Answer:
Grendel was going back to the castle once more. He had attacked Heorot several times successfully, always being able to bring fresh meat to his lair. Once there, he would have his own feast with his mother.
"They must have some kind of trap prepared for me," he thought. He didn't care much about that, as the previous times no one had been able to stop him, no matter what they tried.
He entered the castle easily and attacked one of the men without much opposition. However, the next warrior was different. The monster was surprised to be attacked by a man with his bare hands.
"This cannot be. I shall tear him to pieces," he decided.
Grendel pursued the mighty warrior through the halls, but he was also attacked back by him. He hadn't seen such bravery and strength in a human before. But he was also lonely, too different from the rest.
"It's too dark, and the other warriors won't let me focus on him. I shall escape and return later."
However, without noticing, the warrior had his arm trapped. The monster tried to free himself as much as he could, but couldn't free himself.
In the end, he had to lose his arm and run away. The strong determination of the warrior had shown Grendel that he could not defeat him. He didn't seem to have something to lose, and this made the monster feel afraid for the first time in his life.
