But then Heorot is threatened by the appearance of a monster, a demon, who cannot bear to hear the sounds of the banquet and the songs of the harpers that come from the mead-hall. (Mead is an alcoholic drink made of fermented honey, malt, spices, and water, to which yeast has been added.)
The name of the monster is Grendel. He prowls around the desolate heath and fens. He is identified as one of the clan of Cain, the Biblical character who killed his brother Abel and was made an outcast by God.
At night Grendel sets off for the mead-hall. The men are all asleep following their evening's drinking. Grendel grabs thirty men and rushes back to his lair with their corpses.
In the morning, the men wake and realize what has happened. They go into mourning. Hrothgar is stunned by the destruction wreaked by Grendel.
The following night Grendel strikes again. He murders more men.
And so it goes on. For twelve years Grendel raids and ravages the hall. The whole world hears about it. Heorot becomes deserted, except for the throne itself. As an outcast of God, Grendel is prevented from reaching it.
The desperate Danes try to come up with a plan to repel Grendel's constant attacks. They make offerings to their pagan gods, since the one God of Christianity is unknown to them. But still the raids go on.
Beowulf is largely a pagan poem to which has been added elements of Christianity and three mythological monsters. The warrior society of the Danes is a pre-Christian one (as these lines show), but the poem was written centuries later, in recently Christianized England, so a Christian framework has been grafted on to it.
Grendel is the first of the three monsters. The poet gives him a genealogy that links him to Christian ideas about the origin of evil. This is why he is presented as being descended from Cain, who was cast out by God for having killed his brother Abel. There is no reason given for Grendel's murderous acts other than the fact that he is evil. He is cursed by God and is referred to as a demon. He therefore comes close to fulfilling the role allocated by Christianity to the devil: he is evil and he perpetually struggles against good. In the poem, the "good" are the righteous Danes under Hrothgar, and Beowulf who comes to their aid.