The zanpakutou, "Soul Slayer or Soul Cutter", is a Shinigami's primary weapon. The zanpakutou is a reflection of the Shinigami's skill and self. All zanpakutous have a unique name and spirit that dwells in the owners inner world. Most zanpakutous remain in sealed state also known as an "Asauchi". Most low-class shinigami wield these swords because they are not yet able to summon open the zanpakutou's name. .
To release the seal from the zanpakutou, the user must say a command and the zanpakutou's name. There are two levels of soul release for the zanpakutou. When released, the the zanpaktutou takes upon a new form and may have several abilities. The first release is the "Initial Release" and the second release is the "Ban Release". When the ban release occurs the owners power increases five to ten times.
In order to maximize one's strength the user and the zanpakutou must be able to share one anothers powers and must trust each other. To be able to reach the rank of Captain one must be able to perform the initial release and the ban release. The only exception to this is 11th division captain, Zaraki Kenpachi