Well, technically it's only a weapon against things like robots and ghosts because a ghost knife only cuts unliving matter (it does sap your energy if it hits you, though), but still, this is the actual prop knife used on the trailer for burger_eater's (Harry Connolly) novel Circle of Enemies. And you can only get the ghost knife if you win this auction that Patrick Rothfuss is holding for his Worldbuilders fundraiser.
If I had the cash, I'd bid. This is cool. Any of my more solvent readers, go take a look. :)