A knife with two edges: one that can cut through any material in the physical world, and another that can cut windows between parallel worlds. Forged three hundred years ago by the philosophers of Citta'gazze, the knife is also called Aesahaettr ('God-destroyer' in the language of the bears). It can only be wielded by a bearer chosen by the knife itself - Will Parry becomes its bearer after losing two fingers in a struggle for it. The knife's world-cutting has unintended consequences: each window allows Dust to leak away.