Elder Gods' Greater Power

Limits of the Elder Gods' Power