Boris I, d. 907, khan [ruler] of Bulgaria (852–89). Baptized in 864, he introduced Christianity of the Byzantine rite among the Bulgarians. There followed a rivalry between Rome and Constantinople for the loyalty of the Bulgarian church. In 889, Boris abdicated and retired to a monastery. His son was called Czar Simeon I.
"Boris I." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (June 27, 2019). https://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/boris-i
"Boris I." The Columbia Encyclopedia, 6th ed.. . Retrieved June 27, 2019 from Encyclopedia.com: https://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/boris-i