From publisher blog:
[A] set of tailored RPG rules and a collection of detailed scenarios — both of which are designed to explore a range of different “Lovecraftian” Post-Apocalypse settings.
H.P. Lovecraft’s fiction (and that of his many collaborators and successors) is full of hints that mankind’s dominance over this planet is ephemeral. Something is coming that will sweep away all our accomplishments and nullify our position at the top of the food-chain on Earth. Lots of different stories have vague suggestions or allusions to the possible causes of such an Apocalyptic change of circumstances, although there are precious few tales that describe the state of the world as it exists after the cataclysmic change has come to pass.
APOCTHULHU aims to rectify this by allowing for game scenarios set in devastated versions of our world broken by the “Mythos Apocalypse”. One of the first things that we faced when deciding to create such a thing is the problem of dealing with the many, MANY possible ways in which the forces of the Cthulhu Mythos might (deliberately or accidentally) herald the end of humanity. After all, even the classic Mythos tales provide no real consensus — does humanity’s fall occur because Nyarlathotep goads us into developing weapons capable of wiping out our own species? Or does it come when the cosmic cycles turn and Cthulhu and his ilk can once again walk upon the surface of the land? Or is some future Apocalypse triggered by a need for the Great Race to inhabit the bodies of post-apocalyptic beetles?