Dark History! The Worst Shinigami is Born

In the distant past, the three officials who framed Kōga Kuchiki gloat over his imprisonment until he appears in their doorway with Muramasa at his side and cuts them all down. Word of the jailbreak reaches Captain Ginrei Kuchiki, who confronts his kinsman outside an abandoned building. Kōga admits he slew the men because they betrayed him on purpose and laughed at what they had done, yet the killing left the hollow feeling untouched. Ginrei warns that by striking down unarmed men, Kōga has thrown away any chance of clearing his name and now faces execution.

Convinced that Ginrei secretly despises him and envies his strength, Kōga turns on his captain, only for his released blade to pass harmlessly through a Reiatsu barrier. He severs the hair from his kenseikan, hurls the ornament at Ginrei, and departs to keep killing. A rampage follows: Kōga slaughters dozens of Shinigami and later butchers innocents in the Rukongai, telling Muramasa he means to broadcast Soul Society's cruelty and his own supremacy. When Muramasa gently urges him to rest and to look forward, Kōga seizes his collar, dismisses him as a mere sword, and beats him.

Ginrei explains to the assembled captains that Kōga's illusion-type Zanpakutō can only be resisted by someone with immense Reiryoku and an iron will who seals their heart the instant their blade is seized. Yamamoto volunteers to help enact the sealing. In the rain, Kōga rants that despair unlocks his true power and that might alone defines right, dismissing Ginrei's warnings. During the ensuing clash, Kōga's release inexplicably fails, and Ginrei binds him with Hainawa while four halberds pin his limbs. Yamamoto invokes Shisō Keppū, and darkness engulfs Kōga into a chained, halberd-pierced coffin.

In the present, Muramasa reels as Kōga blames him for never answering his calls during the sealing. Muramasa insists he never heard a single word and had labored ever since to break the seal, reaching out in desperation, but Kōga runs him through and kicks him aside, branding him a conceited tool. As Kōga raises his blade to finish his own spirit, Byakuya Kuchiki intercepts the strike, declaring that a man who calls his Zanpakutō a tool has no right to the name Shinigami.

Kōga Kuchiki murders the three officials who framed him, then turns against Ginrei Kuchiki after convincing himself that everyone envies and hates him. A killing spree across the Seireitei and Rukongai follows. Ginrei outlines the sole defense against Kōga's illusions, and Yamamoto agrees to help seal him. When Kōga's release fails during their battle, the two elders bind and seal him inside a chained coffin. In the present day, Kōga stabs Muramasa for supposedly abandoning him, and Byakuya blocks the finishing blow, condemning Kōga's contempt for his own spirit. A closing segment shows a young Byakuya learning about the kenseikan from Ginrei.

This installment is an anime-only episode within the Zanpakutō Rebellion arc, tied to no manga chapters. It expands Kōga Kuchiki's backstory, first hinted at earlier in the arc, and frames his downfall as a spiral of paranoia and pride that severed his bond with Muramasa. The Shinigami Illustrated Picture Book short features Ginrei promising to teach Byakuya about the kenseikan before slipping away with Shunpo without revealing where he wears his own.