From the campaign:
RAIDERS of R’LYEH is a stand-alone roleplaying game and sourcebook in which mercenary rogues EXPLORE FORBIDDEN FRONTIERS, UNEARTH ANCIENT ARTIFACTS, and OUTWIT VILLAINOUS SCUM. Derived partly from the Runequest rules that have been released under the Open Game License, the game is crafted to emulate not only adventure in the Cthulhu Mythos of H.P. Lovecraft and his circle of influences, but especially the savage and evocative feel of Robert E. Howard’s Mythos and Weird menace stories.
Attention has been made to keep the game mechanics compatible with other percentile systems — while innovating additional rulesets unique to the setting and atmosphere — so those players wishing only to use RAIDERS of R’LYEH as an Edwardian era adventure sourcebook will have a massive global setting to explore and use in their preexisting Mythos campaign, as well as modular adventure, monster and scenario resources for gaming in 1900-1913. Those players wanting a standalone rulebook with its adventure modes of play, will have a complete toolkit for interacting with the Cthulhu Mythos.