The Viking shield maiden.

According to legend, Lagertha was a Viking shield-maiden and ruler from what is now Norway, and the onetime wife of the famous Viking Ragnar Lodbrok. Ladgerda, a skilled Amazon, who, though a maiden, had the courage of a man, and fought in front among the bravest with her hair loose over her shoulders. All marvelled at her matchless deeds, for her locks flying down her back betrayed that she was a woman.

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, 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. Hearing of this, Ragnar Lodbrok came with an army to avenge his grandfather Siward. Many of the women Frø had ordered abused dressed in men's clothing and fought on Ragnar's side. Chief among them, and key to Ragnar's victory, was Lagertha. Saxo recounts:

Impressed with her courage, Ragnar courted her from afar. Lagertha feigned interest and Ragnar arrived to seek her hand, bidding his companions wait in the Gaular valley. He was set upon by a bear and a great hound which Lagertha had guarding her home, but killed the bear with his spear and choked the hound to death. Thus he won the hand of Lagertha. According to Saxo, Ragnar had a son with her, Fridleif, as well as two daughters, whose names are not recorded.

After returning to Denmark to fight a civil war, Ragnar (who, according to Saxo, was still annoyed that Lagertha had set beasts against him) divorced Lagertha in order to marry Thora Borgarhjört (Þóra Borgarhjǫrtr), daughter of King Herraud (Herrauðr) of Sweden. He won the hand of his new love after numerous adventures, but upon returning to Denmark was again faced with a civil war. Ragnar sent to Norway for support, and Lagertha, who still loved him, came to his aid with 120 ships, according to Saxo. When at the height of the battle, Ragnar's son Siward was wounded, Lagertha saved the day for Ragnar with a counter-attack:

Ladgerda, who had a matchless spirit though a delicate frame, covered by her splendid bravery the inclination of the soldiers to waver. For she made a sally about, and flew round to the rear of the enemy, taking them unawares, and thus turned the panic of her friends into the camp of the enemy.