In Beowulf, Hrunting is the name of the sword that Unferth, one of the Danes, gives to Beowulf when he goes to fight Grendel's dam (mother).
And another item lent by Unferthat 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.
We know from the story that when Grendel tries to use the sword, it fails him; however, nearby lies another, ancient blade. Beowulf picks this up and kills Grendel's dam. Her blood melts the blade of the ancient sword, and when Beowulf appears out of the bloodied waters, he holds not only the head of Grendel's dam, but what is left of the melted sword.
Unferth is at first critical of Beowulf when the Geat arrives. His gift of Hrunting to Beowulf might be a gesture of goodwill to a stronger warrior, which is a change from Unferth's previous attitude. It was not unusual for one warrior to give a weapon to another. However, in doing so, some essence of the sword's history would travel with it. Though it at first appears a noble gesture on Unferth's part, it was rumored that Unferth had used this same blade to kill his family; if this was the case, this negative essence traveled with the blade, so Unferth may, in fact, have been trying to lead Beowulf to defeat and death.
The ancient sword in the monster's cave proves to be a worthy weapon, and it is with this that Beowulf kills the "human monster," and returns through the waters of the fen to his men.