The character of Lagertha appears in the ‘Gesta Danorum’ (Deeds of the Danes) of the 12th/13th-century Danish historian, Saxo Grammaticus. Saxo introduces her as the kinswoman of a Norwegian king killed by a hostile foreign force, who then forced the women of the royal family into prostitution.
The semi-legendary 9th-century Viking, Ragnar Lodbrok, the dead king’s grandson, comes for revenge, and is joined in battle by the women, who come to his camp in male attire. Of these women, Saxo names only Lagertha: his brief portrait of her is based on the amazon warrior-women of the classical world. On the battlefield, her long hair betrays the fact that she is a woman.
Ragnar attributes his victory to Lagertha, and seeks her hand in marriage. He goes to find her at home in the Gaula valley in western Norway, but she is unwilling, and has set a bear and a dog to ambush him. Ragnar kills both and wins Lagertha’s hand. Together they have two daughters and a son.
Ragnar later divorces Lagertha, but she continues to assist him on the battlefield, at one point flying like a Valkyrie around the enemy and attacking them from behind, turning the tide of the battle in Ragnar’s favour. After the battle she returns to her new husband, and slays him in the night with a spearhead she has hidden in her gown.
Lagertha is not attested in any other contemporary sources, so the extent to which she is based on a real woman or women is not known.