What is "hrunting"

Hrunting was a sword given to Beowulf by Unferth in the ancient Old English epic poem Beowulf. Beowulf used it in battle against Grendel's Mother.

Beowulf is described receiving the sword in lines 1455-1458:

"And another item lent by Unferth at that moment of need was of no small importance: the brehon handed him a hilted weapon, a rare and ancient sword named Hrunting. The iron blade with its ill-boding patterns had been tempered in blood. It had never failed the hand of anyone who hefted it in battle, anyone who had fought and faced the worst in the gap of danger. This was not the first time it had been called to perform heroic feats.

However, although the sword possessed great power and was claimed to have never failed anyone who used it, when Beowulf descended to the bottom of the lake to fight Grendel's mother, the sword proved ineffective. As the "fabulous powers of that heirloom failed," Beowulf was forced to discard it.