Simeon I of Bulgaria
The earliest occurrence of Michael in the sources is from 912. Venetian chronicler John the Deacon recorded that at that time Pietro, son of the Venetian doge Ursus Particiacus II (912–932), was treacherously captured on his return from a diplomatic mission to Constantinople by Michael, "a prince of the Slavs" ("dux Sclavorum"), when he wanted to pass through the lands of the Croats. Before Pietro entered Croatia, on his way home, when he entered Zahumlje, or when he entered province of Narenta or Pagania, Michael "dux Sclavorum" had him captured and sent as a gift to Simeon I of Bulgaria. Since 912, Michael was a close ally of Simeon I of Bulgaria, who had been mounting a number of successful campaigns against the Byzantine Empire.