Chapter 193: Use Sun Breathing on Muzan
"You really are generous... forging dozens of Nichirin Blades at once. I believe the Master will be very pleased when he hears this news."
On the journey back, the Kakushi attendant known as Butterfly practically skipped along the dirt path. Her mood was visibly soaring.
The reason for her elation hung securely at her waist: a brand-new Nichirin Blade. It was a direct gift from Kobe Hikaru, forged with the tacit approval of the Swordsmith Village elders as a reward for her guidance—even though escorting guests was strictly her basic duty.
Despite being relegated to the logistics division as a Kakushi, Butterfly harbored a deep, quiet longing to wield a Nichirin Blade. She dreamed of stepping onto the blood-soaked front lines to become a true demon-slaying swordswoman, but her physical constitution simply lacked the necessary talent. Now, her fingers brushed against the wrapped hilt, tracing the woven silk with quiet reverence.
If Hikaru had not firmly refused, the master blacksmiths of the village would have thrown themselves to their knees, begging him to stay and instruct them in his forging techniques.
Hikaru, however, remained entirely pragmatic about his own limits. Stripped of his Strategy System's assistance, his pure metallurgical technique did not necessarily surpass the lifelong dedication of those seasoned artisans. He had naturally declined their frantic offers.
He left the bulk of the newly forged swords behind for the Demon Slayer Corps, keeping only the most crucial, core blade for himself.
And one other—a defective product.
Shortly after, Hikaru returned to the Demon Slayer Corps headquarters. Having just acquired his personal Nichirin Blade, he needed time to familiarize himself with its weight and temper before proceeding with his overarching strategy.
In the quiet backyard of the Ubuyashiki estate, a sprawling wisteria trellis formed a natural, cascading canopy overhead. Pale purple spikes of flowers swayed gently in the crisp morning breeze, casting dappled shadows across the stone courtyard.
Hikaru sat cross-legged on a raised stone platform, a sword resting horizontally across his knees.
His Nichirin Blade.
The scabbard was a deep, lacquered red, while the hilt was tightly wrapped in pristine white silk cord. A thumb-sized, unpolished scarlet ore sat embedded in the pommel. The blade itself possessed a highly unusual hue. It lacked the standard silver-white gleam of folded steel or the pitch-black void of mysterious iron; instead, a faint, pulsing orange luster ran along the edge.
This coloration was the direct result of sunlight energy. The scarlet iron sand and scarlet ore, having baked under the scorching sun for centuries, allowed that solar radiation to seep deep into the metal's atomic structure.
The Nichirin Blade was widely known as the Color-Changing Sword. It stood as the iconic weapon of the Demon Slayer Corps, yet this specific blade had been forged by Hikaru's own hands. By using raw materials harvested directly from Sun Mountain and applying his unique forging techniques, he created a weapon whose very strikes carried the burning properties of sunlight. It remained the only known tool capable of entirely severing a demon's immortality, even when wielded by an ordinary human.
As he ran a thumb over the guard, a translucent blue panel materialized in his vision.
[Nichirin Blade (Ownerless)]
[Quality: Semi-Demonic Tool]
[Introduction: A demon-slaying sword forged from scarlet iron sand and scarlet ore. The blade contains the sunlight energy accumulated over the years. Because of the materials, it surpasses ordinary quality tools, but it does not possess a sword soul and does not belong to the category of Demonic Tools.]
[Current favorability: 15 (Close)]
Because he had personally manipulated the iron and stone through his system's strategy, the weapon naturally harbored a baseline level of favorability toward him. It also firmly registered as a Semi-Demonic Tool.
The raw materials were extraordinary. The scarlet ore had gorged itself on centuries of direct sunlight, nurturing a rich, dormant spirituality within the rock. Combined with Hikaru's unique forging method—which forcefully retained almost all of that ambient energy—the weapon teetered on the very edge of becoming a living 'demon' itself. It fell just short of true sentience, but it undeniably possessed supernatural attributes far beyond ordinary steel.
It was a natural Semi-Demonic Tool, primed to undergo a full transformation at any moment. This was a trait entirely absent in standard Nichirin Blades, even the dozens he had generously donated to the Corps.
Hikaru placed his pale palm flat against the red scabbard. Slowly, he allowed a thin thread of his Yao Qi to seep into the wood and metal.
The blade vibrated once. The tremor was incredibly faint, like a sleeper twitching in the middle of a dream, before it settled back into stillness.
As a mere Semi-Demonic Tool, the sword's reaction to his demonic energy was far less sensitive than that of a fully realized Demonic Tool. It had been born into this world with the express purpose of slaughtering demons. While the concentrated sunlight energy trapped inside did not violently repel his Yao Qi, the two forces remained polar opposites.
Even with a foundation of basic favorability, strategizing and romancing this particular sword would require finesse.
But Hikaru was nothing if not adaptable. He possessed the Blood-Fire Transformation.
Fire belonged to the Yang alignment. Even though the demon fire coursing through his undead veins originated from Totosai and was fundamentally a byproduct of Turbid Qi, the core characteristics of fire remained universal: temperature, light, and combustion.
These elemental traits held the potential to connect with the dormant sunlight energy inside the blade. He simply needed to locate the correct frequency.
He adjusted the temperature of his Yao Qi, mimicking the steady, rising heat of dawn.
A moment later, the panel chimed.
[Nichirin Blade: favorability + 1]
[Current favorability: 16 (Aware)]
It had begun.
His lips twitched in a dry, deadpan expression. He was getting entirely too proficient at this. Hikaru couldn't shake the distinct feeling that he was walking further and further down the path of a specialized, weapon-seducing 'scumbag demon'.
At the exact same time, on the opposite side of the sprawling backyard.
Tsugikuni Yoriichi sat quietly on the polished wooden floor of the open corridor. His bare feet rested lightly on the edge of the stone steps. A sword lay across his knees.
It was not the chipped, heavy firewood knife he usually carried for his daily chores. It was a Nichirin Blade.
Technically, Yoriichi had not officially joined the ranks of the Demon Slayer Corps, meaning he had no right to possess such a weapon. However, principles and regulations tended to bend around a man of his caliber.
This sword was the 'defective product'Hikaru had forged. Though labeled a reject by the system's standards, its physical durability and edge retention far surpassed standard Corps weaponry. It remained an ordinary item, devoid of spiritual resonance, but its dense structure perfectly accommodated Yoriichi's overwhelming martial'intent'.
Ubuyashiki, the master of the estate, had offered his tacit approval. The head of the Demon Slayer Corps projected an aura of gentle warmth, but beneath that serene smile lay a calculating, almost chilling decisiveness.
Yoriichi's pregnant wife, Uta, had already been moved into a quiet, secure room deep within the inner courtyard. The estate's maidservants attended to her every need. An experienced midwife remained on standby, ready for the birth at a moment's notice. The headquarters housed not only elite combatants but also their families, ensuring top-tier medical care was always available.
Uta was safe. Their unborn child was safe.
For this deep peace of mind, Yoriichi felt an immense, quiet gratitude toward Hikaru, and by extension, the entire Demon Slayer Corps. Upon learning that this hidden organization existed solely to eradicate the demons plaguing humanity, he decided it was only right to offer his strength in return.
The red-haired youth tilted his head back, his deep, crimson eyes tracking the slow descent of a wisteria petal.
He closed his eyes.
Instantly, his perception shifted into the Transparent World. The invisible currents of the estate mapped themselves across his mind: the exact trajectory of every passing breeze, the rhythmic expansion of every human lung within a mile radius, the microscopic cellular growth of the wisteria vines above him.
But the Transparent World did not merely observe the present. It analyzed the past.
His thoughts drifted back to the night of the attack. He analyzed the nature of the demons. He remembered the terrified old man he had subdued and spared—a mere 'minion', a human puppet violently controlled by a higher power. The existence of such thralls proved these creatures possessed intelligence, hierarchical organization, and deliberate goals.
He then recalled the demon Hikaru had obliterated with a single, devastating slash. Arriving at the scene, Yoriichi had found nothing but a scorched, smoking trench and a lingering stench of ash. The creature had died far too cleanly, evaporating in a manner that reminded him of the crumbling, ancient anatomical scrolls he had studied in the Tsugikuni dojo as a child.
These monsters feared the sun.
The Corps swordsmen had explained the mechanics to him. Demons operated exclusively under the cover of night; the moment direct sunlight touched their flesh, they crumbled into ash. That was the entire basis for wielding swords saturated with solar energy.
Yoriichi lowered his gaze to the cold iron resting in his palms.
A Nichirin Blade. A weapon forged from sun-drinking ore. It was undoubtedly an effective tool.
But Yoriichi's analytical mind found it insufficient. Relying entirely on the material properties of a sword was a fragile strategy. According to Hikaru and the man named Rengoku, the mortality rate among the swordsmen remained staggeringly high.
What if... he used his natural breathing rhythm to simulate the sun itself? What if he forced his own human body to become a vessel of solar heat?
If he combined a sun-forged blade with a breathing rhythm that mimicked solar flares—burning the oxygen in his lungs to absolute maximum capacity at the exact microsecond of the swing, pushing his muscular and vascular systems past their biological limits—would the resulting slash carry the literal temperature of the sun?
The hypothesis crystallized in his mind, sharp and flawless.
He closed his eyes again. He began to adjust his breathing.
This was entirely different from the primitive, instinctual breaths he took during combat. He drew the air down, deep into the lowest chambers of his lungs, dragging the inhalation out until his chest felt tight. When he exhaled, he forced every microscopic trace of waste gas from his system.
The rhythm stabilized. It became perfectly even, the cadence constant and heavy, like ocean waves violently crashing against a cliff face, or a gale howling through a narrow mountain gorge.
This was a biological technique he had unconsciously used since childhood. He had no idea it could be classified as a 'Breathing Style', nor did he realize he was the only human alive capable of performing it. He only knew that when he breathed like this, gravity lost its hold on his limbs, his kinetic vision sharpened to a razor's edge, and his physical output skyrocketed.
An epiphany struck him. He stood up smoothly, his movements devoid of wasted kinetic energy.
He raised the defective blade. The dark iron looked utterly unremarkable in the morning light.
But the man holding it had transformed.
His breathing grew dense and resonant. As he inhaled, his expanding chest seemed to swallow the surrounding atmosphere. As he exhaled, intense, radiating heat bled from his pores, distorting the air around his skin into a shimmering, visible heat haze.
He slowly raised the sword above his head.
He swung it down.
A single, experimental strike. It was executed slowly, without a fraction of his true speed.
Yet, the air directly in the blade's path violently scorched.
A sharp, crackling hiss echoed across the courtyard. Along the exact trajectory of the steel, the oxygen superheated, flashing into a brilliant, fleeting phantom of orange-red fire.
It looked exactly like a physical afterimage of the sun.
It was not a visual trick, nor a mere illusion of speed. Yoriichi's strike had forcefully dragged the ambient energy of the world into his blade. In this mixed reality, his newly formed Breathing Style actively manipulated the laws of physics.
A sharp, ragged gasp broke the silence from the far end of the wooden corridor.
Rengoku Juro stood frozen by the pillar, his crow mask hanging loosely from his fingers. His eyes, framed by thick, expressive brows, were blown wide in absolute shock.
"Mr. Tsugikuni... what in the world was that?"
Though Yoriichi currently dressed as a humble woodcutter, Juro had witnessed the man's terrifying martial prowess firsthand and treated him with the utmost respect.
Yoriichi turned his head, his expression placid. He paused, searching for the correct terminology to explain the biological mechanics.
He settled on a simple title.
"Breathing Style."
His voice was calm and direct. "Using controlled respiration as a foundation to exponentially amplify the body's physical limits."
He looked down at the faintly smoking blade. "And what I just performed... perhaps it can be called Sun Breathing. A specific Breathing Style designed to mimic the shifting intensity of sunlight."
Juro's jaw went slack.
Though he had not yet attained the rank of Hashira, Juro was widely recognized as one of the most lethal combatants in the current generation of the Demon Slayer Corps. Yet, in all his years of bloody combat, he had never witnessed anything like this. A technique that allowed a fragile human to generate elemental power through pure respiration.
The samurai clans and elite swordsmanship schools of the era only used breathing exercises to manage stamina, steady their nerves, or maintain a rhythmic footwork during prolonged duels.
Juro's calloused hand, so accustomed to gripping a Nichirin hilt, trembled violently at his side.
He realized the magnitude of what he had just seen. This was the key. This was the evolutionary leap that would finally allow frail, mortal humans to stand toe-to-toe with regenerating demons—and perhaps even true monsters. They would no longer need to rely on desperate sieges, cowardly traps, or mountains of human corpses to secure a single kill.
They could fight with their own innate power.
"Can you... teach me?" The words tore from Juro's throat before he could stop them.
Yoriichi looked at the trembling swordsman.
There was no hesitation in his crimson eyes, no desire to hoard his newfound strength. He simply acted on his immediate logic. The Demon Slayer Corps had protected his wife and unborn child; passing on this technique was a fair and appropriate return for their hospitality.
Yoriichi smoothly reversed his grip, offering the hilt of the Nichirin Blade toward Juro. "It begins with the lungs. Take a deep breath. Force the air into every neglected corner of your chest."
Juro stepped forward, his eyes locked on the steel.
"Then, exhale slowly," Yoriichi instructed, his voice an even, guiding cadence. "As the air leaves your body, focus on your blood. Feel it flowing through your veins. Force it to accelerate. Force it to heat up."
"Memorize that internal pressure. That is the starting point."
Juro took the heavy sword. He planted his feet, closed his eyes, and took a massive, shuddering breath.
He attempted to force the oxygen deep into his bloodstream, straining his diaphragm.
A second later, he bent double, hacking and coughing violently as his lungs rejected the unnatural pressure.
He had failed, which was entirely unsurprising for a first attempt. But as Juro wiped the saliva from his chin and looked up, his eyes burned with a terrifying, fanatical brilliance. They were so bright it was almost painful to look directly at him.
He had felt it. For a fraction of a second, he had truly felt the heat ignite in his veins.
Meanwhile, on the raised stone platform across the courtyard.
Hikaru continued to steadily feed demonic power into his own Nichirin Blade, his face an unreadable mask. But his Crimson Demon Eyes were narrowed, tracking every microscopic movement on the wooden corridor.
He was watching. And he was learning.
Hikaru felt no surprise that Yoriichi had casually invented Sun Breathing on a quiet Tuesday morning. Even disregarding his meta-knowledge of the original timeline, simply observing Yoriichi's daily habits provided a clear understanding of the man's existence.
Yoriichi was an anomaly of strength. He possessed an inherent, god-like talent, but his true power lay in his adaptability. His martial 'intent' was pure nothingness—he harbored no greed, no ego, no desperate desire for supremacy. He simply existed in a state of infinite, passive progression.
His upper limits were entirely dictated by his environment and the external pressures applied by those around him. In his previous, isolated life in the mountains, he had no reason to push his boundaries. But now, surrounded by the supernatural threats of this mixed world, his evolution had triggered.
As for the Breathing Style itself, Hikaru found it highly intriguing. Respiration techniques were a universal concept across countless worlds and disciplines. He had witnessed many variations. Even his lover, Kikyo—widely revered as the most powerful shrine maiden of the Warring States period—used specific breathing patterns to mobilize her vast reserves of power.
However, Kikyo's breathing manipulated a far higher tier of pure spiritual energy, allowing her to project her soul's power outside her physical vessel.
Yoriichi's Breathing Style, by contrast, operated in the murky middle ground between pure spiritual energy and Turbid Qi. It manipulated the raw, biological energy of living flesh.
The difference in purity was obvious, making the two systems incredibly difficult to merge.
But that did not render Yoriichi's invention useless. Far from it. While it lacked the divine purity of Kikyo's spiritual arts, it was the ultimate equalizer for the masses. It was a power that ordinary mortals could grasp through sheer, agonizing effort. By instantly amplifying the physical body's output, it effectively functioned as a mortal equivalent to a demon's Physical Transformation.
Hikaru wasn't entirely certain if a biological Breathing Style would function within his undead, Yao Qi-driven anatomy.
But that certainly wasn't going to stop him from trying.
Adapting a physical breathing technique was far more feasible than attempting to wield Kikyo's pure spiritual energy, which his demonic nature currently violently rejected.
And so, he sat in silence.
Every minute shift in Yoriichi's posture, every rhythmic fluctuation in his respiration, the exact amplitude of his expanding ribcage, the pressurized flow of oxygen rushing through his trachea—
His Crimson Demon Eyes, permanently enhanced by the Sharp Eye Transformation, recorded the data with flawless, mechanical precision.
He only needed to watch the demonstration once.
The foundational mechanics of Sun Breathing were already permanently burned into his memory.
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