Bakkōtō

Bakkōtō are a rare type of Zanpakutō used by Gyōkaku Kumoi's assassins. This type of weapon differs from the Zanpakutō in several ways. While the Zanpakutō is a symbiotic being, working and growing more powerful as the shinigami develops a relationship with the sword's inner avatar, the Bakkōtō is more of a parasite, gradually consuming the user as it grows in power, until it completely devours him. Also, the Bakkōtō do not have any sort of known release states, instead evolving with the user's reiatsu. Finally, while Zanpakutō can only be wielded by their respective Shinigami, a Bakkōtō can be wielded by any person with reiatsu and varies in power and ability depending on its wielder. It is unknown how a Bakkōtō is formed, however, it is known that Bakkōtō are forged through a secret technique of the Kasumiōji clan. Rukia noted that the Bakkōtō are highly illegal. The eye is shown to be the nucleus of a Bakkōtō.

Saiga
It has the ability to momentarily paralyze opponents by reflecting moonlight into their eyes and can create an alternate dimension in which the opponent is left conscious but unable to move. If the user channels more reiatsu into the weapon, he/she can create illusions based on the targets' memories, trapping them until their souls are crushed. At full power, it is able to shoot energy bolts similar to getsuga tenshō blasts. Saiga was first used by an unnamed assassin, and then by Hanza Nukui against Ichigo Kurosaki.

Kakuyoku (Horned Wings)
Kakuyoku appears as an oversized guandao. When swung, it fires energy blasts which solidify into rock walls. It also allows the user to merge with the walls and hide for a surprise attack. Kakuyoku was used by an unnamed assassin, and then by Genba against Chad.

Retsurai (Furious Thunder)
Retsurai takes the form of a "reaper" scythe, with protrusions over the edges. When eatten it causes blades to form on the user's back which can be fired at will, if eatten again it will refill/add to the blades. It will still keep its normal form, but with bite marks on it. It also seems to increase the user's speed tremendously, and cause spikes to protrude out of the user's body. Retsurai was used by Jinnai Doukou against Rukia Kuchiki.

Shiragiri (Pale Mist)
Shiragiri takes the form of a "normal" katana with protrusions along the edge of the blade. Shiragiri has the ability to create mist, and turn the user into mist as well and solidify so it can attack. Shiragiri was used by Kuzuryuu against Uryū Ishida.

Makoto Kibune
The nucleus of Kibune's bakkōtō grows on his left arm, while the rest of it covers his zanpakutō, allowing him to telepathically control the blade at any direction and spin it like a saw blade.

Shūsuke Amagai
Amagai's bakkōtō takes the form of a tuning fork with the nucleus at the hilt. When used it emits large waves of reiatsu powerful enough to destroy one of the dimensional cleaners in the dangai. Its true form, however, is shown where it covers the right arm and shoulder in black armor, when in use it extends down and forms a blade of highly concentrated reiatsu. Its true ability is the negation of all other zanpakutōs except Amagai's.