The Gesta Danorum, a 13th-century text on Danish history, features the most prominent mention of Lagertha, per the World History Encyclopedia. The text talks about Ragnar going after King Frø, who killed his grandfather, for vengeance. On his way, he is met by a group of women warriors who want to join forces with him to take down King Frø. Lagertha is among these women, and she leaves the biggest impression on Ragnar.
Ragnar courted Lagertha for a while, and she ultimately sets a bear and dog to fight him when he visits her home. Despite being surprised by them, Ragnar defeats both of them, and the two get married, finally having two daughters and a son. However, Ragnar maintains a certain distrust of Lagertha due to her willingness to set animals on him, and when the opportunity arises to potentially marry King Herodd’s daughter Thora, Ragnar divorces Lagertha.
Lagertha, however, returns later on in the story, responding to Ragnar’s plea for aid during the Danish Civil War with 120 ships, and proves to be the deciding factor in the battle, turning the tide by outflanking the enemy when they seemed to be winning. She ultimately murdered her husband and took up the ruling throne in his place Lagertha’s legacy in history is being that of a Viking shield-maiden, and a warrior woman in ancient times when not many women played that role.