Kakuyoku

The Bakkōtō are a forbidden class of weapon carried by the assassins in Gyōkaku Kumoi's service, and Kakuyoku is one of the named examples. Where a Zanpakutō grows alongside a Shinigami as a willing partner, a Bakkōtō behaves like a parasite: it strengthens by draining its bearer's Reiatsu and, without enough spiritual power to keep it in check, eventually devours the wielder's mind and body. Rukia Kuchiki notes that such arms are strictly illegal.

Every Bakkōtō carries a living organic core, an eye that acts as its nucleus fused to a fleshy mass that spreads across the user as it is fed more energy; an ordinary weapon is then attached onto that flesh, and the whole thing can be made stronger by absorbing the nuclei of others. These arms are produced through a technique kept secret by the Kasumiōji Clan and can be taken up by anyone possessing Reishi, not Shinigami alone, so their potency shifts with whoever holds them. Kakuyoku itself takes the form of an outsized guandao; swinging it looses energy bolts that harden into walls of rock, and the wielder can sink into those walls to lie in wait and strike by surprise.

Kakuyoku is wielded by Genga, one of the assassins fielded during the Kasumiōji conspiracy, and an unnamed assassin is also linked to it. Like the rest of the Bakkōtō, it features only in the anime.