Shiragiri

Shiragiri belongs to the Bakkōtō, a scarce and thoroughly illegal breed of weapon wielded by the assassins serving Gyōkaku Kumoi. The Bakkōtō appear only in the anime and are produced through a secret craft guarded by the Kasumiōji Clan, and Rukia Kuchiki flatly brands them as contraband.

A Bakkōtō is nothing like a Zanpakutō. Where a Zanpakutō is a partner that grows stronger as its bond with a Shinigami deepens, a Bakkōtō is a parasite that feeds on its bearer, swelling in power while slowly consuming them, clouding the mind and finally devouring the wielder outright. Only a sufficiently strong spiritual power can keep that hunger in check. It carries no release states, evolving instead in step with the user's Reiatsu, and unlike a Zanpakutō it answers to anyone possessed of Reishi, its strength and abilities shifting from one wielder to the next. Each takes the shape of a weapon fused with an organic core, an eye that acts as its nucleus set amid a mass of flesh resembling a mollusk's innards, which spreads across the user's body as it is fed more Reiatsu. A Bakkōtō can also be strengthened by consuming the nuclei of others of its kind.

The Bakkōtō, Shiragiri among them, are the tools of Gyōkaku Kumoi's assassins, forged by the Kasumiōji Clan. Because the weapon will serve any being carrying Reishi, its potency rests entirely on whoever takes it up.