The correct answer is
798 to 805.
These are the lines from the poem:
798. their praised prince, if power were theirs;
799. never they knew,—as they neared the foe,
800. hardy-hearted heroes of war,
801. aiming their swords on every side
802. the accursed to kill,—no keenest blade,
803. no farest of falchions fashioned on earth,
804. could harm or hurt that hideous fiend!
805. He was safe, by his spells, from sword of battle,
As you can see in these lines, there are some spells that Grendel cast which protect him from blades. Beowulf managed to kill him only using his bare hands.