Beowulf killed Grendel’s mother using a sword he found in her cave. It was an ancient sword forged by giants, Cain’s descendants, like Grendel and his mother. When Beowulf killed the mother and decapitated Grendel, the sword melted away because of the mother’s poisonous blood.
In that battle, Beowulf went underwater to fight Grendel’s mother in her cave. During his descent, the mother attacked and dragged him to her lair. He tried to hit her with Unferth’s sword, Hrunting, but it did not even scratch her skin.
Enraged and frustrated, Beowulf went on her with bare hands. He managed to knock her down, but she was too swift. She sat on top of him, trying to kill Beowulf with her dagger. Thankfully, the hero’s armor protected him.
At some moment, Beowulf threw the monster off. He spotted “a victory-bright blade made by giants.” Beowulf stroke Grendel’s mother with the sword. He broke her collarbones and sliced her in two. When she died, a light illuminated the cave, disclosing her numerous treasures and Grendel’s corpse.
Beowulf cut Grendel’s head off to give it to Hrothgar. Then the new sword’s blade melted to the hilt because the blood of Grendel’s mother was hot and poisonous.
Beowulf did not take any of the treasures. He swam to shore with the monster’s head in one hand and the hilt in the other. His fellow warriors were waiting for him. Four of them carried Grendel’s head to the mead-hall to celebrate the victory.