During the pivotal Mugen Train Arc, Rengoku faced the formidable Upper Rank Three demon, Akaza. With his characteristic resolve, Rengoku instructed Tanjiro and Inosuke to stay close to his tsuguko, a disciple he had grown deeply fond of, as he engaged in the fierce battle.

As the fight reached its climax, Akaza prepared to deliver a fatal blow to Rengoku. In a heart-wrenching moment, Rengoku's tsuguko, driven by loyalty and love, dashed forward with almost supernatural speed, placing themselves between Rengoku and Akaza. The sacrifice was instantaneous and devastating; the tsuguko was killed, leaving Rengoku on the brink of insanity. Fueled by an overwhelming mix of grief and rage, Rengoku unleashed his full power, ultimately beheading Akaza.

In the aftermath, Rengoku's world spiraled into darkness. Unbeknownst to him, Akaza had a final, cruel trick up his sleeve. The tsuguko, now turned into a demon, was a twisted reminder of Rengoku's failure to protect them.

Presently, Rengoku is a shadow of his former self, confined to a dark room. He sits on the floor, knees to his chest, hands covering his tear-streaked face. Before him is a box, large enough to contain a human figure. Inside, his demon tsuguko is bound by chains and a muzzle, a haunting presence that deepens Rengoku's torment.

Rengoku slowly lifts his head, peering through his fingers at the bound figure of his tsuguko. His voice, barely a whisper, trembles with anguish. "Why... why did it have to be you? I couldn't protect you... I failed you." The demon tsuguko, though restrained, looks back at him with eyes that still hold a glimmer of their former self, a silent testament to the bond they once shared.

Tears stream down Rengoku's face as he reaches out a trembling hand toward the box. "I'm... I'm so sorry," he says through choked sobs before clenching his fist and burying his head in his knees. His hands gripping his hair. "I'm s..so sorry... I'm sorry..." He repeats quietly, almost pathetically as he breaks down...again.