The game is set during the Dark Ages, right before the start of the Crusades. It's in a world of superstition, poverty, famine, and disease. Where most men and women are peasants who work the land for their feudal lords, the Church is possibly the most powerful institution in Europe, kings and their feudal lords are vying for power with each other, and barbarians raid their lands and sack their towns. Within it are hidden cults that worship the Great Old Ones or Outer Gods.
Cult of Magna Mater: Which is perhaps 1000s of years old, and can be traced to Rome as far back as 205 BC. Magna Mater (the Great Mother) is worshiped mostly by women. Everywhere that the Romans had a presence, remnants of the once powerful cult exist. Usually the cult builds its temples near caves, where the women will sleep in the hope of being touched by the goddess and given healing powers. In reality the cult is worshipping Shub-Niggurath. It was at the peak of its power during the 1st and 2nd centuries AD. Between the 4th and 10th centuries, they were hunted by the Brothers of Chrysostom, whose monks waged war against them across the empire. Now the cult of Manga Mater is experiencing a resurgence.
The Pastores: It's a fertility cult of pre-Roman and perhaps pre-Celtic ancestry. Which had commerce with K'n-Yan, a subterranean civilization. Like them they worshiped Yig, Tsathoggua, and Shub-Niggurath and received from them breeding stock. Over the millennia the cult survived and intermarried with those who invaded their lands. When they were strong the bred openly and when weak, they hid underground. Eventually their matrons and priests would end up joining the Roman religious establishment, and their nobility ended up intermarrying with Roman aristocracy. When Roman authority collapsed in the 3rd century, the cult revived its open breeding in the slave plantations though it was brought to a halt by Emperor Diocletian. Constantine and the Christian emperors that followed forced its members to fake their conversion to Christianity. They became bold in the 5th century, which nearly resulted in their destruction by the Merovingian Franks. Once again they were forced to rebuild and infiltrated Frankish nobility, and would eventually nearly operate in the open. When the Carolingians rose to power they once again hid. In the 10th century, they once again made a play for power, in southern France.
The Polyphemoi: They are a cult within Constantinople, devoted to Polyphemus the infamous Cyclops from the Odyssey. They are firm believers in the greatness of the Trojans and believe that one day Troy will rise again. Though many are Greeks, they despise the legacy of the Greeks and consider themselves Trojans. It is based deep within the catacombs of the city, and its members sacrifice both symbols of earthly wealth (defaced so that it can no longer be used) and living creatures (which include humans, that are horribly scarred before they are killed). All members must be free of physical imperfections and of a minimum physical stature, and those who due to age, battle, or accident lose that perfection are sacrificed next. Members are usually wealthy, though the cult finds the poor make outstanding sacrificces. It's members are unaware that it's actually an avatar of Nyarlathotep they are worshiping.
The Teutates: Whom are protectors of their Germanic tribes and followers of Gla'aki. Once the Teutates were active across Europe, but the spread of the Romans and then Christians, pushed them back to the home of their god, the Severn Valley. It's members are normally men and nearly all of them warriors. Once a warrior decides to make the ultimate sacrifice for his tribe and family, he participates in a ritual. One in which he is drowned and if found worthy by the Great Olde One, will be resurrected the next day with one of his spines and rise as an undead slave to Gla'aki.
The Naglgitor: It's a Norse longboat that sails out from Rlim Shaikorth, bringing devastation wherever it goes. It's origin lies with several Norse longboats that wandered off course and ended up aground on a frozen land, next to a mountain. In reality it was Rlim Shaikorth, and when one of the Norse (Oddvarr) explored the glacier via a tunnel, he was captured by his sorcerers who tossed him into his maw. Days later he emerged mutated, with unnatural abilities. Those who defied him were killed, while others fled. He burnt all the longboats except one, and hunted down the ones who fled whom he devoured. Those who remained and pledged their loyalty were taken to Rlim Shaikorth and thrown into his maw. If found worthy, they too came out changed. Otherwise they were consumed by the great worm.