What is the charm that helps Grendel defeat Hrothgar's Danes in Beowulf?
In Beowulf, Grendel's skin is protected from being pierced by the Danes' swords. I'll quote the lines from Burton Raffel's translation:
...All of Beowulf's
Band had jumped from their beds, ancestral
Swords raised and ready, determined
To protect their prince if they could. Their courage
Was great but all wasted: they could hack at Grendel
From every side, trying to open
A path for his evil soul, but their points
Could not hurt him, the sharpest and hardest iron
Could not scratch at his skin, for that sin-stained demon
Had bewitched all men's weapons, laid spells
That blunted every mortal man's blade. (368-378)
The world of Beowulf and Grendel is a world that includes magic. Epic poems are not known for their realism, and this one is no exception.
I'm not sure what you mean by "charm." Do you mean Grendel, the monster, is beguiling or that he has a magical ability? My answer is that he has neither leading him to victory over Hrothgar's people. Grendel is very large and scary looking. We are told he can pick up 12 men with one hand and devour them. He has thus intimidated Hrothgar's men to the point where they flee rather than fight back. When Beowulf comes to the land of the Danes to fight Grendel he tells Hrothgar (after Unferth's taunts questioning Beowulf's abilities) that one reason he is there is because no one fights back against the monster so it is able to freely kill. Also, in the first part of the story, we are told that Grendel is a descendant of Cain, who slew his brother making him the first murderer. This creates an evil base for the monster, but it is not a magical base.