 In 680 Bulgar tribes under the leadership of Asparukh moved south across the Danube and settled in the area between the lower Danube and the Balkan , establishing their capital at Pliska . A peace treaty with Byzantium in 681 marked the beginning of the First Bulgarian Empire . The Bulgars gradually mixed up with the local population , adopting a common language on the basis of the local Slavic dialect . 
