Demon Blood and Breathing Techniques

Butterfly Mansion.

In the silent laboratory, only the faint glow from the microscope's bulb illuminated Kashu's focused expression.

He narrowed his eyes, his gaze fixed firmly on the sample of Nezuko's blood beneath the lens.

At that moment, he was trying to unravel a mystery.

Why was Nezuko so different, despite being a demon like the others?

As for the results of his analysis, they could only be described as bizarre.

It was not that demons themselves were bizarre; Nezuko herself was an enormous anomaly.

Under the microscope, the components of her blood seemed alive, continuously shifting and pulsing at an astonishing speed.

She had clearly become a demon, unable to eat normally, and had never consumed human flesh or blood.

Yet Nezuko seemed to possess an inexhaustible source of energy within her body, sustaining these incomprehensible transformations in her blood.

This completely defied everything he knew!

Even the alchemy he excelled at had to operate under the iron law of equivalent exchange.

With that thought, Kashu reached for the materials provided by the Ubuyashiki clan beside him.

There was not much written on the yellowed pages, only one cruel fact.

Every demon currently in existence originated from the blood divided by Muzan Kibutsuji.

Kashu had also examined other demon blood samples stored in the Butterfly Mansion.

Although those samples contained vitality far beyond that of humans, compared to Nezuko's blood, they were like stagnant pools—nowhere near as active as hers.

Demons possessed immense regenerative abilities. Even severed limbs could quickly regrow, so the possibility of the blood deteriorating after being separated from the body could basically be ruled out.

Combined with Tanjiro's account—

His entire family had been attacked. Only his younger sister Nezuko had survived after coming into contact with Muzan Kibutsuji's blood, though she had turned into a demon.

The answer seemed ready to reveal itself.

Nezuko's mutation had originated from Muzan Kibutsuji's blood, but had it somehow merged wondrously with some unknown and unique constitution of her own?

"Something like... a human who can rapidly adapt to becoming a Dead Apostle?"

Kashu's thoughts leaped back to his original world.

That world was bizarre and fantastical, filled with endless monsters and demons. His way of thinking was far broader than that of the people in this world.

Once the idea clicked, he immediately called over the Butterfly Mansion's medical staff and asked for a small favor.

Before long, a fresh sample of Tanjiro's blood was delivered to him.

After quickly determining its blood type with a centrifuge and saline solution, Kashu carefully mixed the two blood samples.

The results were both encouraging and disappointing.

The instant it came into contact with Tanjiro's blood, Nezuko's blood mutated at a drastically accelerated rate. Its activity doubled at once, as though oil had been poured onto a fire.

Unfortunately, that astonishing eruption lasted less than a few seconds. The rapidly changing blood seemed to exhaust all its energy, quickly falling still and completely losing its activity.

Clearly, the experimental sample was too small to sustain such a drastic change.

But the results were enough to explain the issue—

Tanjiro, Nezuko's elder brother by blood.

His blood could provide powerful fuel for the mutation within Nezuko, helping it evolve.

"This really is..."

Kashu shook his head with a bitter smile. An absurd thought took root in his mind and refused to leave.

If Tanjiro had been the one left at home back then, then after coming into contact with the Demon King's blood, might he have become a terrifying existence far surpassing Muzan himself?

Further investigation into Nezuko's condition would have to wait until Kagaya Ubuyashiki and Tamayo arranged their meeting.

After all, Tamayo's identity was special. It was inconvenient for her to appear at Demon Slayer Corps headquarters, so Kashu would have to visit her personally.

He carefully recorded the experimental data and his conclusions, then rose and left the laboratory thick with the smell of medicine.

The moment he pushed open the door, a voice carrying an obvious false smile drifted over.

"My, Mr. Kashu. Has your research made any progress?"

Shinobu Kocho, who had been waiting for some time, leaned against the doorway. Her gentle smile did not reach her eyes.

Kashu's gaze fell on Shinobu Kocho's overly perfect smile, but what flashed through his mind were the containers in the laboratory still carrying traces of poison.

After a moment of silence, he could not help but offer a warning.

"Wisteria poison has limited effect on demons in the end. Everything you are doing now is no different from a moth flying into a flame. It simply isn't worth it."

The smile on Shinobu Kocho's face vanished in an instant, replaced by an impenetrable coldness.

"I'm terribly sorry, but I don't understand what you mean."

Kashu shook his head helplessly. He understood the taboo of speaking too deeply with someone he barely knew, so he said nothing more and turned to leave.

Watching Kashu's retreating back, Shinobu Kocho clenched the fingers around her sword hilt with all her strength. Her expression was so dark it seemed water could drip from it.

"A moth flying into a flame... is it?"

She murmured softly, resolute flames of hatred burning in her eyes.

"As long as I can have my revenge, what does it matter if I burn in the blazing fire?"

When he returned to the temporary resting room arranged by the medical staff, the first thing Kashu saw was Fou lying on the bed, reading in a strange posture.

That was right—reading.

That pure white young beast, normally interested only in fighting and delicious food.

At that moment, it sat on the edge of the bed in an almost humanlike manner, its front paws carefully holding down an ancient book. Its furry head was lowered close, as though it were concentrating intensely on recognizing the words.

The book was among the materials Kagaya Ubuyashiki had specifically ordered the Kakushi to deliver, and its contents covered all manner of subjects.

They recorded the thousand-year history of the Demon Slayer Corps' struggle against demons, along with various secrets.

Kashu knew that Fou could read. Seeing it so engrossed, he did not disturb its rare refined pastime.

He walked straight to the desk and sat down. His eyes swept over an ancient book at the top of the pile, where four large words were written on the cover—

Water Breathing

"What is this?"

Kashu paused slightly, then immediately understood.

This should be the foundation Demon Slayer Corps swordsmen relied upon to fight demons—Breathing Techniques.

When he had fought Shinobu Kocho and Sanemi Shinazugawa earlier, he had keenly noticed something unusual.

Their breathing rhythms were highly distinctive. They inhaled astonishing amounts of air, yet exhaled relatively little.

That breathing was accompanied by an explosive increase in their physical abilities.

Kashu came from a world where mysteries far surpassed martial arts, and he found this method of gaining supernatural power through pure physical training quite novel.

After all, in his original world, a martial artist who had trained in boxing for twenty years could still not withstand a barrage of magecraft bullets.

Naturally, few magi would study the limits of the body's potential.

As for reinforcement magecraft and close-quarters combat, those were more like required courses for modern magi dealing with sudden situations, and could not be lumped together with this.

With curiosity in his eyes, he opened Water Breathing and began to read carefully.

The "Breathing Techniques" of this world were completely different from the breathing methods he knew.

In his world, breathing methods mostly referred to adjusting one's breathing rhythm and frequency during exercise to improve oxygen efficiency, steady the mind and body, and increase endurance.

But the Breathing Techniques here pushed the function of the heart and lungs to the absolute limit!

Through specific breathing patterns, one frantically took in oxygen far beyond the limits of ordinary people, forcing the blood to become highly active in a short time and unleashing physical strength astonishing enough to contend with demons.

Swordsmen who mastered Breathing Techniques to an even greater degree could even maintain this extreme state for twenty-four hours a day, continuously improving their physical condition like an ever-burning furnace.

"The person who created Breathing Techniques was definitely a genius."

After understanding their wondrous nature, Kashu offered sincere praise.

After quickly going through the methods of Water Breathing, Kashu closed the ancient book and sat upright before the desk.

He closed his eyes and began attempting to circulate Water Breathing as described in the book.

"Hiss—"

"Hoo—"

A large amount of air rushed into his lungs. He could clearly feel his heartbeat accelerate and his muscles subtly swell.

But after repeating it only several dozen times, Kashu frowned and opened his eyes.

He shook his head, then immediately picked up Flame Breathing from beside him.

Over the following period, he tried and experienced each of the Demon Slayer Corps' five most basic Breathing Techniques—

Water, Flame, Wind, Thunder, and Stone.

In the end, he came to a conclusion.

"These Breathing Techniques designed for ordinary people have very little effect on me."

Kashu leaned back in his chair, his fingers unconsciously tapping the desk.

"Relying solely on strengthening heart and lung function to improve physical abilities? It's far less efficient than directly using my reinforcement mage—"

Halfway through his sentence, Kashu's voice abruptly stopped.

His expression changed drastically as he sank into deeper thought.

Among the Divine Blessing he possessed was a skill called [Red Dragon Emblem].

Aside from greatly enhancing his magical resistance and mental perception beyond normal limits, its more important effect was granting him an incomparably powerful magical energy furnace core!

That furnace core allowed him to continuously generate enormous amounts of magical energy through nothing more than breathing!

People in this world had no magical energy, so they could only take a different path, strengthening their hearts and lungs by inhaling enormous amounts of oxygen, indirectly increasing their physical abilities.

Then what about Kashu, who possessed a magical energy furnace core?

His reserves of magical energy had already far exceeded those of ordinary magi. After receiving Divine Blessing, they had continued growing like a snowball rolling downhill.

Breathing Techniques used a specific rhythm of breathing to guide breath and strengthen the body.

A magical energy furnace core generated endless magical energy through breathing.

Reinforcement magecraft used magical energy as fuel to directly strengthen bodily functions.

What if... he could combine all three?

Could he create an entirely new Breathing Technique that directly guided magical energy through breathing, as precisely and efficiently as guiding breath, continuously strengthening his body?

Brilliant light burst from Kashu's eyes, like an explorer discovering a new continent.

"The magical energy furnace core... the principles behind reinforcement magecraft... the guidance techniques of Breathing Techniques..."

He muttered to himself, his voice filled with irrepressible excitement.

"I have every condition I need. Come on! Let me try—"

"Try to create a magical energy Breathing Technique that belongs only to me!"

Kashu suddenly sat upright, closed his eyes, and cast aside every distracting thought.

Following the preliminary path rapidly deduced in his mind, he began attempting to guide the endlessly surging torrent of magical energy born from his breathing.

The next moment, an immense force that seemed ready to burst him apart suddenly awakened and surged through him!

Agonizing pain struck!