Chapter Text
Chapter 1: Demon Summoning, Know Who Did That!?
The moon hung overhead, illuminating the grotesque remains of battle. Kokushibo, Upper Moon One of the Twelve Kizuki, stood amidst the carnage he had wrought. His six eyes surveyed the scene dispassionately, the bodies of fallen Demon Slayers scattered around him like broken dolls. His long hair fluttered in the night breeze as he sheathed his nichirin blade.
"Pathetic," he murmured, his voice carrying in the still night air. "Not one of them could even force me to use beyond the third form."
He had hoped that one day he would encounter someone worthy, someone who could push him to his limits. But century after century, disappointment was all he found. Even his brother, Yoriichi, was long gone—the only swordsman who had ever made him feel fear.
As Kokushibo prepared to depart, something peculiar occurred. The air around him shimmered, distorting like heat waves over hot stone. He narrowed his eyes, all six tracking the anomaly with perfect clarity.
"What is this?" he questioned, his hand instinctively moving to the hilt of his sword.
A blinding light erupted beneath his feet, forming a complex magic circle with arcane symbols he did not recognize. The light intensified, bathing everything in a white glow. Kokushibo attempted to leap away, but found himself rooted to the spot, as if invisible chains bound him to the center of the circle.
For the first time in centuries, Upper Moon One felt something akin to surprise. This was not Muzan's doing, nor was it any technique he had encountered from the Demon Slayer Corps.
The world around him began to fade, dissolving into particles of light. As darkness claimed his consciousness, one thought burned in his mind: Whatever force dared to bind him would soon learn the meaning of true terror.
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Consciousness returned to Kokushibo in waves. His senses, infinitely sharper than any human's, detected multiple presences around him. Four others, to be precise. The scent of humans, magic, and something ancient filled his nostrils.
He opened his eyes—all six of them—to find himself in a grand chamber, standing on a stone dais. Three other men stood beside him, each looking as disoriented as he imagined he appeared. Before them stood an elderly man in ornate robes, flanked by soldiers in medieval-style armor that Kokushibo had not seen in centuries.
"O brave heroes!" the elderly man proclaimed, arms spread wide. "Please save our world!"
Kokushibo's eyes narrowed as he took in his surroundings. Stone walls, torches, the clothing of those present—this was not Japan, certainly not the Japan of his era. His hand instinctively moved to check for his blade, relieved to find it still at his side. However, he noticed something unusual—a peculiar sword had materialized in his grasp. It appeared to be a longsword rather than his familiar nichirin blade, with a jewel embedded in its hilt that pulsed with an internal light.
One of the other summoned men—a youth with curly blonde hair—shouted in confusion, "What the hell is this? Where are we?"
Another, a black-haired man with a shield, looked around with a mixture of apprehension and concern. "Is this... some kind of fantasy world?"
The third, a blonde handsome man with a spear looked visibly dumbfounded.
Kokushibo observed them with disdain. Humans. Young, inexperienced, and weak. Not worth his time.
The elderly man continued, unperturbed by their confusion. "You four have been summoned to our world according to an ancient prophecy. You are the Four Cardinal Heroes, each wielding one of the legendary weapons: the Sword, the Spear, the Bow, and the Shield. Our world faces destruction at the hands of calamitous waves of monsters, and only you can save us."
As if responding to these words, the sword in Kokushibo's hand glowed brighter. Information flooded his mind—knowledge of this world, its systems, and the role he was expected to play. He was the Sword Hero, chosen to wield the Legendary Sword. The others were similarly designated: Spear Hero, Bow Hero, and Shield Hero.
"This is ridiculous," the Shield Hero muttered, a young man with dark hair and an ordinary appearance. "I was just reading a book in the library and now I'm in some game-like world?"
Kokushibo remained silent, processing the situation. A different world with different rules. He was bound to this sword somehow—he could feel the connection, similar yet different from his bond with his nichirin blade. Curiously, he could sense that the sword was responding to his demonic nature, neither rejecting nor fully embracing it.
The king—for that was apparently who the elderly man was—continued explaining about waves of calamity, rewards, and their duties as heroes. Kokushibo paid minimal attention. Power structures were always the same, regardless of the world. What interested him more was understanding the limits and capabilities of this new weapon he'd been bound to.
He focused on the sword. Immediately, a translucent interface appeared in his vision, displaying information:
[Legendary Sword: Level 8]
Forms Available: Basic Sword
Skills Locked: 79%
Status: Bound to Sword Hero (Michikatsu Tsugikuni/Kokushibo)
Note: Abnormal Soul Detected - Compatibility Assessment In Progress
Interesting. The sword recognized both his human name and his demon name. More importantly, it was analyzing his demonic nature, presumably to determine what abilities to grant or restrict.
When Kokushibo glanced up again, he noticed the Shield Hero—Naofumi Iwatani, according to the brief introductions that had been made—being regarded with suspicion by the others. The king's demeanor had noticeably cooled when addressing him, and the other heroes seemed to be keeping their distance.
Discrimination existed in every world, it seemed. Not that Kokushibo cared for human politics or prejudices. His only interest was in power and perfection of his swordsmanship.
As the audience with the king concluded, they were informed they would be provided with companions to aid them on their journeys. They would need to recruit party members, as the heroes were apparently stronger together with others than alone—a concept Kokushibo found amusing. He had never needed assistance, not even against multiple Hashira.
The four heroes were escorted to separate chambers for the night, with the promise of being introduced to potential party members in the morning.
Once alone in his assigned quarters—luxurious by the standards of this world—Kokushibo examined his new form more carefully. His body remained unchanged: pale skin, six eyes arranged in pairs down his forehead, muscular build, and the distinctive marks of Upper Moon One in his eyes. His nichirin blade was secured at his side, but the Legendary Sword appeared to be intrinsically linked to him. When he willed it away, it seemed to dissolve into particles of light, only to reappear in his hand when summoned.
With a thought, he transformed his face to its more human appearance—the face he had worn as Michikatsu Tsugikuni. Two eyes, normal skin, the countenance of a swordsman rather than a demon. It wouldn't do to cause a panic among these humans, not before he understood the full extent of this world's capabilities and limitations.
"Upper Moon One of the Twelve Kizuki, now reduced to playing...the role of hero," he murmured, his voice carrying a hint of dark amusement. "Muzan-sama would find this... intriguing."
But Muzan was not here. For the first time in centuries, Kokushibo was truly free of his master's influence. This realization dawned on him slowly, like the rising sun he had not seen in three hundred years.
Freedom. A concept he had nearly forgotten.
With methodical precision, Kokushibo began cycling through the forms available to his new weapon, familiarizing himself with its weight and balance. It responded fluidly to his movements, almost as if it were an extension of his will rather than a physical object.
"Perhaps," he mused, executing a perfect arc with the blade, "this.. world will offer challenges …worthy of my skills."
Skills: Moon Breathing, Regeneration, Absorption, Irregular physical ability(demon tier), Blood Demon Art: Kokushibo possesses a Blood Demon Art that allows him to manipulate his body and create powerful crescent-shaped blades from his flesh, Intimidatio, Demon Slayer Mark, Transparent World(lets him see weaknesses….
A menu opens up giving all kinds of information about the swords admittedly useful features. Even spatial storage thst might come in useful. The sword has the ability to give me unique drops from monsters I kill, It can be used to craft items if I unlock the recipes and relevant skills, but the biggest one is that it can absorb items and copy other, similar weapons to unlock new forms it can take. These new forms each have their own abilities that can be ‘mastered’ by keeping them equipped and any sword I’ve mastered will allow me to keep the benefits of the weapon no matter what form it takes.
…Kokushibo zeroed in on his reflection and his eyes with the Upper One Moon mark.
With deliberate slowness, Kokushibo brought his hand to his face. His six eyes—three on each side—tracked his movements with perfect clarity. He selected the lowest eye on the right side of his face, and without hesitation, thrust his fingers into the socket.
There was no pain—or rather, pain had long ceased to be a meaningful sensation to one who had endured centuries of demonic transformation. With a swift motion, he extracted the eye, a perfect orb trailing thin strands of demonic essence.
Blood dripped from the empty socket momentarily before the wound began to close. Regeneration was one of the many gifts his demonic nature provided—the eye would regrow within hours.
Holding the extracted eye before him, Kokushibo admired how it pulsed with inner light, the pupil shifting to track movement even disconnected from his body. Such was the nature of demons, that even severed parts retained aspects of their consciousness.
"The essence… of an Upper Moon demon," he declared, "fused with the Legendary Sword... Let us see …what emerges."
He pressed the eye against the gemstone embedded in the Legendary Sword's hilt. For a moment, nothing happened. Then the gem began to glow with an ominous crimson light, pulsating like a heartbeat as it absorbed the demonic essence.
The sword's entire form trembled, then began to transform. The straight blade curved slightly, taking on characteristics reminiscent of Kokushibo's nichirin blade. The metal darkened to a deep gunmetal gray, with subtle patterns that resembled flowing water—or perhaps blood—etched along its length.
Most significantly, at the junction where blade met hilt, an eye identical to the one Kokushibo had sacrificed materialized, embedded into the metal itself. The eye swiveled, taking in its surroundings before fixing its gaze on Kokushibo.
[Legendary Sword: Level 10]
New Form Unlocked: [Blood Moon Blade]
Special Ability: Omnidirectional Vision
Skills Unlocked: 75%
Status: Enhanced Synchronization - Demonic Essence Integrated
Warning: Unknown Transformation Detected
Kokushibo lifted the transformed sword, feeling its perfect balance and the newfound connection between them. The eye in the blade seemed to function as an extension of his own vision, providing a perspective that eliminated all blind spots when wielding the weapon.
"Impressive…," he acknowledged, executing a series of precise cuts that left glowing crescents in the air—reminiscent of his Moon Breathing techniques. The Blood Moon Blade enhanced these attacks, adding a crimson tint to the energy and increasing their cutting power substantially.
"The eye grants omnidirectional vision," he mused aloud. "But what other… aspects of my demonic form… might enhance the blade?"
After consideration, Kokushibo brought the sword to his forearm and, with a single fluid motion, sliced deeply into his own flesh. Dark blood welled from the wound—blood that contained centuries of accumulated power, far different from human vitae.
He allowed his blood to flow directly onto the Legendary Sword's blade. The weapon trembled, then began to absorb the liquid as if thirsting for it. The embedded gem in the hilt pulsed crimson, and the metal of the blade darkened to a deep, lustrous black that seemed to absorb rather than reflect moonlight.
[Legendary Sword: Level 15]
New Form Unlocked: [Bloody Void Crescent]
Special Ability: Blood Consumption
Skills Unlocked: 60%
Status: Enhanced Synchronization - Demonic Integration Accelerating
Warning: Weapon Evolution Detected
The transformation was completed, revealing a new blade that curved more dramatically. Along its edge ran a channel designed to collect and store blood—both from enemies and potentially from Kokushibo himself. The sword had taken on a blood red appearance that seemed to move on its own, what looked like dark veins spread across the entire blade.
Testing the new form, Kokushibo executed a series of cuts through the air. The blade left trails of darkness rather than light, and each swing seemed to pull ambient energy from the surroundings. When he channeled his demonic power through it, the blade consumed and amplified the energy, resulting in attacks of devastating potency.
"A suitable… evolution," he approved, watching as his forearm wound regenerated completely. "But… still incomplete."
Kokushibo systematically experimented with incorporating different aspects of his demonic biology into the Legendary Sword.
Each addition unlocked new forms and abilities:
From strands of his hair, woven into the sword's hilt, came the [Midnight Tress] form—a blade that could extend and retract at will, changing length mid-swing to confuse opponents.
From fragments of his nails, ground and fused with the metal, came the [Crescent Talon] form—a weapon that could split into multiple blades connected by invisible force, allowing for attacks from multiple angles simultaneously.
From his own fangs, embedded in the crossguard, came the [Demon Fang] form—a sword that inflicted wounds which spread corrupt energy, hindering regeneration and healing.
With each addition, the Legendary Sword grew more attuned to Kokushibo's unique constitution. The warnings in his status screen about "unknown transformations" became more frequent, suggesting the weapon was evolving beyond its intended parameters.
[Legendary Sword: Level 17]
Forms Available: 87 (5 Demonic Forms)
Skills Unlocked: 46%
Status: Advanced Synchronization - Demonic Integration Reaching Critical Threshold
Warning: Weapon Transformation Approaching Irreversible State
The ultimate test came moments later, when Kokushibo decided to create his masterpiece. Standing in the moonlight of the open window.
Kokushibo raised the Legendary Sword skyward, its form currently neutral. Then, with ceremonial precision, he brought the blade to his chest and thrust it directly into his heart.
Pain—sharp and immediate—radiated through his body. Even with his demonic resilience, this was no minor injury. Black blood poured forth, enveloping the blade entirely. Kokushibo maintained perfect stillness, focusing his centuries of discipline on remaining conscious and in control.
The Legendary Sword pulsed violently, absorbing not just blood but concentrated demonic essence from the very core of Kokushibo's being. The blade began to transform more dramatically than before, reshaping itself as it drew on its wielder's nature.
Lightning crackled around them, drawn to the surge of power. The moon above seemed to brighten in response, its silver light concentrated on the ritual taking place.
With a final surge of energy that cracked the floor beneath his feet, Kokushibo withdrew the transformed blade from his chest. The wound sealed immediately, his demonic regeneration working overtime to repair the grievous injury.
In his hand now rested a weapon beyond anything this world had seen. No longer merely a sword, it had become a manifestation of Kokushibo's demonic essence. The blade was midnight black yet seemed to contain a universe of stars within its metal.
The single eye had multiplied—six eyes now adorned the weapon, arranged precisely as they were on Kokushibo's true face. The hilt had transformed into an intricate pattern of interlocking crescents, and the blade itself could shift between solid and semi-corporeal forms at will.
[Legendary Sword: 23]
New Form Unlocked: [Upper Moon Blade]
Special Abilities: Multiple - Partially Unknown
Status: Complete Integration - Demonic Essence Fully Synchronized
Note: Cardinal Weapon Parameters Exceeded - Classification Updated
"The culmination," Kokushibo breathed, feeling the perfect resonance between himself and the weapon. It was neither fully the Legendary Sword nor entirely his demonic creation—it had become something unprecedented.
Kokushibo extended the blade toward the night sky and channeled his power through it. The mountain trembled as a perfect crescent of energy sliced upward, parting clouds for miles before dissipating.
"This…," he stated with cold satisfaction, "will suffice… for the coming Waves."