The proof of Beowulf’s victory, was the arm that he tore from Grendel’s body. Never before had Grendel met anyone with strength enough to subdue him, and it surprised him when Beowulf pinned his arms back and held him firm in his grip. Grendel tried his hardest, but he could not break free and in their struggle, Beowulf fatally wounded him by tearing Grendel’s arm off. Then Grendel crawled back to his swampy den, and drowned himself. Hope this helps.