Career
According to Paul the Deacon, he was appointed as Bavarian rex by Childebert II, Frankish king of Austrasia, in 591, ending the war with the Franks. The war began during the reign of Tassilo"s predecessor, Garibald I, when Garibald concluded a marriage alliance with the Lombards. We do not know whether Garibald died or was deposed.
The fact that Childebert named Tassilo king shows Frankish control over the Bavarian state.
Paul the Deacon also tells us that Tassilo soon moved into the lands of the Slavs (probably the recently conquered eastern Tyrol and Carinthia), and returned victorious with much plunder. This victory proved to be short-lived as Paul tells us of 2,000 Bavarians, who were slain to a man in 595 when invading the lands of the Slavs to help the Kakan (chief of the Avars).