The story begins long ago, in the distant Sengoku era.
In a respected samurai family, twin sons were born. The elder brother, being the firstborn, became the family's heir. The younger, however, was shunned by their father for one peculiar reason—he bore a strange "mark" on his forehead.
Disgusted by this omen, the father even tried to have the child killed. Only the desperate pleas of the mother spared the boy's life.
But the father did not relent. He decreed that when the younger child turned ten, he would be sent to a temple to live out his life as a monk, far removed from the world of swords and samurai.
Everything about their lives was different. The brothers lived in separate rooms. The elder ate fine meals and trained in the art of the sword, while the younger was given plain rice and rags for clothing. But the younger boy clung to his mother with fierce affection whenever he could. And perhaps it was this contrast—this unfairness—that stirred a deep compassion in the older brother's heart.
The elder would sneak away from his father's watch to play with his younger sibling, even though discovery always ended with a beating. He once carved a small flute and gave it to his brother, telling him, "Blow this when you're in danger. I'll come for you no matter what."
The elder's dream was noble—to become the greatest swordsman in the land. He practiced diligently, day after day, convinced that dedication would lead to strength.
But fate had other plans.
One day, the younger brother picked up a wooden training sword—and immediately defeated one of their father's adult retainers. With no training, no experience, he displayed raw, terrifying talent.
The elder brother's heart sank.
Everything he had worked for—his long hours, his pain and suffering—meant nothing in the face of this genius.
Their father, too, saw it. And just like that, the roles reversed.
The family heir became the unwanted child.
The younger brother was now the center of their father's attention. He inherited the family's martial legacy. Meanwhile, the elder brother was thrown into a cramped room barely large enough to kneel in, and his future was rewritten: he would be the one sent to the temple.
Still, the younger brother came to him before he left.
"I'm going to the temple now," he said quietly. "I'll treasure the flute you gave me."
But later, the elder brother found their mother's journal. It revealed the truth: his younger brother had known everything—the unfairness, the betrayal—and yet continued to act with innocence and humility.
To the elder brother, that humility felt like mockery.
It was then that the seed of jealousy turned to poison.
He began to hate his brother.
Not just the person—but the gift, the talent, the effortless superiority.
And so, the bond of brotherhood was broken by the chasm called "talent."
Eventually, the younger brother vanished. He never went to the temple. He disappeared from all records.
More than a decade passed.
One night, while camping in the wilds, the elder brother was attacked by a demon. As death neared, his younger brother reappeared—now a fully realized warrior. With ease, he slew the demon and saved him.
But instead of gratitude, the elder brother's heart twisted further.
He abandoned his wife and children, joined the Demon Slayer Corps, and swore to surpass his brother.
Though his brother taught all members the breathing techniques, the elder brother could never learn the original form—Sun Breathing.
He settled for a derived style.
Eventually, mysterious marks appeared on his body. His power surged, but it still paled compared to the legend of his brother.
Then came the final betrayal.
To escape the fate of those who bore the Mark—a short life—he made a deal with the Demon King, Kibutsuji Muzan.
He brought Muzan the severed head of the current Demon Slayer Corps leader and drank the demon's blood.
He became a demon.
Decades passed. The elder brother, now unaging, finally met his brother again—old, frail, yet radiating the same invincible presence.
Enraged, he attacked. His brother did not fight back with hatred, only tears.
"How pathetic, brother..."
His brother drew his blade, not to kill—but to try and stop him.
In the end, the younger brother died of old age, standing with his sword raised.
The elder brother could no longer surpass him.
In a final act of resentment, he cut his brother's body in half, and accidentally sliced through the tiny flute that had always hung from his brother's belt.
Seeing the broken flute, the elder brother wept.
The memories returned—the bond they once had, the promise.
Since then, the flute has remained at his side.
Now, centuries later, Kokushibou—once Tsugikuni Michikatsu—stood with one hand touching his waist, where the broken flute still rested.
"When did I forget all this...?" he murmured.
He was so lost in thought, he barely noticed the young swordsman standing before him.
Tokitou Muichirou.
Six eyes looked up, scanning the boy with the clarity of the Transparent World.
"You are my descendant," Kokushibou said, his voice distant.
"That's nonsense," Muichirou replied coldly.
"When I was human, I was Tsugikuni Michikatsu, head of the Tsugikuni family. My younger brother was Yoriichi. I left behind a bloodline. You are part of it."
Muichirou's grip tightened on his Nichirin Blade.
"So what? You think sharing blood means I'll spare you?"
Kokushibou said nothing.
What Nakime hadn't predicted was how slowly the Upper Moons were recovering from their battle with James.
First, second, and third rank demons were all gravely wounded.
Kokushibou, in particular, was barely functioning.
Half of his body had disintegrated, with twisted flesh attempting to knit itself back together—but each time, a burning force halted the regeneration.
Muichirou noticed it too.
A red blade… stronger than Yoriichi's.
Kokushibou acknowledged his state.
"Yes. I am very weak now. So come—if you believe you can kill me."
His tone was calm, even bored, but his six eyes exuded deadly pressure.
Muichirou didn't rush in recklessly.
He had seen Kokushibou's ability—swords could erupt from his flesh at any moment.
But this chance is rare. I must try!
Mist began to spread across the battlefield.
Muichirou vanished within the dense fog.
"Mist Breathing – Seventh Form: Obscuring Clouds!"
He moved like vapor, speed fluctuating to disorient the senses. With blade in hand, he surged forward.
Bang!
Kokushibou's left hand gripped his weapon, while blades burst from his body, spraying sword energy in all directions.
Through the Transparent World, Muichirou's movements were visible—but Kokushibou lacked the strength to act on what he saw.
If that boy reaches me… I'll die.
Muichirou dodged the blades with uncanny agility. His Nichirin Blade was glowing red now.
He kicked off the ground, cracked the floor beneath him, and soared forward with a thunderous sonic boom.
"The red blade…" Kokushibou muttered, eyes narrowing.
Suddenly, a long blade shot out from his shoulder—twice as long as the others. He whipped his head to the side and slashed across Muichirou's path.
But—
"Wind Breathing – Second Form: Claw-Purifying Wind!"
Shinazugawa Sanemi burst into view, intercepting the strike. His red blade, accompanied by swirling blue air currents, crashed into Kokushibou's attack. Metal shattered, and fragments flew across the battlefield.
"Stone Breathing – Second Form: Upper Smash!"
The Stone Pillar arrived, riding his chain, swinging a massive spiked red hammer from above!
Too late to react.
Is this… the end?
In that final moment, Kokushibou saw a man in white, crying.
"How pathetic, brother..."
"Why was I born into this world… Yoriichi, tell me..."
BOOM!
The hammer smashed his skull.
Kokushibou's head flew into the air, only to be obliterated into dust.
His left hand still held the broken flute.
It disintegrated in his grip.
The last sound it made was a soft, distant chime—
Upper Moon One – Kokushibou: DEAD.
Øóffer going on for diamond tier
pàtreøn (Gk31)