Heian period. Inside the birthing chamber of the Ubuyashiki clan, a mother was in labor.
Outside the chamber stood a man, nervously glancing about, desperate to peek inside. But the tightly shut doors barred any glimpse within, intensifying his anxious curiosity.
Minute by minute passed, but the chamber remained as it had since the beginning—apart from the mother's anguished cries, the sound of a baby's first wail never came.
Gradually, the mother's voice weakened. At last, the midwife emerged, her face grave.
The anxious husband rushed forward, seizing her arm, his voice filled with hope:
"Tell me—was the child born safely?"
The midwife looked at him with sorrow, unable to speak. She let out a long sigh and shook her head.
The man's eyes darkened instantly. He burst into the chamber.
Inside, his wife had already fainted from exhaustion. In the cradle lay their two newborns, faces bluish and silent, while four physicians worked desperately to save them.
In the end, each one sighed, shaking their heads in regret.
As head of the Ubuyashiki clan, this man had never shed tears no matter the hardships he faced—but now, his eyes brimmed uncontrollably.
His voice trembled as he asked the nearest physician:
"Both children… are they gone?"
The physician looked at the new father with pity—what should have been a joyful occasion had turned into a cruel misfortune.
"One of the children… was born without breath, without a pulse—a stillbirth. The other… is desperately weak. I fear he will not live much longer."
Just then, the mother awoke from her faint, gazing at her husband with exhausted yet hopeful eyes:
"Takuto… are they boys or girls?"
Ubuyashiki Takuto looked at her worn face, his lips unable to form words.
Seeing his expression, she knew something was wrong with her children. But she refused to accept it. Though her body was frail, she forced herself up and gathered the two silent infants into her arms.
When she saw her babies lying lifeless against her chest, the dam broke—she wept bitterly.
"Mikako, you mustn't cry. Your body is still too weak—you need to rest."
Takuto comforted her as best he could, but grief overwhelmed her. Her eyes rolled back, and she fainted again.
Takuto gently placed the child with a faint breath back in the cradle, while taking the stillborn into his arms.
In the Heian period, a stillbirth was considered a terrible omen. In great clans, a child born dead was to be cremated immediately, a ritual known as tabi—the funeral fire.
Takuto raised a torch, preparing for the rite. But then—
The lifeless infant stirred. His tiny body convulsed, and from his lips came the sharp, unmistakable cry of a newborn.
Startled and overjoyed, Takuto extinguished the torch and held the child tightly. The baby soon fell into slumber, his breath growing steadier.
Takuto rushed back to the birthing chamber, telling his wife the miraculous news. Husband and wife each cradled a child, their tears now of relief and joy.
He named the two boys Ubuyashiki Muzan and Ubuyashiki Yosuke.
Takuto summoned renowned physicians to nurse his children back to health. Yet after examination, their verdicts were grim:
Muzan, they declared, was afflicted with a terminal illness and would not live beyond twenty.
Yosuke, meanwhile, was born with organs so weak that even the simple act of breathing—a thoughtless act for others—demanded his full strength each time.
Five years passed.
Both Muzan and Yosuke lay bedridden in separate rooms. Thanks to careful treatment, the doctors' earlier prediction of Yosuke's imminent death had not come true—he endured stubbornly.
But his life was far from easy.
At this moment, Yosuke wore a face of despair, utterly defeated.
He had soiled himself again. His frail body offered him no control; incontinence had become a daily humiliation.
A maid removed his soiled clothes, cleaned him carefully, and withdrew.
Yosuke was not of this world. He was a transmigrator.
Only two years earlier, when his infant brain had matured enough, had his past memories returned.
But this "honor" as a transmigrator had brought him nothing but shame. He had crossed into a new life only to be bedridden, powerless, sustained by medicine alone.
Worse still, he had been reborn into the world of Kimetsu no Yaiba—a world where human life was as fragile as grass.
Though he had yet to hear of demons appearing, he knew from the anime that the boy lying in the next room—Ubuyashiki Muzan—was none other than Kibutsuji Muzan, the future King of Demons.
When Yosuke's memories returned at age three, he longed to crawl from his bed and end Muzan's life while he was still an infant.
But reality was cruel. Even speaking a single word was beyond him.
Just then—swish! The sliding door opened.
A maid entered carrying a milk bottle.
It was feeding time.
Ordinary children began eating solid food by age two or three. But Yosuke's esophagus could not swallow. Even at five, he survived only on milk.
"Young Master Yosuke, time for your milk~" The maid cradled him gently.
Yosuke shut his eyes in humiliation, enduring the forced feeding.
"Young Master Yosuke smiled! You must really like being in my arms~"
Five more years passed. At ten years old, Yosuke's condition had not improved. He could swallow some liquid food and barely control his bladder, but his voice was gone, his body limp.
Muzan, though terminally ill, fared slightly better. He could speak, even walk a little.
Yet Yosuke often heard his brother's voice filled with bitterness—resentment toward the parents who had cursed him with such a frail body.
Earlier that day, Yosuke overheard Muzan quarreling fiercely with their parents. Not long after, Takuto and Mikako entered Yosuke's room.
Takuto's face was heavy with guilt. Mikako's eyes brimmed with tears, her expression sorrowful.
Kneeling beside her son, Mikako forced a smile, her voice soft:
"Yosuke, did you eat well today? Does anything hurt?"
Yosuke longed to wipe away her tears, to tell her he had eaten properly, that though his condition had not improved, it had not worsened either.
But he could not. His lips produced only a weak whimper.
Takuto and Mikako's hearts ached. Since their children's birth, they had never known health—only constant torment.
"Let him rest, Mikako," Takuto said firmly. "I swear—I will find a physician who
can cure them, no matter the cost!"
His voice carried unshakable resolve. He would stake everything to save his sons.