HE WHO WAS the earl's defender would in no way allow the murderous stranger to live, and he did not consider his days or years useful to any man on earth. Now many of Beowulf's band brandished ancestral blades, wanting to save the life of their leader, the proud prince, if such they could do. They did not know as they neared their foe, those stalwart warriors who thought to hack him on every side and kill the accurséd one, that not even the keenest blade or the best falchion fashioned on earth could hurt or harm that hideous fiend! His sorceries made him safe from the victorious sword and all iron edges. His departure from life and his end would be full of woe, and his departed spirit would wander far off into the fiends' domain. He, who in former days had wrought such murder on many men, whose heart was full of harm and hatred of God, soon found that his mortal body now failed him. The valiant kinsman of Hygelac now held him by the hand; each one's life was loathsome to the other. The foul bandit took a mortal wound, and a fatal tear appeared on his shoulder. His sinews ripped apart and his bone-frame broke. victory was now given to Beowulf, and Grendel, sick unto his death, went hence and sought his den in the dark moors, that vile abode; he knew full well that his life had reached its end and that the last of his days on earth had come. The fulfillment of the desire of all Danes had come through the bloody battle.
He who came from afar, stalwart and wise, had purged Hrothgar's hall of ravage; his night work–brilliant and honorable–had succeeded. The valiant Geat had made good his boast to the Eastern Danes, assuaging all their sorrow and ills, and the harrowing struggle which they had endured for so long, forced to suffer that great indignity. Proof of this—the hand, arm, and shoulder of Grendel, his full, strong grip —was displayed beneath the high gabled roof.
Due to evil magic, Grendel cannot be harmed by weapons, and Beowulf's earlier decision to match Grendel's strength with his own arms crucial to achieving victory. The poet never revealed this fact earlier in the story, likely because there were no survivors to confirm this claim.— Wesley, Owl Eyes Editor
We learn that Beowulf considers his life to be worthless should he not defeat Grendel. This line reinforces the code of honor that Beowulf lives by: His duty is to kill Grendel. If he fails, his life is not worth living.— Wesley, Owl Eyes Editor