Zanpakutō Unknown Tales arc

The is the term used to refer to all the events that took place during the anime-exclusive story arc focusing on the rebellion by the spirits of the Shinigami Zanpakutō.

Soul Society Invasion
A mysterious man informs a group of strange beings that it is the time to strike. Ichigo Kurosaki wakes up in his inner world, where Zangetsu asks if Ichigo can hear anything, but thinks it is okay that he cannot. In Seireitei, the captains and lieutenants of Gotei 13 are confused as to why their Zanpakutō are not working properly and feel detached. At night, a group of Ryoka attempt to enter Genryūsai Shigekuni Yamamoto's office. Yamamoto and his lieutenant Chōjirō Sasakibe confront the intruders but what they see leaves them in shock. Later, Yamamoto calls all captains and lieutenants to the Sōkyoku Hill. There they are met by the mysterious man, who calls himself Muramasa. Sajin Komamura tries to attack him with his Zanpakutō, Tenken, but Tenken appears in a Human form and turns on him, ending the battle by knocking him unconscious with one swing of his blade. The other Shinigami attempt to attack, but are also unable to release their Zanpakutō. The Zanpakutō assume Human-like forms and join Muramasa, who states that the Shinigami Zanpakutō have renounced them and are declaring war.

Ichigo's Arrival
Ichigo finds Rukia Kuchiki injured in the Human World, whilst a strange woman, who uses Rukia’s abilities, is attacking her. It is revealed that this woman is in fact Rukia's Zanpakutō, Sode no Shirayuki. Ichigo successfully forces the renegade Zanpakutō into submission, and Sode no Shirayuki retreats to Soul Society. Ichigo brings Rukia to the Urahara Shop, where she reveals that the Zanpakutō spirits have attacked their masters, and that Byakuya Kuchiki was seemingly defeated by his own Zanpakutō, Senbonzakura. Yoruichi Shihōin returns from investigating Soul Society, and reveals that everyone who met with Muramasa has survived. The remaining Shinigami of the Gotei 13 have regrouped at the 4th Division headquarters while they try to re-establish their defenses and learn more about the Zanpakutō rebellion. Kisuke Urahara observes that only the Zanpakutō in Soul Society are affected and that those in the living world are unaffected, and decides to try to capture one of the materialized Zanpakutō in an attempt to learn more about their new enemy. In the middle of the night, Rukia leaves for Soul Society, apparently in search of Byakuya. Ichigo and Yoruichi leave in an attempt to go after Rukia and to save Soul Society.

The Ice Princess: Rukia Kuchiki vs Sode no Shirayuki
Rukia rushes back to the 6th Division barracks and is reunited with Renji Abarai. However, Byakuya's status remains unknown after the night of the Zanpakutō' attack. Ichigo and Yoruichi arrive in Soul Society and split up to search for clues. Rukia returns to Byakuya's compound and finds Sode no Shirayuki attacking and freezing Shinigami. She tries to reason with her Zanpakutō, but Sode no Shirayuki demands her freedom. After a fierce battle, Rukia loses to Shirayuki and is about to be frozen to death when Ichigo arrives to rescue her. However, due to his reluctance to harm Rukia's Zanpakutō, he too is defeated by Sode no Shirayuki. Rukia attempts to get the Zanpakutō to return to her by telling the Zanpakutō how important she is to her. Using her Kidō powers to restrain Sode no Shirayuki and herself, Rukia attempts to sacrifice them both in order to convince her that one could not live without the other. The resulting explosion injures Rukia severely. Sode no Shirayuki has a change of heart and attempts to reach out to her master, but Muramasa arrives and holds her back. Muramasa introduces himself to Ichigo and states that he is a Zanpakutō.

An Inner Battle Fought Outside: Ichigo Kurosaki vs Zangetsu
Muramasa sends Sode no Shirayuki away, and Ichigo asks why he wanted the Zanpakutō to gain independence over the Shinigami. Although he does not give the reason, Muramasa replies that Zanpakutō can separate themselves away regardless of being the soul of a Shinigami, and he is a good example of one. Ichigo cannot accept this view and attacks him. Muramasa uses illusions to overwhelm Ichigo and pins him to the ground, where he proceeds to drag Zangetsu out of him. Once Zangetsu manifests, he immediately attacks Ichigo. Meanwhile, Mayuri Kurotsuchi begins experimenting on himself in order to research the Zanpakutō's manifestation. After Zangetsu seemingly defeats Ichigo, Ichigo transforms into his Hollow form and strikes back, putting Zangetsu on the defensive. Muramasa tries a sneak attack but the Hollow immediately regenerates and attacks him. Before the Hollow form can do anything else, Ichigo manages to return to his Shinigami form and falls unconscious at Muramasa's feet.

Tough Heart, Tough Battle: Renji Abarai vs Zabimaru
Rukia is found by the 4th Division and sent to recover from her injuries. Shūhei Hisagi and his men patrol Seireitei and are attacked by the sadistic Kazeshini. Renji searches the ruins for any clues of the missing Byakuya, who was last seen hit by Senbonzakura's attack. He instead encounters Zabimaru in its baboon and snake humanoid form. Both entities of Zabimaru have no intention of returning to their original owner and attack Renji. According to Zabimaru, the reason it attacks is that as a sword it is its nature to attack. Kyōraku and Ukitake discuss the possibility that defeating the Zanpakutō spirits will destroy the Zanpakutō themselves. When Zabimaru threatens to kill Rukia, Renji remembers Ichigo's reason for fighting and then his own, which in turn enables him to regain his resolve, and manage to activate Zabimaru's Shikai. Zabimaru takes the advantage when they are able to activate Bankai while Renji, no longer able to control them, cannot. Renji considers his own weaknesses, and tricks Zabimaru into taking his sword, then breaks it with a Shakkahō spell and then he defeats Zabimaru by firing the broken shards of his Shikai at them.

Berserker Charge: Hollow Ichigo vs Muramasa
Ichigo wakes up in Zangetsu's dimension, but Zangetsu is missing. He sees Muramasa and attacks him, but he immediately incapacitates Ichigo and summons his Hollow form out as a separate entity. Muramasa has begun to take an interest in the Hollow form and uses his powers to freeze it in place. Thinking that it has been subdued, Muramasa approaches it but the Hollow form lashes out straight at him. Ichigo's inner Hollow fights with Muramasa inside his mind until Muramasa uses his powers to restrain him and through that Ichigo sees through Muramasa's powers and saves his Hollow. Ichigo then steps up to fight, stating that the Hollow is a part of him, too. Ichigo manages to force Muramasa outside of his mind and continues the fight with him there.

Fear and Death: Shūhei Hisagi vs Kazeshini
Hisagi fights with Kazeshini, and, disgusted by his desire to take life, plans on killing him even if he loses his Zanpakutō. Kazeshini wounds Hisagi in a surprise attack, but Izuru Kira intervenes to help him, and using Kidō, binds Kazeshini long enough for the two of them to escape.

An Unexpected Alliance: Ichigo and Hollow Ichigo vs Zangetsu - Round Two
Ichigo fights Zangetsu, and struggles when Zangetsu activates Bankai. He uses a new Getsuga Tenshō working together with his inner Hollow to defeat Zangetsu, and forces Muramasa to retreat. Zangetsu reveals that Muramasa is using a technique to awaken the Zanpakutō' instincts and by doing so, he can control them.

Surrounding the Zanpakutō
Ichigo reports his findings of Muramasa's abilities to Jūshirō Ukitake, Shunsui Kyōraku and Retsu Unohana. In order to find out about Muramasa's effects on Zanpakutō, Ichigo and Ikkaku Madarame patrol Seireitei to seek out the entities. Meanwhile, someone breaks into a compound and knocks out the Shinigami guarding the area. Marechiyo Ōmaeda and his men stumble on the unconscious guards and investigate, only to encounter his Zanpakutō spirit, Gegetsuburi and Hōzukimaru. Ōmaeda and Gegetsuburi are just about to fight when Ichigo and Ikkaku interrupt. Ōmaeda is knocked out of the fight, leaving Ichigo to battle Gegetsuburi, while Ikkaku fights Hōzukimaru. Ichigo defeats Gegetsuburi but Ikkaku is wounded by Hōzukimaru's Bankai. Just as he attempts to continue the battle, Ukitake, Shunsui, Soifon and her Stealth Corps arrive to surround Hōzukimaru, but he uses his Bankai to get away. However, they manage to obtain Gegetsuburi. Ichigo goes after Hōzukimaru, but the entity escapes when Senbonzakura's petals block Ichigo's route. Ichigo discovers that the attack was from Byakuya, who disappears. Ichigo continues searching for Byakuya, and encounters the spirits of Tobiume and Haineko, who are looking for Hyōrinmaru. Momo Hinamori and Rangiku Matsumoto arrive to fight their Zanpakutō while Ichigo continues the search. Momo uses a Kidō spell to knock Tobiume into Haineko, causing the two to begin arguing. Ichigo encounters Hyōrinmaru, who cannot remember his name or his master. He attempts to freeze Ichigo and leave, but Tōshirō Hitsugaya confronts him. Elsewhere, Muramasa encounters a large group of Menos Grande.

Where he Belongs: Tōshirō Hitsugaya vs Hyōrinmaru
Ichigo is still trapped in the ice prison Hyōrinmaru imprisoned him in as Tōshirō and Hyōrinmaru confront each other. Meanwhile, Momo and Rangiku fight their Zanpakutō in the woods and finally manage to overpower them with kidō spells. As Hyōrinmaru and Tōshirō begin to fight, Tōshirō tries to seal him but is shocked as Hyōrinmaru effortlessly dissolves the seal into fragments of ice. As their fight continues, Hyōrinmaru's tremendous power turns the whole of Soul Society's sky black. Tōshirō began to recall his past during his first encounter with Hyōrinmaru to convince him. Tōshirō managed to activate Hyōrinmaru's Shikai, thus managed to convince Hyōrinmaru that Tōshirō is his master. Despite freezing himself along with his Zanpakutō and losing consciousness, Tōshirō manages to convince Hyōrinmaru of his identity.

Byakuya's Betrayal
Mayuri experiments on Gegetsuburi, and discovers that the Zanpakutō only return to their owners' control if they are defeated by their owners. Momo and Rangiku send their Zanpakutō back to the 4th division barracks, but an unknown assailant attacks the cart carrying them. Ichigo meets with Momo and Rangiku, and then reaches Muramasa, who is having difficulties using his abilities in his current body. Ichigo attempts to attack Muramasa but is stopped by Senbonzakura. Ichigo fights Senbonzakura, and gains the upper hand before Byakuya arrives. Byakuya reveals that he has joined Muramasa as part of following his internal desire to protect his pride, and then disappears with Muramasa and Senbonzakura.

The Shinigami discuss Byakuya's betrayal, and the discovery that if a Zanpakutō is killed by anyone other than its owner, it will be unusable. Byakuya returns to the Zanpakutō's hideout, and proves his loyalty by defeating and seemingly killing Sode no Shirayuki, leaving Rukia unable to use her Zanpakutō. Rukia and Renji investigate Byakuya's office, but Byakuya arrives with Senbonzakura, tossing Rukia the broken remains of her Zanpakutō. Zabimaru arrives to help Renji fight Byakuya and Senbonzakura, but they are overpowered and several Zanpakutō, including Tenken, Hōzukimaru, Kazeshini, Ruri'iro Kujaku and Gonryōmaru, arrive. The Second Division surrounds the Zanpakutō, the Zanpakutō's owners come out to fight them, and Ichigo confronts Byakuya.

Shinigami and Zanpakutō, Total Sortie
Ichigo goes to confront Byakuya, but Senbonzakura intercepts him, and he and Ichigo fight each other with their Bankais. Renji and Zabimaru move to prevent Byakuya from leaving. Izuru begins fighting Kazeshini, suspecting that its personality is based on Hisagi's suppressed negative impulses. Soifon fights Gonryōmaru and Tenken at once, but is surprised by a smoke attack. Yumichika Ayasegawa faces Ruri'iro Kujaku, and the two exchange insults. Ikkaku fights Hōzukimaru, and although Hōzukimaru unleashes his Bankai, Ikkaku is able to break it by repeatedly attacking, and manages to withstand its final attack and claim victory. Fourth Division lieutenant Isane Kotetsu and Seventh Division lieutenant Tetsuzaemon Iba go to assist the wounded at the scene of the battle when Ashisogi Jizō appears.

Soifon fights Tenken and Gonryōmaru, making some progress, but then her Zanpakutō, Suzumebachi appears and attacks her. Ruri'iro Kujaku gains the advantage over Yumichika, and begins to use its special ability on him. Iba and Isane attempt to retreat from Ashisogi Jizō, but Tobiume and Haineko arrive and Ashisogi Jizō paralyzes Isane's legs. Rangiku and Momo then return to help them against their opponents. Izuru manages to capture Kazeshini by tricking him into thinking he could still use his Zanpakutō in order to unsettle him, but is soon attacked by his own Zanpakutō, Wabisuke. Byakuya manages to defeat Renji and Zabimaru. Ichigo is desperately fighting Senbonzakura when Kenpachi arrives.

Wabisuke is about to finish off Izuru when Kenpachi appears. The Zanpakutō turns to strike him but is killed off by Kenpachi instead, leaving Izuru unable to use Wabisuke. Yachiru appears as well and tells the Shinigami present that she and Kenpachi were off hunting Hollows and got lost on the way back, providing an explanation for their disappearance throughout the crisis. Senbonzakura takes on Kenpachi but is overwhelmed by the latter's enormous spiritual power, which destroys a wide radius around them. Byakuya intervenes and fights Kenpachi while Senbonzakura resumes his fight with Ichigo. Yumichika breaks out of Ruri'iro Kujaku's binding vines and defeats him, reclaiming his Zanpakutō.

Ichigo continues fighting Senbonzakura while Kenpachi fights Byakuya, but Ashisogi Jizō appears and using its Bankai, spraying poison gas over the battlefield, enabling Senbonzakura and Byakuya to escape. Yachiru chases Ashisogi Jizō and he gets captured by Zaraki. Mayuri blows up Ashisogi Jizō, causing it to return sealed form and break. Soifon has difficulty hitting the small Suzumebachi, but manages to defeat her, and Yoruichi congratulates her. Unohana arrives to treat the victims of Ashisogi Jizō's poisoning, including Momo, Isane, Rangiku and Iba. Yoruichi meets with Ichigo and the captains, and reveals that she knows Yamamoto's location. Yamamoto is then shown as a prisoner of the Zanpakutō, with Sōgyo no Kotowari, Katen Kyōkotsu, and Minazuki encasing him in a barrier.

Flowers and Waves: Shunsui Kyōraku and Jūshirō Ukitake vs Katen Kyōkotsu and Sōgyo no Kotowari
The Captains begin planning on how they are going to rescue Yamamoto, knowing that they are going to run into high class Zanpakutō. Meanwhile Kazeshini comes back to the hideout and faints from exhaustion. Kenpachi, Ukitake, and Kyōraku head into the cave while Yoruichi and Ichigo head in another way. In the cave, Ukitake and Kyōraku meet their materialized Zanpakutō. Zaraki and Yachiru get lost in the caves, and come across Gonryōmaru and Tenken. In the meantime, Ichigo and Yoruichi encounter Haineko and Tobiume, and both are defeated by Yoruichi. Ichigo heads on, and is lead into Kazeshini by Minazuki. Kazeshini is easily defeated, breaking, as Ichigo moves on to find the Captain-Commander.

Muramasa stands in Ichigo's way of rescuing Yamamoto, and goads him into releasing his Hollow powers against him. Meanwhile, Yoruichi, Ukitake and Shunsui realise that something is amiss and break away from their opponents. Having used the Zanpakutō entities to distract Ichigo's companions, Muramasa tricks Ichigo into firing his most powerful Getsuga Tenshō at him, but deflects the attack into Yamamoto's barrier, shattering it just as Yoruichi and the rest arrive to prevent this from happening. Muramasa reveals that Yamamoto encased himself in the barrier to prevent him from stealing Ryūjin Jakka. But with Ichigo's Hollow-based attack, it allows Muramasa to easily obtain Yamamoto's Zanpakutō entity. When Muramasa says that he has no need for the other entities anymore, Katen Kyōkotsu and Sōgyo no Kotowari attack him but end up having their physical forms destroyed by Ryūjin Jakka. Muramasa makes his escape.

Escaping Ryūjin Jakka
Ichigo, Yoruichi, Ukitake, Shunsui, and Yamamoto are surrounded by a wall of flames created by Ryūjin Jakka. Yoruichi escapes underground through a fissure she opens using Shunkō, but Tobiume uses her fireball to cave it in behind Yoruichi, preventing the others from following. Ichigo attempts to escape by jumping over the wall of flames but is unsuccessful. Yamamoto reveals that Muramasa's plan is to destroy Karakura Town in order to free his owner, Kōga, who has been sealed there. Upon arriving at Karakura town, Muramasa stumbles upon Inoue and blacks out. After waking up and asking Orihime Inoue who she is, Uryū Ishida and Yasutora Sado arrive to tackle Muramasa. Ichigo makes a final attempt at jumping over the wall of flames and just before he is defeated and completely enveloped in fire, Hyōrinmaru and Hitsugaya arrive to save him. Hitsugaya and Hyōrinmaru both activate their Bankai and, together, get Ichigo out of the flames so that he can head to Karakura Town to stop Muramasa.

Unsealing Muramasa's Master
Kenpachi continues his fight against Tenken. Yoruichi outmaneuvers Haineko and Tobiume while trying to convince them that Muramasa betrayed them. Yamamoto, Ukitake, Kyōraku, Hitsugaya and Hyōrinmaru plan to defeat Ryūjin Jakka together. Uryū and Chad arrive to fight Muramasa, but he evades all their attacks and Senbonzakura arrives to help him. Muramasa tries to use his ability on Chad, and incapacitates him due to his not being a Shinigami. Orihime asks Muramasa about his motive, and Muramasa reveals that he wishes to be free of solitude. Rukia arrives as Muramasa tries to escape to unseal Kōga. Uryū continues to fight Senbonzakura, managing to hit it with Seele Schneider, but it is still able to keep fighting. Muramasa manages to unseal Kōga as Ichigo arrives to face him.

Turn Back Time: Kōga and Muramasa
In a flashback several centuries ago, it is revealed that Kōga was part of the Kuchiki Clan as Ginrei Kuchiki's son-in-law, and was overconfident in the abilities of his Zanpakutō. He helped put down a rebellion of Shinigami, but some grew suspicious of his power, and had him attacked. He cut down his attackers, and was imprisoned as a result. In the present, Kōga awakens, repelling Ichigo and Rukia, then suddenly stabing Muramasa.

Trivia

 * Tite Kubo involved himself in the creative thinking of this arc, particularly when it came to the appearance of the Zanpakutō spirits themselves.
 * Despite Muramasa claiming to have released all the awakened Zanpakutō spirits in Soul Society, several Zanpakutō spirits have yet to be seen (ex: Isane Kotetsu's Itegumo).