I think you are talking about the special sword that belonged to Grendel's mother. It been created and hammered/jeweledby the giants. It was special also because it was stronger than she was. Hrunting (Beowulf's sword) could not cut Grendel's mother when he went after her to kill her. She could not be harmed by normal swords and knives. But this sword that hung in her dwelling could. Beowulf grabbed this special sword and used it to defeat her. He chopped her head off with it, then went and found Grendel's lifeless body and chopped his head off as well. This rids the Danes of any more deaths caused by Grendel or his mother.