The ogre who has menaced Hrothgar's people for 12 years is a huge, powerful descendant of the biblical Cain, the son of Adam and Eve, who killed his brother Abel out of jealousy (Genesis 4). Cain's name in Hebrew is Qayin, meaning "creature," and, according to legend, the monsters of the earth are his descendants. Grendel is envious, resentful, and angry toward mankind, possibly because he feels that God blesses them but that the ogre himself never can be blessed. Grendel especially resents the light, joy, and music that he observes in Hrothgar's beautiful mead-hall, Heorot. The scop's "Song of Creation" (90-98) especially enrages him because it tells of the beauty and light of God's creation.
Although Grendel looks something like a man — having two arms (or claws), two legs, and one head — he is much larger and can defeat dozens of men at a time. He is protected from man's weapons by a magic charm. He devours some of the dead on the spot and carries others back to his lair, the cave he shares with his mother beneath a mere in a dark fen. The first night that Beowulf is with the Scyldings, Grendel stomps up from the swamp, bashes open the mead-hall's door with a single tap, and quickly wolfs down one of the Geats inside.
The passage describing Grendel's ascent from the fen (710-727) is one of the finest in Anglo-Saxon poetry. The drama increases as the poet describes Grendel's approach in set stages. Dark skies contrast with "the shining wine-hall" (715), a source of joy to men and the symbol of civilization. Grendel has ruled the hall for 12 years, often spending his nights there as the Danes hid elsewhere. He expects to rule again this night but meets a human warrior equal to the ogre's strength and superior in his tactics.
Beowulf observes the monster's method as one Geat is slaughtered and devoured. Grendel has no chance after that. Although the battle is furious, Beowulf has won as soon as he is able to grasp his enemy's claw. The ogre is vulnerable because Beowulf uses no weapons, and the hero has the strength of 30 men in his grip. Beowulf rips the monster's arm from its shoulder. Mortally wounded, Grendel flees to the swamp. The giant claw later hangs from Heorot's roof as a trophy.
In many ways, Grendel is the most interesting character in the epic. He is a mix of man and beast; his fury is based on very human feelings of resentment and jealousy. The novelist and Anglo-Saxon scholar John Gardner explores the inner conflicts of the character in his 1971 novel, Grendel, an intensely moving, funny, and perceptive book.