Tōjū

The are rampant Zanpakutō spirits that were materialized by Muramasa, and collectively serve as the main antagonists of the anime-exclusive Beast Swords arc, after Muramasa's death.

Overview
These Zanpakutō spirits emerged from the Zanpakutō of rank-and-file Shinigami, and went rampant once their masters were killed, after failing to force them into submission. Normally, a Zanpakutō dies when its master dies, but these rampant Zanpakutō spirits suffer an emptiness at the realization that they no longer have a master, and thus no true identity, which results in an abnormality that has them exhibiting berserker rage-type behaviour.

Mayuri Kurotsuchi, Captain of the Twelfth Division, was the one to dub these rampant Zanpakutō as Tōjū and notes how they are not Zanpakutō, much less Shinigami or Hollow; in essence, Tōjū are part of an entirely-new phenomenon. He also states that a rampant Tōjū can use its Zanpakutō-based powers to its fullest capacity; essentially, a rampant Tōjū is in constant Bankai, which means that even a low-level Zanpakutō will be a force to be reckoned with.

The Tōjū first appear shortly after Muramasa's death, at the Kuchiki mansion where Byakuya Kuchiki is holding a flower-viewing festival to watch the sakura trees in full-bloom. Rukia Kuchiki is the first to encounter two of the Tōjū, and it is only with the assistance of Sode no Shirayuki and Senbonzakura that she survives the initial encounter. One of the Tōjū is killed, while the other escapes. The escaped Tōjū reappears that night and is successfully subdued by Rukia Kuchiki and given to Mayuri Kurotsuchi for study.

Jūshirō Ukitake states that the appearance of the Tōjū may be why the Zanpakutō spirits of the Captains and the Lieutenants have inexplicably reappeared, and the mastermind behind this new phenomenon has yet to be revealed. Another phenomenon is that Tōjū have demonstrated the ability to fuse with other spirit beings such as Hollows. Hitsugaya deduced that such a fusion makes the Tōjū stronger and even more difficult to deal with.

Some Tōjū have been shown to be driven by the desire to find their masters. One Tōjū who escaped into the Human World, continuously asked for his master. However, as their masters are dead, the Tōjū have no place to go and are so consumed by longing. Just the same, there exists other Tōjū that relish in the fact that, with their masters dead, they have the "freedom" to live as individuals, not as weapons of war.

When an unknown thief, suspected to be a Tōjū, was stealing supplies, Ikkaku Madarame, Nanao Ise, Hōzukimaru and Hanatarō Yamada were sent to hunt down the thief, who was last spotted in the sewers. Along the way, a tentacle monster attacked the group and captured Ikkaku; it's later revealed that the thief was Hanatarō's materialized Zanpakutō, Hisagomaru, and the tentacle monster was a Tōjū, which was soundly disintegrated by Hisagomaru's Akeiro Hisagomaru ability.

Around this time, a group of Tōjū form a 'gang' of sorts and begin raiding the Rukongai districts, murdering the civilians on sight, prompting Izuru Kira, Shūhei Hisagi, Ikkaku Madarame, and Tetsuzaemon Iba to run to the Rukongai in order to quell the raids, and spent the majority of their time hunting down, fighting against, and killing them.

During this time, Kazeshini, still wanting to kill Hisagi, had met and effortlessly killed one who, upon realizing that he wasn't on their side, had called him a "Shinigami dog". Another attacked Kazeshini from behind, but was similarly-killed by him. As with the other Tōjū before them, they were all killed off by Hisagi's group.

These Tōjū had fused themselves with the Rukongai residents, similar to the Hollow Tōjū. They all had a roughly humanoid appearance, and had virtually the same type of face: all were bald, had large red eyes, and a fairly-deranged expression on their faces.

Tōjū can also fuse with Humans as well, which allows them to conceal their reiatsu from Shinigami senses.

Given what happened with Narunosuke, the Tōjū transformation takes time to occur, and the process is shown to be quite painful.

It's revealed that Tōjū only have a limited amount of reiatsu, and once that's expended, they're reduced to their sword forms again. In an effort to stave off that inevitability, the Tōjū identified as Kirikaze, by Kazeshini, attacks other Tōjū and drains their reiatsu, growing more powerful in the process.