Short Summary
A short filler arc that takes place after the Zanpakuto Rebellion Arc. Despite Muramasa now gone, the Zanpakutos still remain in their corporal human forms for some reason. While Mayuri tries to figure it out, owner-less Zanpakutos, dubbed Beast Swords (Toju in the Japanese version), are running rampant through both Soul Society and the human world, forcing the protagonists to track them down.
Main Summary
Muramasa is dead, and his mind control is gone. But the zanpakuto he manifested still remain, though on better terms with their wielders now. Everyone's relaxing and rebuilding Seireitei when a new Muramasa-related problem occurs. While all the named characters were able to defeat their Zanpakuto or at least survive until Mayuri broke Muramasa's hold over them, offscreen, not everyone was so lucky. Many shinigami were killed by their own swords. These zanpakuto spirits are still manifested, but with their masters dead, they're going insane. And they're constantly in Bankai mode even if their wielder didn't have it before their death, because of course this couldn't be that easy. So, while trying to get back on their feet after Muramasa, the shinigami and zanpakuto have to hunt down these masterless zanpakuto, which Mayuri dubs 'Beast Swords'.
Rukia and Sode no Shirayuki catch one after a drinking party (which was a failed attempt to lure it out), and in the living world, Ichigo discovers they can get more dangerous than usual when one fuses with a Hollow before Hitsugaya destroys it. Zabimaru gets some Character Development while chasing after a Beast Sword, as Chimpette shows that she does care about Snakey and Snakey learns how much he values Chimpette and Renji. Ikakku, Hozukimaru, Hanataro, and Nanao discover Hisogumaru, Hanataro's zanpakuto, down in Soul Society's sewers, and incidentally solve the problem of who was stealing stuff from shinigami.
Kazeshini also gets himself some character development, as he learns to care for a baby he finds in Rukongai. Haineko befriends a Beast Sword called Narunosuke, but is forced to put him down when he finally goes insane from his master's death. Senbonzakura and Zabimaru find themselves in a spot of trouble in the 12th division barracks, and Nanao has some cute moments with the ninja half of Katen Kyokotsu.
All these shinigami-zanpakuto interactions have been fun, but alas, this is a filler arc, so they can't stay. One final Beast Sword, Kirikaze, has started absorbing the power of its fellows to strengthen itself, and it's too much for the zanpakuto or the shinigami to fight on their own. Eventually, the zanpakuto sacrifice their manifested forms to allow Renji to destroy Kirikaze. They're still with their Shinigami as swords, but there'll be no more running around Seireitei.
Beast Swords Arc has the following tropes
- A Day in the Limelight: The arc more or less just gives some screen time to the Zanpakutos in a less antagonistic light.
- Breather Episode: The majority of the episodes are just Slice of Life ones about the Zanpakuto spirits interacting with their owners and figures in the real world, coming down off the action-citric previous arc.
- Bittersweet Ending: In the final battle, the Zanpakutos give up their human forms to help their owners.
- Morality Pet: Kazeshini learns how to protect life when he finds a baby whose father was killed by a Beast Sword.
- Out of Focus: Ichigo only shows up in a handful of episodes.
- Tragic Villain: Naranosuke is actually quite nice, but as a Beast Sword, his sanity inevitably goes downhill and Haineko has to put him down.
- What Does This Button Do?: When confronted with a surveillance system of Seireitei, Senbonzakura starts pushing buttons at random and causing various misfortunes to befall other shinigami (Renji gets his bathwater superheated, Omaeda's barbecue explodes on him, the treadmills Matsumoto and Haineko are on suddenly stop, and so on).
