"boris" started its worldwide spread with its adoption by rus' slavs from the first bulgarian empire. bulgarian cultural missions intensified in the 10th century, during the reign of tsar petar and with them the spread of bulgarian culture to what would become ukrainian and russian lands continued. it is speculated that the name of the bulgarian saint tsar boris i reached the rus in the late 10th century, likely during the reign of boris ii of bulgaria (969–977), great-grandson of boris i. in 967 the byzantines instigated the rus to attack the first bulgarian empire and it is probably around this campaign that the marriage of vladimir i of kiev to a bulgarian noblewoman, who is assumed to be a daughter of peter i, i.e., sister of boris ii, was arranged.