Lagertha

[1] 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.

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.

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".

[3] In portraying the several warrior women in these tales, Saxo drew on the legend of the Amazons from classical antiquity, but also on a variety of Old Norse (particularly Icelandic) sources, which have not been clearly identified.

[4] Hilda Ellis Davidson, in her commentary on the Gesta, also notes suggestions in the literature that the name was used by the Franks, for instance by Luitgarde of Vermandois (c. 914–978), and that the tale of Lagertha could have originated in Frankish tradition.

[4] When Saxo describes Lagertha as "flying round" (circumvolare) to the rear of the enemy, he ascribes to her the power of flight, according to Jesch, indicating a kinship with the valkyries.

[6] Davidson deems it possible, as Nora K. Chadwick considered very probable,[1] that Lagertha is identical with Thorgerd (Þorgerðr Hölgabrúðr), a goddess reflected in several stories.

[7] Finally, the description of Lagertha coming to Ragnar's aid with flying hair is similar to how the Flateyjarbók describes Thorgerd and her sister Irpa assisting Haakon.

Broadly based on Saxo's account, the series portrays her as a shield-maiden and as Ragnar's first wife, who later rules as a jarl and then as a queen in her own right.