In Beowulf, Grendel is the monster who attacks Heorot before Grendel’s mother. He is introduced as a frightening enemy of King Hrothgar’s Danish warriors, who celebrate in the great hall.
The poem says that Grendel is tormented by the noise of singing and feasting in Heorot. For twelve years, he repeatedly comes at night and kills the sleeping warriors. Beowulf eventually travels from Geatland to Denmark to confront him.
Beowulf fights Grendel without weapons and tears off the monster’s arm. Grendel escapes to his marshy home, where he dies. The victory does not end the danger, because Grendel’s mother then attacks Heorot to avenge her son.
Although Beowulf uses Germanic heroic tradition, Grendel is not a named figure from the surviving Norse mythological poems. His story belongs specifically to the Old English epic.