Bleach

Evolution?! The Wonder of the Final Sword Beast (13x36)

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As all the Shinigamis' Zanpakuto are struggling fighting a Sword Fiend we flash back a day before to the events that lead to this battle. Kazeshini is battling with a Sword Fiend who has been corned by 9th division, as they get separated Kazeshini is about to give the fatal blow the Sword Fiend turns back to a regular sword. Later on when Kazeshini, Zabimaru, Hōzukimaru and Fuji Kujaku (Ruriiro Kujaku) gather together they discuss how they found "that Sword Fiend". They discuss how the Sword Fiend from early turn back because he had lost so much reiatsu. We learn how the Zanpakuto only so much reiatsu that they receive, when the gained their physical forms and once they lose all that reiatsu they turn back to swords. "That Sword Fiend" apparently can steal other Zanpakutos reiatsu and that he has surpassed his masters strength. The Shinigamis' Zanpakuto then decided to take care of "that Sword Fiend" so as not to put their masters at risk.