Grendel is a monster, identified in the text as a descendent of Cain. In the Bible, Cain was the first murderer. He killed his brother because of jealousy. Grendel is awakened by the celebrations going on at Herot and is jealous of the celebrating. That's when he attacks Herot and kills many men. As a result, Herot is left empty for 12 years and Hrothgar, the king, is very upset. Beowulf hears of Hrothgar's plight and comes to help. Beowulf succeeds in killing Grendel, but then must deal with Grendel's mother.