Beowulf's sword Hrunting fails him in his battle with Grendel's mother by proving ineffective against her naturally-armored hide. In the original Old English poem "Beowulf," Hrunting is touted as an infallible sword, defeating every enemy encountered. Against Grendel's mother, however, Beowulf has to discard the weapon.
While in the lair of Grendel's mother, Beowulf finds another sword after Hrunting proves useless against her skin. Whereas Hrunting was forged by man, this new, nameless sword was forged by magical giants. When Beowulf uses it on Grendel's mother, it cleaves her in two. The magical sword melts afterward because of the heat and poison of the monster's blood.