In Beowulf, Grendel’s ferocity, which is expressed through physical description, sets the stage for Beowulf’s heroism. As an epic hero, Beowulf has exceptional strength and courage. Where dozens of men are unable to defeat Grendel, Beowulf can, even without the use of weapons.
The post above shows how Grendel is physically characterized before his battle with Beowulf. It is also important to look at how is described near the end of this encounter, as Beowulf gains the upper hand and begins to defeat the monster.
The monster’s hatred rose higher,
But his power had gone. He twisted in pain,
And the bleeding sinews deep in his shoulder
Snapped, muscle and bone split
This vivid imagery (look at the verbs: twisted, bleeding, snapped, split, broke) brings Beowulf’s power to the forefront. By describing Grendel’s defeat in such detailed and painful terms, the reader sees that Beowulf has an unmatched ability to defeat evil and make life safe for humanity, at least in Herot. Grendel is no longer fearsome; he is broken.
I have always found Grendel an interesting character, because he isn't described with a huge amount of imagery in regard to his physical description. I believe it was the responsibility of the original listeners to use their imaginations to help flesh him out. Still, though, there are some descriptions of him throughout the text that help the reader to develop his own more individualistic views of him.
When Grendel is first introduced, he is called a "fiend of hell" and his lineage is traced back to Cain, the evil young man who killed his own brother and was cursed by God for his sins. Later, Grendel is described as a "dark death-shadow" who "lurked and swooped" about. Once Beowulf arrives, Grendel is again described, this time as a "shadow-stalker, stealthy and swift," "greedily loping," as having "talons" and "claws." When Beowulf presents the gift of Grendel's arm to the kingdom, it is described as " brutal and bloodcaked." Through these well-placed details, the listener or reader can create an individual, imagined picture of Grendel.
Grendel, as described in Beowulf, is horrible indeed. A clawed monster of great strength and cunning, he can move quickly and quietly in the night, as he did when he invaded Herot to carry off thirty warriors:
Thoughts were as quick as his greed or his claws:
He slipped through the door and there in the silence
Snatched up thirty men, smashed them
Unknowing in their beds and ran out with their bodies,
Grendel's eyes reflect his evil nature as they "[g]leamed in the darkness, burned with a gruesome Light." He has "swift hard claws," "powerful jaws," and "great teeth." Furthermore, Grendel's skin was impenetrable in battle: "[T]he sharpest and hardest iron / Could not scratch at his skin."