 Lagertha's tale is recorded in passages in the ninth book of the "Gesta Danorum", a twelfth-century work of Danish history by the Christian historian Saxo Grammaticus.According to the "Gesta" (¶ 9.4.1–9.4.11), Lagertha's career as a warrior began when Frø, king of Sweden, invaded Norway and killed the Norwegian king Siward.Frø put the women of the dead king's family into a brothel for public humiliation.