Tōjū

The are rampant Zanpakutō spirits that were materialized by Muramasa.

These Zanpakutō spirits emerged from the Zanpakutō of mere rank-and-file Shinigami, and went rampant after their masters were killed after failing to force them into submission.

Normally, a Zanpakutō dies when its master, but these rampant Zanpakutō spirits appear to be suffering from an abnormal composition and do not seem to exhibit any type of behavior save that of a berserker rage.

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; 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. Ukitake Jūshiro states that the appearance of the Tōjū is why the Zanpakutō spirits have inexplicably reappeared.

The mastermind behind this new phenomenon has yet to be revealed.

History
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.

But the other Tōjū reappears that night, after a party that was supposed to lure it out, and is successfully subdued by Rukia Kuchiki and given to Mayuri Kurotsuchi for study.