Six gruesome blades forged from dwarven blood in the pits of ancient bol-Pakash. Six knives the dwarves wish never existed.
Maddoc sat amid a pile of unrecognizable corpses. Men, Dwarves, Orcs, you couldn't tell. He periodically took a stab at one with his new knife and screamed, "STOP LAUGHING AT ME!"
A dark dagger of song, forged for vengance. It shall have that vengance.
A dagger that makes the user think that he is invisible.
If you want to add a "supernatural" touch to your campaign, define the rules of magic and the universe. Make your players comfortable with those rules. Then your new supernatural creatures must then break those rules.