At the center of Infinity Castle, the main hall was shrouded in pitch-black mist. Kibutsuji Muzan floated in midair, his crimson eyes sweeping over the demons assembled below.
To the left stood Kokushibo, gripping his long blade; to the right, Akaza stood with his fists clenched. Tomioka Giyu had just stepped through the doors, clad in a new black haori.
"Mushiki."
Muzan's voice echoed through the hall—emotionless, but laced with faint satisfaction.
"The Blood Battle of Position Exchange. You won."
Before the sound of his words could fade, Muzan's right hand extended. His fingers morphed into black claws that pierced straight into Giyu's back.
A violent wave of pain tore through his body—followed by an overwhelming surge of power.
Muzan's blood flowed into him like molten lava, racing through his veins and flooding every muscle and bone. His body felt as if it were being reforged, reshaped from within by that infernal power.
Giyu gritted his teeth, not uttering a single sound.
He could feel it—his demonic energy swelling uncontrollably, his already immense strength deepening into something nearly boundless. His Blood Demon Art, Water Vortex Domain, pulsed with new potential; he knew instinctively that with a single thought, he could summon water dragons even stronger than before.
Moments later, Muzan withdrew his claws. The wound on Giyu's back closed instantly, leaving no trace behind.
He lifted his gaze to the mirrored wall of the hall. In the reflection, the kanji for Upper Moon Six in his dark-blue vertical pupils slowly faded—replaced by two new characters: Upper Moon Two.
His rank now stood just below Kokushibo.
"The position of Upper Moon Six is empty."
Muzan's voice came again, heavy with contempt.
"Those Lower Moons are worthless. They can't even kill a single Hashira, let alone deserve promotion. From now on, you'll find me a new Upper Moon Six. My only requirement—strong enough, and absolutely loyal to me."
"Yes."
Giyu bowed respectfully, though his mind was already turning rapidly.
Being tasked to recruit meant one thing—freedom to move outside Infinity Castle. That would give him opportunities to contact the outside world, pass along information, and obtain Tamayo's medicine.
The meeting ended quickly. Akaza was the first to leave, clearly eager to hunt a Hashira and prove his strength.
Kokushibo turned next, walking toward the exit with steady steps. His long blade swayed slightly in his grasp, glinting coldly in the dim light.
"Lord Kokushibo, please wait."
Giyu's voice broke the silence, calm but deliberate.
Kokushibo paused and turned slowly. Six eyes locked onto Giyu, radiating quiet pressure.
"Do you intend to challenge me to a Blood Battle for my position?" he asked, voice low and steady. His grip on the hilt of his sword tightened slightly.
"I suggest you think carefully," he added. "Douma's power was nothing compared to mine."
Giyu shook his head, his gaze filled with something he had rehearsed carefully—a sincere, burning desire for mastery.
"I wish to learn your swordsmanship," he said. "You've trained from the Sengoku era to the present. You've long surpassed the limits of mortal swordsmen."
Kokushibo's six eyes narrowed.
For an instant, a long-buried memory surfaced in his mind. Centuries ago, when he was still Tsugikuni Michikatsu—a man, a brother—he had cast away everything to chase his younger brother Yoriichi's shadow. His wife, his children, his humanity itself—all abandoned for the sake of surpassing the one who stood at the peak of swordsmanship.
And now, the look in Giyu's eyes mirrored that same obsession.
Kokushibo was silent for a while. Then his hand relaxed slightly on the hilt of his blade.
"Come with me," he said.
He led Giyu through the labyrinthine corridors of Infinity Castle to a hidden stone chamber—his personal dojo.
The room was barren, its only feature a massive bronze mirror reflecting the flicker of dim lanterns. The floor was carved with countless blade marks, the scars of centuries of relentless training. In one corner lay dozens of broken swords, relics of weapons that had failed to endure his power.
"Your Sun Breathing," Kokushibo said at last, his tone calm yet edged with faint disdain, "is incomplete."
Kokushibo turned toward Giyu, his six eyes fixed on him as he spoke with certainty.
"I once witnessed Yoriichi use Sun Breathing. The precision of your forms, the way your breath flows—it all falls short of the original Sun Breathing."
Giyu's heart trembled slightly—so he did know what the original looked like.
Feigning surprise, he said, "You've seen the very first Sun Breathing? The style I learned came from an old dance scroll. Could it be that the scroll itself is incomplete?"
Hearing the formal tone, Kokushibo interrupted him. "You don't need to use honorifics with me. Just call my name."
Then he answered calmly, "Most likely, yes."
He stepped to the center of the stone chamber and lifted his long sword.
"Sun Breathing is the origin of all other Breathing styles. Moon Breathing is my own creation—born from Sun Breathing, but adapted to suit my body. You've mastered Water, Wind, Flame, and Sun Breathing. If you can learn Moon Breathing as well, you may be able to compare them—and reconstruct the complete form of Sun Breathing."
With that, Kokushibo swung his blade. There was no wasted motion. The air split with a sharp sound, pale-blue moonlight tracing across the edge.
A chilling aura spread through the chamber. "Moon Breathing, First Form: Dark Moon, Evening Palace!"
The blade flashed like a ghost under moonlight, the trajectory twisting unpredictably, too fast for Giyu to see clearly.
This was Moon Breathing—the strongest style aside from Sun Breathing itself, perfected through hundreds of years of Kokushibo's refinement.
Giyu watched with total focus, memorizing every motion, every shift of breath and energy.
He knew now why the Kamado family's Sun Breathing had become incomplete—it had been transformed into a dance by their ancestor. Though most sword forms were preserved, the finer details of breathing control and power focus were lost. For instance, the starting stance of Sun Breathing: Dance—Tanjuro only taught the rotation, never explaining that the power must come from the abdomen to guide the entire flow of breath.
Moon Breathing, being born from Sun Breathing, shared many similarities in its control of rhythm and breath.
If he could master Moon Breathing and combine it with the four other styles he had already learned, he might truly restore Sun Breathing to its original form—the power once used to stand against Muzan.
"Did you see clearly?"
Kokushibo lowered his sword, his six eyes steady on Giyu.
"From today onward, you will come here at this time every day to practice. I'll teach you every form of Moon Breathing and point out the flaws in your other styles."
Giyu met his gaze calmly. "Thank you for your guidance."
Kokushibo didn't respond. He turned back toward the massive bronze mirror, his expression unreadable.
Centuries had passed since he last stood alongside an equal. Since Yoriichi's death, no one had reached his level of swordsmanship. But now, seeing Giyu's focus and determination, he felt something stir within him—a faint glimmer of anticipation.
Perhaps soon, this young swordsman would grow strong enough to face him in a true duel. Perhaps, after all this time, he wouldn't have to stand alone at the peak.
Giyu looked at Kokushibo's back, his dark-blue eyes burning with resolve.
He knew he was walking a dangerous path—drawing close to Kokushibo not only to learn and restore Sun Breathing, but also to gain his trust, study his weaknesses, and prepare for the day the Demon Slayer Corps would strike down the Upper Moon One.
The deep cuts carved into the stone floor glimmered under the faint light. Here, in this quiet chamber, a duel of swords and hidden intentions began.
Giyu tightened his grip on his Nichirin Sword. He knew the road ahead would be hard. But for his comrades—for the world—and to destroy Muzan, he would keep walking it.
Until the day Sun Breathing was complete.
Until every Upper Moon—and Muzan himself—was cut down.