Atli.
Armor made by this character that was immune to steel and iron weapons was used successfully by her sons until an old man revealed that the armor had a weakness to stones. Those sons of this character were attempting to take revenge for the horse-trampling death of their sister, who was accused of having an affair with Randver. This character attempts to commit suicide by holding stones and jumping into the sea, but the waves carry her to the kingdom of Jonakr, who she marries. In one source, this mother of Hamdir, Sörli, and Svanhild is hacked up by Hildebrand, the mentor of Dietrich of Bern, after she decapitates Hagen. In another source, she takes revenge for her murdered family by killing her sons Erp and Eitil and feeding them to the murderer, her second husband Atli. In the Nibelungenlied, this character is known as Kriemhild. For 10 points, name this sister of Gunnar who marries Sigurd in the Volsunga saga.
Armor made by this character that was immune to steel and iron weapons was used successfully by her sons until an old man revealed that the armor had a weakness to stones.
Gudrun [or Kriemhild until it is read]