Not blade on earth, no blacksmith's art Could ever damage their demon opponent. He had conjured the harm from the cutting edge Of every weapon.

When they joined the struggle
There was something they could have not known at the time,
That not blade on earth, no blacksmith’s art
Could ever damage their demon opponent.
He had conjured the harm from the cutting edge
Of every weapon.

– Beowulf, Seamus Heaney (trans.)

Lines 779-804: Grendel is not capable of being being harmed by any blade or sword.

Beowulf Quotes