Brynhild is a shieldmaiden and a valkyrie in Germanic mythology.
In the Nibelungenlied, Brunhild is instead the queen of Iceland. Gunther overpowers her in three warlike games with the help of Siegfried – equipped with an invisibility cloak. First, Brunhild throws a spear, that three men can only barely lift, towards Gunther, but the invisible Siegfried diverts it. Second, she throws a boulder, that requires the strength of twelve men to lift, twelve fathoms. Finally, she leaps over the same boulder. However, Gunther, with Siegfried's help, defeats Brunhild in these games, and takes her as his wife.
On their wedding night, Brunhild refuses to yield her virginity to Gunther, and instead ties him up and suspends him from the ceiling of their chamber. Siegfried (again invisible) violently subdues Brunhild, by cracking her bones and taking her girdle and ring. Following this episode, Brunhild loses her supernatural strength and becomes a devoted wife to Gunther.
Later, in front of the Worms Cathedral, Brunhild enters into an argument with Siegfried’s wife Kriemhild regarding their husbands’ relative prestige. Brunhild believes Siegfried to be nothing more than a lowly vassal of Gunther’s, but Kriemhild reveals the deception and humiliates Brunhild by showing her the ring and girdle.
The Nibelungenlied also differs from Scandinavian sources in its silence on Brunhild's fate; she fails to kill herself at Siegfried's funeral, and presumably survives Kriemhild and her brothers.
Sources: