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 fact that Childebert named Tassilo duke 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 the last man in 595.
They had invaded the lands of the Slavs, who received help from the Kaghan (chief) of the Avars.