Inside the dojo of the Infinity Castle, pale blue moon-patterned slashes wove together like a web, slicing the air with piercing screams.
Kokushibo stood tall, his long blade steady as a mountain pine. Each swing carried the weight and precision of hundreds of years of mastery.
"Moon Breathing, Third Form: Loathsome Moon, Chains!"
The pale blue arc of his blade curved like a crescent moon, impossible to evade. The blade wind gouged a deep trench into the floor, sending shattered stones flying.
Tomioka Giyu focused, his expression calm as his Nichirin Sword flared with faint moonlight. He tried his best to mimic Kokushibo's movement and the flow of his breathing.
"Moon Breathing, Third Form: Loathsome Moon, Chains!"
The slash resembled Kokushibo's technique in form, but lacked the density of experience, the years of refinement that gave the strike its weight. The air pressure from his swing was weaker, lighter.
"Too slow."
Kokushibo's voice came cold and even. His body blurred like a phantom as he shifted aside, his blade sliding past Giyu's sword and tracing across his waist.
Without pausing, the pale blue glow flared again.
"Moon Breathing, Fifth Form: Moon Spirit Calamitous Eddy!"
A rotating vortex of crescent blades burst forth, instantly drawing Giyu into its center.
The sound of flesh tearing rang sharply. Giyu's body was cut clean in half at the waist, upper and lower halves falling apart as black blood splattered across the scarred floor.
Before the pieces even hit the ground, dark mist surged violently from the wounds, twisting and binding like living tendrils. Within two breaths, his body fused together again—smooth, unmarked, as though nothing had happened.
Giyu gripped his Nichirin Sword and straightened, his breathing calm and steady, as if that clean split had been a mere illusion.
Kokushibo lowered his blade and watched him, all six eyes narrowing slightly. It was the fifth day since he began teaching Giyu Moon Breathing.
Five days ago, Giyu couldn't even perform the basic stance properly.
Three days ago, he barely managed to mimic the First and Second Forms.
Now, he could use the first six forms of Moon Breathing fluently, even keeping up with Kokushibo's rhythm in sparring. The gap was still vast—but his progress defied reason.
"Do you know how long it took me to perfect the first six forms of Moon Breathing?"
Kokushibo's tone carried a rare hint of emotion. "Thirty years. It took me thirty years to make those six forms flawless—without a single weakness."
Giyu's fingers tightened on the hilt. He could feel the weight behind those words, but he didn't reply. He simply waited, knowing the next words would matter.
"Centuries ago, I once met a swordsman of the Demon Slayer Corps with remarkable talent. He took twenty years just to master the first three forms of Moon Breathing."
Kokushibo's gaze drifted to the corner of the dojo, where countless broken blades lay scattered. His eyes flickered with memory.
"Even for those called 'genius' among swordsmen, it takes at least ten years to fully master the first six forms."
He paused, six eyes locking onto Giyu, a note of reluctant awe in his voice. "And you've done it in five days."
Giyu lowered his gaze, unsurprised. After becoming a demon, his body had adapted to the breathing arts in ways no human could match. He could not only memorize forms instantly but also understand the flow and rhythm of every technique at a glance.
Though Moon Breathing was born from Sun Breathing, its essence was colder, sharper—tinged with demonic power. That very energy resonated perfectly with his current form, making learning it effortless.
More importantly, by comparing Moon and Sun Breathing, he could fill in the missing pieces of the latter. Each mirrored the other, allowing him to understand both at a level beyond ordinary comprehension.
"Your gift for swordsmanship far exceeds what I expected."
Kokushibo's voice rose again, this time stripped of arrogance—only filled with genuine fascination.
"I once believed that only Yoriichi's talent surpassed mine. But now... perhaps you stand equal to him."
The name "Yoriichi" struck like thunder in Giyu's ears. His head snapped up to meet Kokushibo's gaze. This was the first time Kokushibo had mentioned Tsugikuni Yoriichi—the man who created Sun Breathing, the one hailed as invincible.
Kokushibo seemed to sense his reaction but didn't explain. Instead, he raised his sword once more, pale moonlight burning brighter than before.
"Continue. This time, use all the forms of Moon Breathing at your fastest speed. Hold nothing back."
Giyu drew a deep breath as demonic energy surged through him. The moon patterns on his blade pulsed darker, sharper.
"Moon Breathing, First Form: Dark Moon, Evening Palace!"
His slash burst forward like lightning, aimed straight for Kokushibo's neck.
Kokushibo neither dodged nor retreated. He raised his long blade and blocked precisely. The moment their blades clashed, sparks burst through the air—
Giyu shifted instantly. "Moon Breathing, Fourth Form: Perpetual Night, Lonely Moon—Incessant!"
The pale blue arcs split into multiple slashes, striking toward Kokushibo's limbs from every angle.
Kokushibo's eyes narrowed, his body gliding through the air as his blade traced a perfect circular path before him—
Kokushibo blocked every slash effortlessly. "Not bad. Your breathing is steadier now, and your transitions are clean. But it's still not enough. The essence of Moon Breathing isn't imitation of form—it's 'wielding the moon as the blade and the shadow as the force.' You haven't grasped that yet."
Before his words even faded, Kokushibo closed the distance in a flash. His long blade shot straight for Giyu's chest, moving several times faster than before.
Giyu's pupils constricted. He instinctively raised his Nichirin Sword to block, but the sheer power behind Kokushibo's grip numbed his wrist, nearly knocking the weapon from his hand.
Seizing the opening, Kokushibo lifted his leg and kicked him hard in the stomach. Giyu was thrown back, crashing into the copper mirror at the far end of the dojo. The mirror shattered instantly, fragments scattering across the floor.
Giyu pushed himself up, wiped the black blood from the corner of his mouth, and his eyes gleamed with new clarity. In that brief instant, he had felt it—the "force" emanating from Kokushibo. It was cold and oppressive, like the stillness of a midnight moon, a presence that blended perfectly with each movement of Moon Breathing to form its true killing power.
"You seem to understand now."
Kokushibo's voice carried a faint curve of amusement, the first trace of expression he had shown since their training began. "Moon Breathing is more than sword light and technique. It's the unity of mind and breath. To sense that during battle proves your talent."
He stepped forward and handed Giyu his own blade. "Try this sword."
The moment Giyu grasped it, he felt the chilling energy coursing through the steel. This was Kokushibo's blade—tempered through centuries, steeped in both the power of Moon Breathing and the weight of his understanding of the sword.
Giyu closed his eyes, recalling the "force" he had just felt. His demonic energy blended swiftly with the rhythm of Moon Breathing.
When he opened his eyes again, the calm focus was gone. In its place was a sharp, cold light—eerily similar to Kokushibo's own.
He raised the sword, moon patterns flowing across the blade, deeper and denser than before. "Moon Breathing, Sixth Form: Perpetual Night, Lonely Moon—Incessant!"
The strike was simple and unadorned, but its pressure was suffocating, blanketing the entire dojo as it rushed straight for Kokushibo's chest.
All six of Kokushibo's eyes flared. He didn't block. Instead, he mirrored the motion exactly. "Moon Breathing, Sixth Form: Perpetual Night, Lonely Moon—Incessant!"
Two pale blue arcs collided at the center of the dojo. There was no explosion, no roar—only silence so absolute it seemed the air itself had been cut away.
Moments later, the light dispersed. Giyu stood covered in blood, his arm trembling uncontrollably, while Kokushibo remained perfectly still, a faint satisfaction on his face.
"Excellent."
Kokushibo sheathed his sword, his tone filled with recognition. "It won't be long before you master all the forms of Moon Breathing. When that time comes, perhaps we can fight in earnest."
Giyu slid his Nichirin Sword back into its sheath. The dark mist around his body faded, and the trembling in his arm ceased.
He looked at Kokushibo silently, thoughts racing. Mastering Moon Breathing was only one step of his plan. By studying it alongside Sun Breathing, he had already begun to restore parts of the latter's missing structure. Next, he needed to learn the remaining forms of Moon Breathing—and eventually attempt to merge it with other styles, completing the true essence of Sun Breathing.
Kokushibo gave him a calm glance and turned toward the door. "Starting tomorrow, I'll teach you the Seventh through Tenth Forms of Moon Breathing. Those are the techniques I spent a hundred years perfecting. They are the core of the style."
Giyu watched his back, his voice low and steady. "Thank you."
As Kokushibo's figure disappeared into the hall, Giyu tightened his grip on the Nichirin Sword. The broken mirror shards scattered on the floor reflected his image—his vertical blue pupils, the small horn at his temple, and beneath them all, a flicker of emotion that hadn't yet faded.
…
Just as Giyu steadied his thoughts, Muzan's cold voice echoed within his mind.
"Mushiki, there's a pest on your territory. Go deal with it."