The people are forced to flee Heorot every night because Grendel always attacks the hall at night and kills the people he finds.
Heorot is built at the request of Hrothgar, and he uses it as a meeting place for his people where he provides entertainment and presents gifts to his people. However, Grendel is distressed by the merrymaking and seeks to disturb the peace. The first night the monster attacks, he takes away thirty thanes as they sleep. Grendel continues his attacks on Heorot until the building is abandoned. The situation drags on for 12 years, and the stories about Grendel spread far and wide. The information reaches Beowulf, and he decides to go to the aid of King Hrothgar.
Beowulf faces off with the monster and emerges victorious, dealing a fatal blow to the beast. Grendel’s mother seeks revenge, but the valiant warrior deals her the same fate as the son. Ultimately, peace returns to Hrothgar and his people.
The Danish King Hrothgar built the monumental mead-hall Hereot as a gathering place for his family and warriors. It was built on a massive scale designed to impress people with the wealth and power of Hrothgar and his men.
As soon as Hereot is built it is attacked by Grendel, an evil monster who is the son of Cain (the evil brother in the Bible). Grendel's motivations are unclear though he seems to resent the Danes moving into territory he considers his own and finds their cheer and laughter infuriating. As a monster and creature of darkness, he attacks at night. He is supernaturally powerful and cannot be harmed by ordinary weapons.
The first night Hereot is inhabited, Grendel kills 30 warriors and the attacks continue on subsequent nights over a period of 12 years making Hereot uninhabitable at night until Beowulf kills Grendel.
Grendel comes every night to Herot to kill men. Anyone who remains there at night is killed by him. The only exception is the king who is protected by God.