The story takes place in the distant past.
During the Sengoku period, a samurai family welcomed twin boys. The older brother became the family's eldest son, while the younger brother, born with a strange "mark" on his head, was hated by his father, who even wanted to kill the child. However, the child's mother stopped him.
But the father did not give up and said that the younger brother must become a monk in a temple when he turned 10 years old.
The rooms the two brothers lived in, the clothes they wore, the education they received, and even the food they ate were completely different. Perhaps for this reason, the younger brother would tightly hug his mother every time he saw her. This made the older brother feel sorry for his younger brother.
The older brother would also secretly go to play with his younger brother, but after their father found out, he would be severely beaten. The older brother was afraid that his father would find out if he gave his things to his younger brother, so he made a flute by himself and gave it to his younger brother. He told him that if he needed help, he should play it, and the older brother would surely come to help him.
The older brother's goal was to become the strongest samurai. He always studied and practiced hard, believing that he could achieve this goal through his own efforts. When the older brother practiced with his sword, the younger brother would watch from the side.
On one day by accident, the younger brother showed amazing talent, beating his father's servant right after picking up a wooden sword.
Only then did the older brother understand that the younger brother, whom he had always felt sorry for, was actually better than himself. He had worked very hard, but compared to his younger brother, his progress was as slow as a turtle.
After the father learned about the younger brother, the positions of the two brothers completely changed. The family's inheritance was passed to the younger brother, and the older brother moved into the room that was only three tatami mats in size. Three years later, the older brother would be sent to the temple instead of the younger brother, unable to achieve his dream of becoming a samurai.
However, the younger brother found the older brother and said that he would be leaving for the temple soon and that he would treasure the flute the older brother gave him. The younger brother then left.
Later, the older brother accidentally saw the diary left by his mother and learned that the younger brother had understood everything long ago, and he had been like a fool showing off in front of him. From that moment on, the older brother was extremely jealous of his younger brother. He hated this genius from the bottom of his heart.
From then on, the bond between the two brothers was also separated by this impossible wall called "talent."
The younger brother did not go to the temple. Instead, he lost contact and disappeared without a trace. When the brothers met again, it was a night more than ten years later.
The older brother was attacked by a demon while camping outdoors. When the demon was about to attack the older brother, the younger brother appeared. He easily killed the demon and saved the older brother.
Seeing his younger brother's powerful appearance, his insides burned again with jealousy and hatred. He left his wife and children without hesitation and joined the Demon Slayer Corps where his younger brother was. His younger brother taught every Demon Slayer Corps member the Breathing Style, but he could never learn his younger brother's Breathing Style, so he could only settle for a different Breathing Style.
Later, strange patterns also appeared on the older brother's body, and his strength grew quickly, but he could never catch up to his younger brother.
To break the short-life curse of the strange patterns, he cut off the head of the current Demon Slayer Corps leader and gave it to the Demon King Muzan Kibutsuji, receiving blood from the Demon King and becoming a demon.
Decades later, the older brother, showing no signs of aging, met his aging younger brother. He was surprised that his younger brother was not affected by the curse, as if he would always be better than ordinary people, and attacked his younger brother out of jealousy.
The younger brother only shed tears, and with a single swing of his blade, he badly wounded the older brother.
"How pathetic, older brother!"
After the younger brother struck, the older brother thought he would be cut down in the next move, but the younger brother had already kept his sword-holding position and died of old age.
The older brother no longer had a chance to beat his younger brother. He angrily cut his younger brother's body in half at the waist, and the short flute hanging at his waist was also cut into two.
Seeing the flute that had been treasured his entire life, the older brother seemed to see his younger brother as he was when he made the promise. He shed tears and wore the flute on his body.
To this day, that flute still hangs at his waist.
Kokushibo's remaining hand touched the remaining half of his waist, where a short flute hung.
"Yes, when did I forget these things...?"
Kokushibo murmured, not even paying attention to Mist Hashira Muichiro Tokito, who was facing him like he was facing a dangerous enemy.
His six eyes lifted, looking at Mist Hashira Muichiro Tokito in front of him. The Transparent World scanned every inch of his muscles.
"You are my descendant..."
"Nonsense."
"When I was human, my name was Tsugikuni Michikatsu, the head of the Tsugikuni family. I left behind my bloodline. And Yoriichi Tsugikuni was my younger brother."
Muichiro could not tell if this was true or false. He simply gripped his Nichirin Sword tightly.
"So what? Do you think that by using the bond of blood, you can make me give up on killing you, who are now extremely weak?"
That's right, what Nakime didn't expect was that the Upper Moons injured by Seiichi recovered very slowly. Upper Moon One, Two, and Three all suffered very bad injuries, especially Kokushibo.
Half of his body had completely disappeared, and muscle fibers twisted together, flesh moving, trying to repair the body, but every time it recovered a little, there would be a violent burning feeling, and the speed simply couldn't keep up.
A Red Nichirin Sword...? And stronger than Yoriichi's.
"You are right, I am very weak now. If that's the case, then why don't you come forward and kill me?"
Kokushibo's six eyes gave off a chilling sense of pressure. His calm tone was as if it wasn't his own life being judged.
Remembering the past, Muichiro had seen him fight before and knew that blades would come out from Kokushibo's body. Getting too close might lead to being killed instead.
But! This chance while he was badly injured was rare. He had to try!
Mist began to spread, and Muichiro's figure disappeared into the slowly thickening mist.
"Mist Breathing—Seventh Form: Obscuring Clouds!"
His first move was a killing blow! In the mist, Muichiro was sometimes fast, sometimes slow, causing confusion to the eyes. He held his Nichirin Sword with both hands, in a running position.
Bang!
Kokushibo held his Nichirin Sword in his left hand, and blades grew all over his body, sending out countless sword attacks in all directions. He could see Muichiro's movements perfectly through the Transparent World, but he was very weak now. Once Muichiro got close, he might lose his head, so he couldn't take risks.
Muichiro turned sideways, dodged, avoiding one sword attack after another. The blade of his Nichirin Sword slowly turned red. He stomped hard with both feet, cracking the floor, and shot out, a sonic boom sounding around him.
"Red Nichirin Sword..."
His six eyes stared at the orange-red blade. Muichiro quickly got closer. Kokushibo slowly opened his mouth, and a blade instantly came out, twice as long as the others. He sharply tilted his head, and the long blade swept sideways in the direction Muichiro was coming from!
"Wind Breathing—Second Form: Claws-Purifying Wind!"
It was Wind Hashira Sanemi, who had arrived. His crimson blade, with a cyan airflow, heavily struck Kokushibo's extended blade. A crisp sound rang out, several pieces flew, and Kokushibo's attack was cut off!
"Stone Breathing—Second Form: Upper Smash!"
Stone Hashira, who had also arrived, stepped on his chain and fiercely swung his glowing red great hammer! A bursting whoosh of wind hit Kokushibo from behind!
Things are hopeless. Is today my death day?
Vaguely, Kokushibo saw an old man shedding tears.
"How pathetic, older brother!"
"Why was I born into this world, tell me, Yoriichi."
Boom!
A flash of mist, and Kokushibo's head flew up, then was crushed by the giant hammer coming from behind. His body, holding the flute in his left hand, caught fire without any flame. The broken flute fell to the ground, making a crisp sound.
Upper Moon One - Kokushibo, dead!