Grendel and his mother were descendants of the biblical Cain. Grendel was the reason Beowulf left Geatland, because Grendel had been attacking the mead-hall at Herot, killing and eating anyone he found there. Grendel's reason for attacking were justified in these times, as the noise from the drunken revellers at Herot's mead-hall was disturbing him.
Beowulf promised the people of Herot that he would kill the monster on even footing. As Grendel fought with his bare hands and without even a sheet of cloth to protect himself, so too did Beowulf. The battle was long and fierce, and ended when Beowulf ripped off Grendel's arm. Grendel retreated to his cave under the nearby swamp, where he bled to death. His death led to attacks by his mother, who Beowulf also killed.