During the Heian period, around 794 AD, on a certain day.
A peculiar figure who would bring calamity to future generations was born into the Ubuyashiki family.
Even before birth, this infant was shrouded in the shadow of death.
His heart stopped beating for a time while he was in his mother's womb due to his weakness.
He was even born into the world as a stillborn infant.
Although he was born by chance, he was diagnosed with a terminal illness and was told he would not live past 20, so he was bedridden most of the time.
A kind doctor developed a medicine to prolong his life.
And it was because of this medicine that the first demon was born in this world.
Muzan Kibutsuji!
Muzan Kibutsuji is extremely cruel and ruthless, lacking human emotions and possessing very low empathy.
To compensate for his weakness as a demon, which is afraid of sunlight, Muzan Kibutsuji created a large number of demons to search for the legendary blue spider lily.
In an instant, demons roamed the world, and all living beings suffered.
Centuries later.
Another extraordinary person has been born into the world.
His name is Yoriichi Tsugikuni.
Yoriichi Tsugikuni seems to be a natural nemesis of Muzan Kibutsuji.
After joining the Demon Slayer Corps, he created the first Breathing Technique, Sun Breathing.
He also imparted all his knowledge to the Demon Slayer Corps' Demon Slayers.
Primarily using the Sun Breathing, the first swordsmen to employ this breathing technique appeared.
The Demon Slayer Corps' power far surpasses that of the demons.
The era of rampant demons was finally brought under control.
Years later, Yoriichi's older brother, Michikatsu Tsugikuni, who possessed the Moon Breathing technique, which, although derived from the Sun Breathing technique, was also considered an initial form of breathing, was seduced by Muzan Kibutsuji and became a demon.
Not long after, Muzan Kibutsuji and Yoriichi Tsugikuni had a fateful encounter.
Yoriichi Tsugikuni understood the moment he saw Muzan Kibutsuji.
He was born to defeat this man.
A battle ensued between a human and a demon.
Yoriichi Tsugikuni, a human, nearly killed Muzan Kibutsuji with his bare flesh and blood.
Just before he was about to be killed.
Muzan Kibutsuji chose to self-destruct. In order to survive, Muzan Kibutsuji shattered his body into 1,800 pieces.
Yoriichi Tsugikuni immediately swung his sword and cut off more than 1,500 of them, but the remaining pieces of meat were too small. All of them together were about the size of a human head and escaped the disaster.
Muzan Kibutsuji survived.
…
Later, because Yoriichi Tsugikuni failed to kill Muzan Kibutsuji, he let Muzan's subordinate Tamayo escape. Moreover, Yoriichi's older brother, Michikatsu Tsugikuni, cut off the head of the then-lord and took the head to join Muzan Kibutsuji, becoming a demon.
For various reasons, Yoriichi Tsugikuni was expelled from the Demon Slayer Corps.
From this moment on, the balance that had previously been maintained by the Demon Slayer Corps over the demons was once again broken.
However, after the battle between Muzan Kibutsuji and Yoriichi Tsugikuni, a researcher from the Demon Slayer Corps who was cleaning up the battlefield secretly preserved a piece of Muzan Kibutsuji's flesh and blood.
This researcher secretly conducted research on Muzan Kibutsuji's flesh and blood without telling anyone.
However, the flesh and blood severed by Yoriichi Tsugikuni's Sun Breathing had completely lost its vitality and was no longer of any research value.
Just when the researchers were about to give up and destroy the remains, they were surprised to find that the lifeless flesh and blood of Muzan Kibutsuji reacted when it came into contact with Yoriichi Tsugikuni's cell tissue in the culture dish.
The researchers became interested and immediately began their research.
Not long after, researchers successfully cultivated and manufactured a drug.
Researchers believe that by taking this potion, one can obtain the combined power of the strongest demon and the strongest human, thus gaining immortality and endless potential.
However, just as the researchers were about to take the medicine, an unexpected incident occurred.
In order to eliminate all swordsmen who knew how to use the Sun Breathing technique and sever its lineage, Muzan Kibutsuji dispatched demons to attack the Demon Slayer Corps' base.
The researchers died in the Northwest before they could even take the medication.
The medicine also fell to the ground and was buried deep underground.
...
Centuries passed, and the Taisho era arrived.
Humans gradually settled on this ruined land, and it developed into a small mountain village.
One day, a woman was washing clothes by the river when she suddenly saw a wooden basin floating on the river. Inside the basin was a newborn baby, still in swaddling clothes.
The woman picked up the baby from the wooden tub and felt pity for the adorable infant who was neither crying nor fussing.
"Poor child, were you abandoned by your parents right after birth? You'll live with us from now on."
The woman took the child home, where her husband was also present.
"Miyako, who is this child?"
"Kenichi, I found this child while I was washing clothes by the river. Look how cute he is. He was abandoned at such a young age. Shall we adopt him?" Miyako pleaded with her husband.
The couple originally had a child, but during the war, they were forcibly conscripted and died on the battlefield not long after.
Disheartened, the couple chose to settle in this remote mountain village.
Kenichi, who has no children, was tempted by Miyako's suggestion.
Looking at the baby in swaddling clothes, Kenichi nodded: "Then let's raise this child together."
Miyako beamed with joy, happily teasing the baby in swaddling clothes.
"Kenichi, what's this child's name?"
After pondering for a moment, Kenichi blurted out, "Katō Mitsuko, what do you think of this name?"
"Do maple leaves symbolize longing, a way of remembering our deceased child, Sōta...?"
Miyako's expression was somewhat sad. Looking at the child in her arms, she smiled gently and said, "Mitsuko, from now on you will be Kenichi and my child, Katō Mitsuko."
Although Mitsuko Katō was an adopted child, Kenichi and Miyako treated her as their own, just like their biological child.
Mitsuko thrives surrounded by the selfless love of Kenichi and Miyako.
…
Six years have passed in the blink of an eye, and Mitsuko is now six years old.
From babbling and taking his first steps to now speaking clearly and walking briskly.
Everything was so beautiful.
Spring plowing season has arrived.
The weather is great today, the sun is shining brightly.
Kenichi and Miyako were loosening the soil in the field, preparing for sowing.
Mitsuko was also playing in the field, and even though his face was covered in mud, he was still having a great time.
suddenly.
Under the sunlight, Mitsuko noticed something glittering in the soil.
Full of curiosity, Mitsuko went to the spot where the light was shining, dug away the soil, and found a well-sealed pottery jar.
Driven by curiosity, Mitsuko opened the earthenware jar, which contained a blue liquid.
The fragrance emanating from the liquid continuously stimulated Mitsuko's sense of smell.
Mitsuko swallowed hard, covered the earthenware jar, and wanted to take it home to show his parents what it was.
But the scent lingered in Mitsuko's mind, constantly tempting him.
As if possessed, Mitsuko seemed to have lost his mind, opened the earthenware jar, picked up the mysterious blue liquid inside, and drank it all in one gulp.
When Mitsuko came to his senses, he suddenly felt hot all over and his whole body ached, as if thousands of ants were gnawing at his body.
Unable to bear the pain, Mitsuko fainted in the field.