Grendel

He comes to the hall because of the noise - the harp, the boasting, the sound of people enjoying themselves - and he has been coming for twelve years, and nobody has landed a blow that mattered because no blade will bite on him. Swords turn. Spears skate off. What finally works is somebody who understands the problem, waits in the dark, takes hold of the arm and does not let go, and is strong enough to make that a real proposition.

Seize: bludgeoning, melee 10 ft, +6 to hit, 3d8+5, and on a hit he has hold and does not release voluntarily. Rend: bludgeoning, melee 5 ft, +6 to hit, 2d10+5, his second attack each round, used on whatever he is already holding.

No Blade Will Bite: he is immune to all damage from weapons of any kind, magical or otherwise, and to all spells that mimic a weapon. This is an enchantment on him and it holds absolutely. Grappled: unarmed attacks, grapples, holds and anything that works by leverage rather than by edge affect him normally and are the only things that do. Takes the Arm: reduced to 0 HP he tears free, leaving the held limb behind, and flees to the mere to die there. He does not come back, and something else will come looking for him.