Originally Posted by Wikipedia
A rare type of weapon known as a bakkoutou has also been revealed. 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 Bakkoutou is more of a parasite, gradually consuming the user. Also, the bakkoutou differ as they lack the release forms (shikai and bankai) of a zanpakutō, instead growing and evolving as it feeds on the wielder’s spirit energy. Finally, while zanpakutō can only be wielded by their respective Shinigami, a bakkoutou can be wielded by any person with Spiritual Pressure and varies its power and ablity depending on its wielder. For example, the bakkoutou Saiga is used by more than one character. When it is first introduced, it is wielded by a common assassin and has the ability to momentarily paralyze opponents by reflecting light from its mirror in their eyes. However, when it is merged with someone more powerful, such as Nukui Hanza, it gains the ability to completely immobilize an opponent in its own dimension. Furthermore, if it is at full power (in which case it engulfs its wielder's whole upper body), it can create "nightmares" from the memories of the victim, even when its mirror is shattered. It is unknown how a bakkoutou is formed since a Soul Reaper's zanpakutō is allegedly formed when a spirit becomes a shinigami. It is, however, known that bakkoutou are forged through a secret technique of the Kasumiōji clan.