The clock turns back three months.
Tokyo, once known as Edo, now the heart of the nation and its political center.
Within a luxurious Western-style mansion somewhere in the city, an extraordinary shadow lay hidden.
Beyond the massive floor-to-ceiling windows, the garden plants were trimmed with meticulous precision, arranged in perfect symmetry alongside several Western-style sculptures, forming an elegant landscape.
Inside, expensive Persian carpets covered the floor. Luxurious yet cold ornaments and paintings were displayed everywhere, revealing the immense wealth of the master of the house.
This was the residence of Taizo Hirota, Director of Research and Development at Suzuki Pharmaceuticals.
The project under his supervision focused primarily on plasma product development, intended for emergency treatment on the battlefield and for exploration into deeper medical fields.
At this moment, he was pointing at a book while explaining to his adopted son, Toshikuni, helping him sort out the pharmacological interactions between different plants.
Suddenly, Toshikuni, dressed in an exquisite white shirt and appearing exceptionally well-mannered, twitched his brow and stretched with a long yawn. "Yaaawn..."
"Haha, are you sleepy, Toshikuni? Well, it is already past midnight. You really are a diligent child." Hirota smiled, patting Toshikuni's head as he closed the book and set it aside. "Get some good rest. We'll continue tomorrow."
To Taizo Hirota, Toshikuni was the perfect adopted son.
Well-read and courteous, kind to others, and especially hardworking.
The only pity was his skin condition. He couldn't be exposed to sunlight and could only study at home. He had no companions to play with either.
At this thought, Hirota's gaze grew heavy.
"I need to push those experimental projects harder and produce a special medicine as soon as possible! I can't let my Toshikuni live without ever seeing the sun!" he thought.
After Hirota left, Toshikuni rose gracefully from the bedside, smoothed his sleek black hair, and walked to stand before the floor-to-ceiling window.
In an instant, his pale purple eyes twisted sharply, revealing crimson pupils beneath.
A figure appeared before him in a wretched state, dropping heavily to one knee on the thick carpet, one hand clutching his neck.
Upper Rank Three, Akaza.
Traces of fierce battle still lingered on his powerful body, especially the grotesque wound at his neck.
Even with an Upper Rank demon's recovery speed, two days had passed and the fatal injury torn open by Asuka's blazing Nichirin Blade had yet to fully heal.
The edges of the wound were unnaturally charred black. Dark red demonic blood formed fleshy buds in an attempt to stitch it closed, yet they were resisted by a faint red-black aura, slowing the healing process to a crawl.
It wasn't that Akaza hadn't considered simply tearing away that chunk of flesh and enduring the pain to regenerate it anew.
But his fighting instinct had stirred a stubborn pride within him. He wanted to overcome this suppressive force with his own regenerative power, and so he had kept it until now.
At the same time, it served as a mark of humiliation branded upon himself. Defeated by another's hand, he would reclaim that honor.
"Akaza." Toshikuni's childlike voice carried no warmth, cold and piercing like a scalpel scraping across glass. "Report."
"Yes... Lord Muzan..."
That was right. The Toshikuni before him was the Demon King, Muzan Kibutsuji.
Once again, he had changed bodies, infiltrating a pharmaceutical household under the guise of a human adopted son, intending to use these scientists to find a way to conquer sunlight.
Muzan was extremely cowardly and selfish. He would not part with even a drop of his blood samples, fearing someone might discover his weakness.
When the value of this household ran dry, he would not hesitate to erase them entirely and conceal his existence.
Akaza did not dare raise his head. In a low voice, he began his report.
"The Mugen Train mission failed. Lower Rank One, Enmu, was slain. The targets, Tanjiro Kamado and the demon girl Nezuko, were not captured..."
"The reason for the failure? Do not force me to rummage through your brain, Akaza," Muzan said coldly.
"...We encountered the Demon Slayer Corps' Flame Hashira and... a swordsmith who called himself Arashi, a Hashira-level swordsman... very strong..."
"Very strong? Just two Hashira. What is strong about that?"
Muzan was truly angered. In his view, demons being superior to humans was a matter of course.
Even if all nine Hashira gathered, they were nothing more than useless fools.
He had driven the Demon Slayer Corps into panic countless times before. Naturally, he was deeply dissatisfied with Akaza's performance.
"Tell me, Akaza... have you grown complacent?"
"You keep talking about pursuing the peak of martial arts. Has it been too long since you tasted fresh flesh rich with vitality?"
"Have you grown weaker? Has your power declined?"
Just as Muzan was about to continue his tirade, his brows suddenly tightened as he noticed Akaza's unnatural gesture of clutching his neck.
With a slight movement of his fingers, Akaza's hand was forced away, revealing the not-yet-healed wound.
"This kind of injury again..." His gaze swept across the strange sensation emanating from the wound as he reached out to touch it.
So hot... His heart jolted as unpleasant memories surfaced.
"Lord Muzan... I..." Akaza began to explain the abnormal wound, but Muzan snapped sharply.
"Silence!"
His small frame radiated terrifying pressure as his slender, childlike hand suddenly seized the unhealed wound.
Rip—!
A tearing sound rang out. Muzan's fingers were like five sharp steel hooks, mercilessly plunging into Akaza's flesh, digging into the obstructive tissue and ripping it out in one brutal pull.
"...Urgh." Akaza let out a muffled grunt, his fist pressed against the ground clenching tightly.
Under Lord Muzan's destructive will, he had to endure not only the agony of flesh being torn away, but also the trembling that originated from the very core of his bloodline.
Yet he dared not utter another cry of pain, breathing only in heavy gasps.
In the blink of an eye, a bloody hole deep enough to expose bone had been torn into his neck, though it soon began to regenerate.
Muzan held the charred piece of flesh that had obstructed Akaza's healing, his pupils contracting slightly.
Beneath the broken fibers and blackened surface, he could sense a faint yet stubborn, peculiar fluctuation.
"This is..." He recalled the discomfort he had felt in his stomach after devouring Lower Rank Two, Rokuro. The sensation was identical to the fluctuation emanating from this fragment.
Without another glance at Akaza, he walked toward an ebony cabinet in the corner of the room, inlaid with brass clasps.
Inside, neatly arranged glassware and chemical instruments formed a miniature laboratory.
He swiftly retrieved a specially made glass flask and carefully placed the charred flesh imbued with that strange power inside, sealing it with a customized cork stopper.
Muzan stared at the flask, crimson eyes gleaming with cold and dangerous light.
He sensed that he had discovered some unknown force of extreme danger and immense research value.
After locking the cabinet once more, he turned back to Akaza. The fleeting surprise on his face had vanished, replaced again by bone-chilling indifference.
Without requiring another word, he extended his icy little finger directly into Akaza's forehead.
"Let me see what kind of thing that swordsman who left you in such a miserable state truly is," Muzan said flatly.
Akaza could neither resist nor refuse.
Muzan's will brutally invaded the depths of his mind, rummaging through every memory fragment of the battle aboard the Mugen Train.
Kyojuro Rengoku... Flame Breathing... the boy with earrings...
Muzan swept through these images one by one, until his gaze locked onto the black-haired youth.
His Nichirin Blade had turned molten red like lava, the blade vibrating and humming under the strain of immense power.
At the corner of his eye, that savage red-black serrated mark—like living flames—stabbed painfully into Muzan's vision.
Boom—!
A boundless terror, buried in the deepest recesses of his soul for centuries, erupted like the most violent volcano within Muzan Kibutsuji's heart.
The scene overlapped perfectly with the deepest nightmare he could never escape.
The man who had frightened him out of his wits, forcing him to split into eighteen hundred fragments to survive.
The nightmare that had dragged him down from the altar of a perfect being and sent him hiding like a stray dog for hundreds of years.
The existence whose very name froze his blood and compelled him, by instinct alone, to flee at any cost—
Yoriichi Tsugikuni.