Emiya discovered many interesting details in these medical texts.
For example, more than four hundred years ago, before the genius Yoriichi Tsugikuni was born, the Demon Slayer Corps did not possess breathing styles.
When fighting demons, they were at an overwhelming disadvantage.
In that era, a single ordinary demon could slaughter several Demon Slayer swordsmen.
To compensate, the Corps relied on numbers.
They would send several-sometimes over a dozen-swordsmen to surround and kill a single demon.
Even then, every victory came at the cost of lives.
At that time, demons were undeniably powerful.
But the Demon Slayer Corps were not fools.
After recognizing the gap in strength, they began researching demons.
Some proposed an extreme idea.
Use demons to fight demons.
In other words, allow humans to accept demon blood, gain power, and use that power against other demons.
So the Corps captured demons in large numbers.
They combined medical research with experimentation, attempting to adapt human bodies to demon blood.
The result was simple.
Failure.
Demon blood was highly corrosive.
Humans who accepted it rapidly degenerated and became demons themselves.
Of course, a very small number of people had partial resistance.
They could maintain their sanity for a short time after consuming demon blood, and their strength would increase dramatically.
This was similar to Genya Shinazugawa, the younger brother of the Wind Pillar.
Genya possessed exceptional bite force, digestion, and a special constitution.
By consuming demon flesh, he could temporarily gain demonic strength and regeneration.
The stronger the demon he ate, the greater the effect.
Looking back, Genya would have been considered outstanding even among those rare cases in the past.
Still, it was a brutal era.
To fight demons, members of the Demon Slayer Corps willingly stepped toward madness.
They constantly hovered on the brink of degeneration.
During that time, the Corps developed many medicines meant to suppress demon blood.
If that path had continued, it was possible that medicine would eventually have stabilized demon transformation while preserving sanity.
Strength without loss of self.
However-
At that moment, the genius of this world was born.
Yoriichi Tsugikuni.
Born with Sun Breathing.
Born with the Transparent World.
A man who could drive others to despair without effort.
While others who awakened the birthmark died before twenty-five, he lived past eighty.
His existence illuminated the entire Demon Slayer Corps.
Because of him, the Corps abandoned the path of suppressing demon blood.
They learned breathing styles.
With refined swordsmanship, they gained absolute dominance over demons.
Even Muzan Kibutsuji nearly died at Yoriichi's hands.
At that time, Muzan had not yet formed the Twelve Kizuki.
He was beaten so thoroughly that he barely escaped.
Afterward, he went into hiding.
Only after Yoriichi died of old age did Muzan emerge again.
He formed the Twelve Kizuki.
He initiated a systematic hunt against the Demon Slayer Corps.
The method of activating the birthmark was lost.
That state continued to the present.
Emiya closed one of the old books and shook his head.
The path of medicine-based demon suppression had been harsh.
But it had been effective.
It was unfortunate that it had been cut off halfway.
Yoriichi's appearance had ended it prematurely.
That left Emiya with some regret.
If Yoriichi Tsugikuni had never existed, the Demon Slayer Corps might look completely different today.
Still, these records were ultimately a dead end.
They held no practical value for Emiya.
So he didn't study them deeply.
They were merely curiosities.
What truly caught his attention were the breathing styles and sword techniques.
Those gave him ideas.
Four hundred years ago, Yoriichi created Sun Breathing.
Aside from Kamado Tanjiro's ancestor, no one truly mastered it.
Later, the Corps derived five basic breathing styles from it.
Flame.
Water.
Wind.
Stone.
Thunder.
But even those weren't suitable for everyone.
Many swordsmen created new styles based on their own habits and physical traits.
Insect Breathing.
Flower Breathing.
Sound Breathing.
Love Breathing.
Mist Breathing.
Serpent Breathing.
After creating them, their users would leave records with the Ubuyashiki family.
To prevent their techniques from vanishing after death.
Over four centuries, countless Pillars had done the same.
Some techniques were lost.
Others were preserved.
And all of them had been handed to Emiya by Kagaya Ubuyashiki.
Nearly a hundred breathing styles.
Each with corresponding swordsmanship.
There were even styles that predated breathing techniques entirely.
At first, Emiya didn't think much of them.
The only one he truly cared about was Sun Breathing.
But after studying the others, he realized something.
They weren't useless.
Although all breathing styles originated from Sun Breathing, time had changed them.
Only the five basic styles still retained clear traces of it.
The others-
Serpent.
Mist.
Flower.
They were barely related anymore.
They had evolved into independent systems.
Each had its own advantages.
Emiya knew this because he had already learned Sun Breathing.
After the Final Selection at Mount Fujikasane, when he accompanied Kamado Tanjiro to Mount Sagiri, Tanjiro had demonstrated the Hinokami Kagura.
Emiya memorized it then.
Before learning Nen, this wouldn't have been possible.
But now-
His physical capabilities had increased rapidly.
His mental power had surged.
His learning speed had reached an abnormal level.
He could master a breathing style after seeing it once.
Effortless.
In Emiya's view, Sun Breathing's greatest strength wasn't raw power.
All breathing styles enhanced strength, including Sun Breathing.
But that boost meant little to him.
What truly impressed him was Sun Breathing's recovery effect.
Tanjiro's father once said that with correct breathing, the Hinokami Kagura could be danced endlessly.
That sickly man danced through an entire night in the snow without exhaustion.
That alone proved its value.
Correct breathing allowed rapid physical recovery.
Even a weak body could endure endlessly.
Other breathing styles also extended stamina.
But none could compare to Sun Breathing.
Still, the others had unique traits.
Stone Breathing emphasized raw power.
Its users could wield massive weapons, each strike capable of shattering the ground.
Thunder Breathing prioritized speed and explosive force.
It demanded extreme physical fitness.
Its strikes were so fast they produced thunder-like sounds.
As Emiya examined these styles, a thought emerged.
What if-
He used Sun Breathing as a foundation.
Integrated the traits of all other breathing styles.
Created something beyond Sun Breathing itself.
And combined it with Nen.
This wasn't idle fantasy.
Emiya knew the upper limit of breathing techniques was extremely high.
For example-
Hamon breathing.
From JoJo's Bizarre Adventure.
Also called Ripple.
It mimicked sunlight.
Allowed ordinary humans to unleash tremendous power instantly.
It offered fine control, anti-aging effects, healing, even plant revival.
If Hamon could achieve that-
Then Demon Slayer breathing styles could as well.
They simply hadn't reached that stage yet.
No one had created it.
If someone was willing to try-
They might create a breathing style that surpassed Sun Breathing.
One that rivaled Hamon itself.
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