Knowledge is power, so the adage goes. To the adepts of the Priory, this is a literal truth as they have been curating their knowledge of Mythos for generations. It is the power by which humanity will be saved from the predations of the Old Ones in the Shadow War.
Over the millennia, shamans, soothsayers, skalds and scholars held the keys to the power of Mythos but were unaware of its greater purpose. Those fortunate few used it sparingly and merely glimpsed behind the curtain to barely retain their sanity.
As humanity became increasingly arrogant in their own manifest destiny, this precious lore of Mythos was discarded in favour of technology, and the illusion of safety created by our increasingly modern world. So the Priory have been charged by their mysterious coordinator Madame Adrestia to remember the old dangers that still lurk in the darkness and to fight it at every turn so that the Priory emerge victorious in the Shadow War.