Lagertha was, according to legend, a Viking shieldmaiden and ruler from what is now Norway, and the onetime wife of the famous Viking Ragnar Lodbrok. Her tale was recorded by the chronicler Saxo in the 12th century. According to modern scholarship, it is likely that Lagertha was not a historical figure, but a reflection of tales about Viking warrior women or about the Norse deity Thorgerd.
Her name as recorded by Saxo, Lathgertha, is likely a Latinisation of the Old Norse Hlaðgerðr (Hlathgerth). It is frequently rendered in English-language sources as "Lagertha", and has also been recorded as Ladgertha, Ladgerda or similar.
Life according to Saxo Grammaticus
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. Hearing of this, Ragnar Lodbrok came with an army to avenge his grandfather Siward. Many of the women Frø had ordered abused dressed themselves in men's clothing and fought on Ragnar's side. Chief among them, and key to Ragnar's victory, was Lagertha. Saxo recounts:
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.
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.
Upon returning to Norway, she quarrelled with her husband, and slew him with a spearhead she concealed in her gown. Saxo concludes that she then "usurped the whole of his name and sovereignty; for this most presumptuous dame thought it pleasanter to rule without her husband than to share the throne with him".