War broke out between the brothers soon after their father's death, but few details are known. About Tassilo's time as duke, next to nothing is known. His existence is confirmed in the "Codex of Salzburg" (Salzburger Verbrüderungsbuch) where he is listed as unmarried, though some surmise that a certain Waldrada, mentioned as a wife of Theobald, was in fact Tassilo's.