Metal that can only be found in the Underworld. And only children of the Underworld can wield it.

[Hell, he may as well just summon the sword so Katsuki can see it for himself. He raises his hand and conjures it from thin air. It's three feet of pure black, faintly glowing an eerie kind of purple.]

I suppose I could if I had to. I never really learned hand-to-hand, though.

[It was too easy to raise dead fighters to do the punching for him.]