From the Chaosium website:
A Malefic Cult for Cthulhu Dark Ages
The Pastores are a fertility cult rooted in an alliance of several noble families who worship Shub-Niggurath in her form of the Magna Mater, and a variety of other Mythos entities under the guise of her sacrificed Son. Their cultic practices involve the breeding, transformation, and periodic sacrifice of the human populace under their control, as well as extensive self-transformation of their ruling houses. Ancient beyond human memory, the Pastores aim to exploit the current lawlessness of France to expand their temporal power and subvert local religion, ultimately instituting the open worship of the Magna Mater in their domains and harnessing all of rural society to their breeding programs. Associated with the Pastores are a handful of independent sects and sorcerers who do not share their theology but recognize them as powerful allies.
The first part of this book describes the Pastores in detail — their beliefs, their practices, and the resources, allies, and leaders with which they threaten the health of the realm and any investigators unfortunate enough to encounter them. It includes advice on how to incorporate and utilize them to best effect in a new or ongoing Cthulhu Dark Ages campaign. The second part of the book contains scenarios featuring the Pastores. Together, they constitute a loosely connected campaign.