Asura

These small kozuka knives carry a flower motif along the handle and were crafted from the cursed sword Asura. First seen in Chapter 163, they pass through many hands, from Saint-Germain to the blackmailed students he and his agents recruit. The handle color signals function: each knife is either black or white, with the black-handled version used far more often.

The knives operate as tools for transferring power. The white-handled blade grants an ability when a person cuts themselves with it, while the black-handled blade extracts a target's power, but only after the victim has consented to yield. Through this mechanism a single physical ability can be relayed across a chain of owners, with the same power tracing a path through users such as Koki Yukishiro, the Lord of the Flies, and ultimately Vlad. Abilities documented as moving this way include microfauna manipulation, jiangshi and Severed Head Dribblers control, Kettetsu transformation, enhanced jumping, body manipulation, and Rin's gravity power channeled through Mai.

Saint-Germain originally amassed the knives and used them to hunt yokai for their powers, dispensing them to subordinates and recruits. Takeshi Murakami wields one to coerce Koki Yukishiro into siphoning Momo's psychic ability, and Mika Adachi turns hers on Murakami to seize the Severed Head Dribblers. Other named bearers include the Red Baron, Hase, who gains Jumping Crone abilities, and the Kettetsu Power User. The knives originate from a cursed sword stolen five years before the series and matter heavily across the Kozuka Knives and Six Curses arcs.