Stalkers of the Elder Dark

Basic Rules for Role-Playing Games of Cosmic Horror

From the introduction:

Cosmic horror, for all its supernatural pretensions, was always naturalistic, doubling as a cautionary tale in a new age of science. For humankind was not alone in the universe. It shared its space with innumerable alien races. Terrible races with unassailable intellects and devious intentions towards our kind. And these, in turn, had religion in the form of doubly alien gods. Elder things seeking entry to our world and stopping at nothing to get there.