Beowulf wounds Grendel in a fight. He first grips Grendel, his attacker, by the hand. Beowulf's strength causes the other man's hand to break and begin to bleed. As the fight continues, Beowulf delivers a mortal injury to Grendel by ripping his arm off.
The fight begins because Grendel attempts to sneak up on Beowulf and his soldiers and kill them. To achieve this, Grendel has armed himself with spells that make him impervious to all man-made weapons. Despite this, Beowulf manages to get the upper hand in the fight through his great strength and is rewarded for his efforts by receiving the arm and claw of Grendel after Grendel's death.