Only two girls out of this family of six survived in the end.
Of them, the girl who survived thanks to her mother's protection was gravely injured, the kind that could turn fatal at any moment.
Fortunately, she ran into Charles. After he poured a vial of panacea into her, the wounds on her body began to knit rapidly.
But the older girl was different; her situation was much more troublesome.
She was badly hurt too, but her biggest problem now was no longer her injuries.
An abnormal power inside her was steadily eroding her body, constantly converting her into a different kind of being beyond human.
Under the influence of that power, even her wounds had begun to heal so she wouldn't die from them. But if the transformation completed, the girl would fully change from human into a man-eating demon.
Charles had already recognized the girl and her family, and in turn knew which world he had entered.
Demon Slayer: Kimetsu no Yaiba, also colloquially called Pillar-Slaying Blade, a world revolving around demons and demon slayers.
So-called demons are man-eating monsters that fear sunlight and can live only in darkness.
In exchange for that weakness, they gain limitless longevity and a powerful vitality that can regenerate from any wound. High-ranking demons can even wield special abilities known as Blood Demon Arts.
And there is only one source of all demons: the Demon King, Muzan Kibutsuji.
Muzan Kibutsuji was born in the Heian period and nearly died several times in his mother's womb, even being delivered as a stillbirth. Surviving at all took luck.
Throughout his growth he was beset by illness, repeatedly deemed likely to die at any time.
Thankfully, a physician refused to give up and promised to cure him.
But after taking the physician's medicine, Muzan did not recover his health, so in anger he killed the physician who had cared for him all along.
Ironically, after the physician's death, Muzan discovered his body truly had regained vigor, and time no longer affected him. He would not age and would grow stronger as time passed. Eventually, he could even stand alone against an army.
The price, however, was a hunger for human flesh and blood, and a fear of sunlight—the latter being a flaw Muzan could not tolerate.
Only after finding the physician's prescription did he realize the medicine he had taken lacked the final ingredient: the Blue Spider Lily.
Sadly, the only person who knew anything about the Blue Spider Lily was the very physician he had killed with his own hands.
To seek the Blue Spider Lily and become a truly perfect being, Muzan shared his blood with others, creating large numbers of man-eating demons.
Those who battled these demons and protected ordinary people were the swordsmen of the Demon Slayer Corps.
The Breathing Styles allowed them to confront physically powerful demons head-on, and blades forged from special ore could eradicate demons by severing their necks.
The story of this world unfolds around the struggle between the two sides.
The malevolent aura Charles had sensed earlier belonged to the Demon King, Muzan Kibutsuji.
And the ones he attacked were the family of this world's protagonist—the charcoal seller Kamado household.
Muzan likely had no idea that his impulsive assault on this family would drastically alter his fate.
This attack produced two consequences. One was the creation of the first demon to overcome the weakness to sunlight—Nezuko Kamado, the Kamado family's eldest daughter now before Charles.
That was, aside from the Blue Spider Lily mentioned only in the physician's notes, another existence that could allow him to overcome his sole weakness.
To devour Nezuko and achieve his final evolution, the climactic battle of the original plot erupted.
The other consequence was setting the protagonist, Tanjiro Kamado, on the path of the demon slayer, ultimately becoming the blade that felled Muzan.
The reason for Muzan's attack on the Kamado family is unknown, but the Kamados were undoubtedly special.
A hundred years ago, Yoriichi Tsugikuni, the swordsman who created the Breathing Styles and came within a step of slaying Muzan, passed his swordsmanship to an ancestor of the Kamado family.
For a century, the Kamados preserved this sword style in the form of the Kagura dance, together with Yoriichi's Hanafuda earrings.
The house where the Kamados lived had once been Yoriichi's residence and was also where his wife—killed by demons—was buried.
The Blue Spider Lily that Muzan had sought all along once bloomed upon that grave, and Kie Kamado, Tanjiro's mother, picked it.
Given children's habit of putting anything they see into their mouths, Nezuko's uniqueness—and Tanjiro later overcoming sunlight even after being possessed by Muzan—very likely came from the two of them having eaten the Blue Spider Lily.
There's even a joke that the Kamado family's pickling jar might have had Blue Spider Lily in it, and Muzan missed it by sheer bad luck.
Charles could feel that the energy eroding Nezuko Kamado was indeed mixing with something inside her and producing a change.
He had no way to stop this change, because it wasn't a lesion or injury but a living being's instinctive evolution toward a higher level.
So with Nezuko, Charles could only watch and wait. After settling Nezuko and the surviving Hanako Kamado, he began to consider how to proceed in this world.
Everything worth Charles's attention here lay with the demons and the Demon Slayer Corps.
This should have been a world without the supernatural, but Muzan's appearance changed that.
Yoriichi's claim that he came to this world because of Muzan may not have been wrong. His innate mission was to slay Muzan, hence the birth of the wondrous Breathing Styles.
As for the demonic power derived from Muzan, Charles wouldn't consider it at all, even putting aside stances and personal views.
Given the world's tier, demonic power held nothing worth Charles's study.
Frankly, even if Muzan could inject his blood into Charles, Charles's own mana would spontaneously purify it until nothing remained.
The blood of such low-grade stock couldn't contaminate Charles's body.
What was worth considering was the Breathing Styles of the Demon Slayer Corps. A power that triggers supernatural phenomena through swordsmanship was something Charles found quite intriguing.
Even by Charles's standards, the ceiling of the Breathing Styles was very high.
Its founder, Yoriichi Tsugikuni—called a man who walked among mortals with the bearing of a god—proved as much.
In his battle with Muzan, he unleashed more than fifteen hundred strikes in an instant. That's an absurd feat; at his current level, even giving his all, Charles estimated he could only manage about seven hundred.
Muzan once said that Yoriichi's very existence was the greatest mockery of the world's common sense; in short, even demons thought he was outrageously beyond human.
Perhaps a single slash from Charles could rival fifteen hundred, but the two were entirely different in nature and not comparable.
It was Yoriichi who was that strong, and no one after him ever reached that height.
Yoriichi also said that those who pursue the Way to its limit all arrive at the same destination. Charles was very interested in that destination of all the strong—the so-called supreme realm.
Precisely because such things exist that surpass the limits of the world, Charles felt this trip would not be for nothing.
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