GENERAL
The Tōjū (刀獣, Sword Beasts) are rampant Zanpakutō Spirits that were materialized by Muramasa.
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 behavior.
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 Captain 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 and given to Mayuri for study. Captain 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 although Ukitake theorized that someone had masterminded this incident with the Tōjū, as there was no such revelation by the end of the arc's run, it seems that this phenomenon was only a side effect from Muramasa's influence.
Tōjū have demonstrated the ability to fuse with other beings, presumably as a means to preserve or prolong their existence. Such as a fusion with Hollows, as Captain Tōshirō Hitsugaya deduced, grants the Tōjū a substantial boost in power and merges their abilities with that of the possessed Hollow, making them far more difficult for the Shinigami to contend with. A fusion with Rukongai residents, however, seems to disfigure and derange the peasant in question, rendering them bald, red-eyed, mindless and near-identical in appearance, implying that either the Tōjū in particular were already in their berserker state or accelerated the rampancy transformation with their fusion. This also weakens the Tōjū's power level and limits their combat capabilities, as the possessed peasants were only shown to have regular weapons that they swung in an erratic fashion, and could be killed without much difficulty by seated Shinigami. Possessing a Human, however, allows Tōjū to conceal their Reiatsu from Shinigami senses, synching their souls with one another, thereby enabling the Tōjū to possess their body and materialize their own form in the place of the Humans', even telepathically conversing with the Human in much the same way a Zanpakutō would to its Shinigami.
Not every master-less Zanpakutō immediately become rage-filled berserkers upon the death of their Shinigami, and would retain their identity for a time, although given what happened with Narunosuke, the absence of a master and inability to preserve their existence causes most Tōjū to gradually and painfully transform into mindless beasts. However, it appears that Tōjū can briefly regain their sanity when shown compassion or through bonding with another, at least in Narunosuke's case.
Some Tōjū 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 no longer have an identity 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.
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. Eventually, with Kirikaze's death and the Zanpakutō all returning to their sword forms, the threat of the Tōjū is eliminated.
