Describe what happened to Grendel when he raids Heorot and encounters Beowulf?

Grendel enters Heorot after ripping the door from its hinges. He looks over the sleeping warriors and eats one of them whole. Grendel is unaware that Beowulf is watching him, and when he attempts to grab Beowulf, the warrior takes hold of his arm and will not let go. Grendel panics and tries to get away, but Beowulf holds on all the tighter. In the end, Grendel escapes..... but he escapes without his arm.