Kuber was Bulgar leaer who ruled over Srem as a vassal to the Avar Khagan. All of what we know comes from the Miracles of St Demetrius. He ruled a mixed Christian population of Bulgars, ‘Romans’, Slavs and Germanic people that had been transferred to the Srem region in Pannonia by the Avars 60 years earlier. He and his people revolted in 670s, and after defeating an Avar punitive attack, the tribe moved to Macedonia. The Byzantines called his people Sermisianoi (after their former settlement- Sirmium), as well as the Keramisians(after their new place: the Keramissian plain in Macedonia).