After the King of the Danes, Hrothgar, builds a mead hall called Heorot, the Danes enjoy singing and playing the harp there. Their rejoicing evokes ire in Grendel, who was a "prowler about the borders of the homes of men, who held the moors, the fens, and fastness" (Clarence Griffin Child translation). Grendel is the descendant of Cain, doomed by God for Cain's slaughter of Abel. He prowls about the moors waiting to attack people.
Grendel spies about Heorot after the Danes are drinking, and he decides after their merrymaking to slay their men. Then, he heads homeward "exultant over his spoil." Night after night, Grendel waits until dark and then ambushes the men. Grendel returns to Heorot to again murder Danes, who endure this situation for "twelve winters' season." The Danes are desperate for help until Beowulf arrives from abroad to slay Grendel.