Grendel terrorizes Heorot for 12 years before Beowulf arrives. The period establishes the extraordinary scale of the Danes’ helplessness and explains why Beowulf’s arrival is treated as a moment of hope.
Heorot is the great mead hall built by the Danish king Hroðgar. Grendel is enraged by the sound of celebration there and begins attacking at night, killing and carrying off Hroðgar’s warriors. The attacks continue for twelve winters, leaving the hall effectively unusable and the Danish court unable to stop him.
News of the crisis eventually reaches Beowulf in Geatland. He sails to Denmark with fourteen companions and offers to fight Grendel without relying on weapons. During the night encounter, Beowulf tears off Grendel’s arm, mortally wounding him; the severed limb is displayed in Heorot.
A common mix-up is to combine this twelve-year period with the later attack by Grendel’s mother. Her revenge begins after Grendel’s defeat and forms a separate episode in the poem.