In one of his letters , Patriarch Nicholas Mysticus threatened to unleash a Rus ' invasion of Bulgaria .
The First Christianization of the Rus ' was reported by Patriarch Photius in the 860s .
In one of his letters , Patriarch Nicholas Mysticus threatened to unleash a Rus ' ' invasion of Bulgaria .
The First Christianization of the Rus ' was reported by Patriarch Photius in the 860s .