Brynhildr was a princess, a shield-maiden, and was said to be a Valkyrie who disobeyed Odin. Stripped of her Valkyrie powers, she was put into a deep slumber, on a castle on top of a mountain surrounded by a magic fire, and can only awakened by a brave man with a kiss, which she was by Sigurd the Dragonslayer. With Sigurd, she had a daughter; Aslaug.
Although the pair fell in love, Sigurd was deceived by Gjuki, King of Burgundy, whose wife, the sorceress Grimhild, prepared a magic potion that made Sigurd forget about Brynhildr so he could marry their daughter Gudrun. When she found out about Brynhildr, they let their son Gunnar go to her castle. Gunnar was only able to cross the ring of fire when he switched places with the enchanted Sigurd and convinced Brynhildr to marry Gunnar instead of Sigurd.
Sigurd later regained his memories. Heartbroken upon finding out the truth from Gudrun, Brynhildr urged Gunnar to murder Sigurd. Gunnar and his brother Hogni had both sworn oaths of blood brotherhood with Sigurd and could not kill him in fear of angering the gods, so they instead incited their young brother Gutthorm to do the deed. Sigurd killed the young Gutthorm, and was also killed, while Brynhildr killed Sigurd’s and Gudrun’s son.
Distraught over her actions, Brynhildr committed suicide by throwing herself on Sigurd’s funeral pyre. They reunited in Hel, but left Aslaug alone in the world.