At the end of part one of Beowulf, Grendel is mortally wounded by Beowulf.
Grendel enters into Heorot after having its doors closed for twelve years. Grendel returns to Heorot upon this night because the celebrations have resumed. A hero, Beowulf, has arrived at Heorot to rid the mead hall of the monster.
Upon arriving at Heorot, Grendel notices that the men sleeping in the mead hall are much larger and stronger looking than the Danes he encountered there before. Regardless, he enters anyway.
After killing one man, the last he will ever kill (as foreshadowed by the quote "Destiny did not permit him to seize any more of mankind after that evening"), Grendel grabs onto Beowulf's arm. Beowulf immediately returns the grab. After a short battle, Beowulf has defeated Grendel. Beowulf has taken Grendel's arm (which is hung "beneath the high gabled roof"). Beowulf, mortally wounded, slinks back off to the lair he shares with his mother.