Vlastimir's three sons successfully fought off an onslaught by Boris I of Bulgaria in 853 or 854 (shortly after the death of Vlastimir), when they captured 12 great boyars and the commander himself, Vladimir, the son of Boris. The Bulgars had sought to avenge the previous defeat of Presian in 842. The two sides made peace, and possibly an alliance. The two younger brothers later revolted against Mutimir for undisclosed reasons. Mutimir sent them as prisoners, a guarantee of peace, to the court of Boris I at Pliska. After Mutimir requested that Emperor Basil I (867–886) baptize his lands, Constantinopolitan priests were sent and a Serbian bishopric was founded. The Christianization is evident in the tradition of theophoric names found in the next generation of Serbian monarchs ("e.g.", Petar Gojniković, Pavle Branović). The three branches of Vlastimir's sons continued a succession war over the decades.