Special One-Shot

Created by Koyoharu Gotoge before the release of the Mugen Train movie and later folded into the second official fanbook, this one-shot is sometimes known as Rengoku Chapter Zero. It dramatizes Kyojuro Rengoku's debut assignment as a slayer, sending him against a demon notorious for using a flute to destroy his prey. Kyojuro reflects on duty and sacrifice before earning the win that secures his confidence as a hunter.

Leaving for the first mission

Shinjuro Rengoku dismisses both his sons as weak and warns Kyojuro not to chase impossible dreams, insisting he cannot inherit the Flame Hashira position. Undeterred, Kyojuro tells his brother Senjuro he is heading out and asks him to guard their home, recalling fellow trainees who looked up to him and wished to protect others from losing family to demons.

The flute demon

Kyojuro finds the demon seated among the corpses of nine slayers, several with their insides devoured. The demon plays a flute that summons dogs and scrambles the nervous system, claiming a single note can undo a slayer's training. Believing Kyojuro cannot fight while covering his ears, the demon presses his attack.

Victory through sacrifice

Surviving comrades arrive to help, and Kyojuro unleashes Flame Breathing, Fifth Form, beheading the demon. The dying demon learns Kyojuro had ruptured his own eardrums to nullify the flute, and that his fallen allies had used sign language to warn him of the ability. The demon disintegrates, furious at failing to join the Twelve Kizuki, while Kyojuro comforts a surviving child.

The chapter ran in Weekly Shonen Jump in October 2020 and was packaged into Rengoku Volume 0. As a prequel to the Mugen Train arc, it grounds Kyojuro's later heroism in his earliest victory, showing both his willingness to maim himself to win and the loyalty he inspires in those around him.