From the Chaosium website:
The Second Cthulhu Invictus Companion
As a rule, men worry more about what they can't see than about what they can.
Fear of the unknown is mankind’s earliest fear, his most basic. What is out there in the dark? What hidden dangers lie within the forest, over those mountains, across that ocean? As the empire expanded, it encountered and absorbed new lands. These areas were filled with new people, speaking their own languages, with their own customs, gods, and mysteries. Just beyond the borders lurked evil entities.
Forces older than our world had dwelled in these places millennia before the arrival of mankind. For their own safety, the local peoples eventually formed legends, myths, and taboos around them. These whispered tales of dread and horror kept people from stumbling into the lairs or paths of these dark and ancient forces.
Unfortunately these safeguards were seen as ignorant barbaric superstitions by the new Roman settlers and masters. As the native people slowly became more and more like their Roman conquerors, the old ways were slowly forgotten … but that didn’t make the danger go away.
Occasionally these loathsome, powerful, and malignant beings would be disturbed by the unwitting, unknowing, or unbelieving. When this happened, the normal, predictable world would be turned upside down as horrors beyond mankind’s worst nightmare suddenly became real. Minds shattered, bodies withered, and souls were torn from their mortal frames to feed an insatiable hunger. Reality came undone, like a tapestry disintegrating beyond which is only darkness. Welcome to Extrico Tabula, Unraveling Borders, the Second Cthulhu Invictus Companion.