Kael walked toward Kibutsuji Muzan, observing the reaction process of the uv light experiment.
These high-intensity uv lights he had crafted were almost capable of perfectly simulating sunlight, yet they still failed to pose a fatal threat to Kibutsuji Muzan.
After all, it wasn't the real sun.
This world, after all, was not entirely materialistic.
Beyond emitting light and heat, the sun carried other meanings.
"You deserve to die! You deserve to die!!"
Kibutsuji Muzan hissed and roared, tearing his disguise to shreds and fully exposing his demon side; his blood-red body looked exactly like a skinned toad.
Although the high-intensity uv lights couldn't deal a fatal blow, the physical and mental damage to Kibutsuji Muzan was immense.
Back when he was still human, he had suffered from illness for years, hovering on the brink of death, which made his fear of death and attachment to life reach a pathological extreme.
Simply put, he was so terrified that his mental defenses had completely shattered.
Ignoring Kibutsuji Muzan's shouting, Kael took out a specially made syringe and plunged it into his heart.
Under normal circumstances, given Kibutsuji Muzan's absorption capabilities, the needle would have been digested and absorbed by his flesh the moment it pierced him.
However, under the suppression of the uv lights, all of his attributes and abilities were severely weakened.
"What are you doing?"
Kibutsuji Muzan didn't understand Kael's actions, but his emotions were becoming increasingly uncontrollable—especially when he saw the indifference in the other's eyes, he felt as if his pride and self-esteem were being trampled underfoot.
For him, this was the most intolerable thing of all.
A mere human?!
"Muzan!!"
A thunderous shout rang out from Sanemi Shinazugawa.
The Hashira of the Demon Slayer Corps were arriving at the battlefield one after another.
Kibutsuji Muzan couldn't care less about his original plan now; the concentrated uv light was causing him excruciating pain, while the hope of eternal life was just within reach.
The sound of a biwa string being plucked echoed through the air.
A wooden door appeared out of thin air beneath Kael and Kibutsuji Muzan, opening the gateway to the Infinity Castle.
"Kael!!"
Seeing this, Shinobu Kocho's heart tightened instantly, and she cried out in shock.
However, in the next second, Kibutsuji Muzan and Kael disappeared from everyone's sight, leaving only the heavy breathing of the Hashira in the Ubuyashiki estate.
Were they a step too late?
Gravity dragged Kael downward.
Without the suppression of the uv lights, Kibutsuji Muzan's condition was visibly improving. The expression of near-collapse on his face shifted into a twisted look of triumph and the satisfaction of getting his wish.
But this expression didn't last long.
Kael slammed his foot onto Kibutsuji Muzan's face.
In an instant, that head burst like a watermelon; the scene was quite gruesome.
Under the counterforce, Kael's downward momentum came to an abrupt halt.
He then landed steadily on the ground.
The headless Kibutsuji Muzan served as his footstool.
This humiliation was more than Muzan could bear. His arms deformed violently, turning into two blood-colored spiked whips that lashed out to kill Kael at a speed the naked eye of an ordinary person couldn't possibly track.
Essentially, by the time one heard the whistle of the air being torn apart, their body would have already been shredded into pieces.
Moreover, these spiked whips carried Kibutsuji Muzan's will, allowing him to change the direction and shape of the attack at will, and the blood they carried was a lethal toxin.
However, faster than Kibutsuji Muzan's spiked whips was the nichirin blade in Kael's hand.
Before the spiked whips could even get within a meter of Kael, they were severed in an instant.
Kibutsuji Muzan didn't even realize what had happened before his body was also minced into strips of meat.
The "Solar Power" within the nichirin blade was fully activated by Kael—the legendary "Bright Red Blade" form, which could greatly inhibit a demon's regenerative abilities.
One only had to look at the wounds where Kibutsuji Muzan had been cut.
The granulation tissue attempting to regenerate and heal kept withering and dying under the influence of this power, only to sprout and regenerate again, over and over.
"You?!"
The moment Kibutsuji Muzan's mouth grew back, Kael stepped on him again.
The surrounding demons watched this scene in a daze, momentarily unsure of what to do.
The Kibutsuji Muzan they feared and revered was actually being trampled underfoot?
But soon, Kibutsuji Muzan's consciousness descended upon all the demons.
It was as if a silent horn had been blown.
All the demons surged toward Kael together, like waves crashing against a coastal reef.
For a moment, the air was filled with demonic wailing and howling.
One could vaguely hear the reluctance within it.
Could demons like them, who couldn't even reach the rank of the Twelve Kizuki, possibly hope to challenge a demon slayer who could trample Kibutsuji Muzan?
Weren't they being forced?
They weren't sure if they would die by Kael's blade if they hesitated, but they would certainly be obliterated by Kibutsuji Muzan's curse.
Kael looked up at the approaching demons, his blade pointing downward.
Then, one could see the swarms of demons, like moths to a scorching sun, being annihilated in batches, radiating outward from Kael's feet.
Even as demons, they couldn't see Kael's attacks; all they could feel was the agony of being incinerated in an instant.
"Five brains and seven hearts... how afraid of death are you, Miss Muzan?"
Kael observed Kibutsuji Muzan's cross-section and couldn't help but comment.
To evade death, Kibutsuji Muzan had certainly put in the effort, even willing to study medical knowledge to change his physiological structure; he was a man of action and practice.
Precisely because of the modification of his organs, Kibutsuji Muzan's regeneration was almost limitless and required no stamina.
To be honest, studying this guy's body had taught Kael quite a bit of biological knowledge; he could afford to slice a bit more carefully.
Just then, Kibutsuji Muzan's lower body whipped out eight more tube-like whips. This was his strongest killing move, with an attack speed far exceeding the whips from other parts of his body.
A bone-shaking sonic boom echoed through the air.
However, the result remained the same. The moment the eight tube-whips were about to strike, they were all severed by Kael.
This was the gap in basic stats.
Kibutsuji Muzan was strong, but only relative to humans.
If he were placed in a different setting without his stat advantage, given his abilities and will, he would at most be a bully relying on immortality.
One only had to look at his changes over the last few centuries.
Compared to when he faced Yoriichi Tsugikuni, there had been no improvement.
Hundreds of years ago, he was one-shotted by Yoriichi Tsugikuni; hundreds of years later, in the same scenario, the result would be the same.
"You cannot kill me!"
Kibutsuji Muzan's triumphant voice rang out from his abdominal cavity.
He had to admit that Kael was the strongest swordsman in the thousand years since Yoriichi Tsugikuni. However, he couldn't kill him; only sunlight and Sun Breathing could threaten his life.
But within this Infinity Castle, there could never be sunlight.
As for Sun Breathing, it had long been lost. Everything related to Yoriichi Tsugikuni had been thoroughly erased by Kibutsuji Muzan; there could be no more successors.
Human stamina was ultimately finite. Once Kael ran out of strength, it would be the moment of his own eternal life.
Kibutsuji Muzan was incredibly smug in his mind.
"Let go of Lord Muzan!"
Akaza shouted angrily.
Along with his voice came the shockwaves from his fists.
(Blood Demon Art: Destructive Death - Air Type)
However, before this attack could even get close to Kael, it was instantly dispelled.
"This guy! What on earth is going on?"
Akaza stared at Kael with a grim expression; he couldn't sense even a trace of an aura from him, as if the man were one with the world itself.
This sense of power was something Akaza had never experienced before.
Arriving with him were the other three Upper Ranks. As for the original Lower Ranks, they had either died at the hands of the Hashira or at the hands of Kibutsuji Muzan.
"Don't look at me, don't look at me..."
Upper Rank Four Hantengu cowered in the corner of a railing, shivering.
This guy looked like an elderly man, appearing cowardly and afraid of death. Kael only glanced at him, and it scared a scream out of him.
Upper Rank Five Gyokko rolled on the ground, his deformed and twisted body squeezing out of the pot's mouth, his eyes darting restlessly at Kael as if he were itching to act.
Finally, Upper Rank One Kokushibo walked toward Kael. He still retained much of his appearance as a human swordsman, though most of his face was covered by six demonic eyes.
He didn't look as inhuman as the other demons, but the pressure and fear he radiated were the most intense.
Kokushibo looked at Kael and asked calmly.
"State your name, swordsman."
"What an impolite demon. You haven't introduced yourself yet."
Kael said without even looking up, glancing at the silent, playing-dead Kibutsuji Muzan.
"Twelve Kizuki, Upper Rank One, Kokushibo."
Kokushibo's tone was calm, like a pool of stagnant water that had long since stopped flowing.
"Kokushibo, is it? I thought you were called Michikatsu Tsugikuni."
Upon hearing this name, Kokushibo didn't react much, but Kibutsuji Muzan's body shuddered first.
"I didn't expect anyone to still remember that name."
A slight ripple appeared in Kokushibo's tone.
"Even though this guy has been trying hard to erase the existence of that first swordsman who created the Breathing Styles, unfortunately, this world produced a bug like me."
Kael stepped on Kibutsuji Muzan and continued.
"You can just call me Kael. Remember to report this name when you go to hell."
"I'll let my fish tear you apart first!"
Ten pots with sticky fish patterns appeared one after another on Gyokko's row of deformed small hands, and a large number of sticky fish spewed from the mouths of the pots.
These sticky fish, formed from demon blood, not only had strong attack power but also carried lethal toxic corrosion.
(Blood Demon Art: Ten Thousand Gliding Slime-Fish!)
However, at the very moment Gyokko raised his hands, a crack of thunder exploded.
Where did thunder come from in the Infinity Castle?
Doubt arose in Gyokko's mind, and before he could react, he realized he had already been attacked as his life began to drain away rapidly.
He strained to glance back and barely saw Kael's back, but that was his limit.
The pot carrying Gyokko's life shattered.
The swarms of sticky fish filling the air also vanished into blood mist with their master's death.
"So fast!!"
Akaza shouted in shock. What chilled him was that the sound of thunder rang out again alongside Kael's footsteps.
This time, the target was Upper Rank Four Hantengu.
To Akaza's eyes, he could only barely see a jagged, fierce flash of fire.
That was the explosive momentum generated by the combination of Thunder Breathing and Flame Breathing.
"Don't..."
Upper Rank Four Hantengu's shrill scream came to an abrupt end.
Only then did Nakime's biwa string sound again.
The spatial architecture within the Infinity Castle began to rotate, attempting to separate Kael from the demons.
Kibutsuji Muzan intended to use his centuries of accumulation to wear Kael down to death.
At this moment, he really didn't dare to face Kael directly.
Even if the other couldn't kill him yet, every swing of Kael's blade seemed to lay bare the deepest fears in his heart.
Watching the Infinity Castle spin like a kaleidoscope, Kael stopped in place and whispered to himself.
"This is actually a bit boring. You're not even as good as the sisters in Yoshiwara; their willpower was firmer than yours."
He had originally thought Kibutsuji Muzan could provide some fun, but he had overestimated him. After only a slight increase in intensity, this guy's mentality had already collapsed.
The fun of the game was gone, so there was no point in playing anymore.
Kael pulled out the Demon King's blood he had collected earlier and injected it into his body.
His perfect potion had long since been developed.
And this was the final catalyst.
Seeing this, Kibutsuji Muzan was first stunned, then ecstatic, and finally, his expression froze.
The will hidden within the demon blood was obliterated the moment it entered Kael's body.
In this brief mental clash, Kibutsuji Muzan was utterly defeated.
However, that wasn't what broke Kibutsuji Muzan the most. Kael changed his starting stance before his very eyes.
"Sun Breathing..."
You actually know Sun Breathing?!
Kibutsuji Muzan watched the blazing sun slowly rising before him, every cell in his body screaming in terror.
Two years ago, Kael had already reproduced Sun Breathing.
He didn't even need Tanjiro to teach him; he had reverse-engineered it by combining the five basic Breathing Styles.
It was just that Kael hadn't used it since then.
Because he couldn't yet perfectly master Sun Breathing as he did with the other styles.
With just a little effort, he wouldn't be able to control the body's liberation.
A Breathing Style was a key to unlocking human potential, and the Markings was the symbol of that liberation.
It was easy to open, but difficult to close again.
In this world devoid of supernatural factors, the result of liberating human potential was self-combustion.
Once the evolution of life began, there was no way to stop or go back.
It was like a boiler; if new water wasn't continuously replenished, the outcome once the water boiled dry was self-evident.
And with Kael's current life level, once liberated, he wouldn't be a water-boiling boiler, but a planetary engine.
Of course, life was not a boiler after all; it would find its own new way out.
Just like Nezuko, but that was a result of forced adaptation.
But Kael was not a demon, and he didn't have that kind of self-adaptability.
So he needed the perfect potion to act as the steering wheel for his body's liberation, and the demons before him were the fuel to drive his life's evolution.
Kibutsuji Muzan was now scared out of his wits, constantly urging Nakime to increase the distance between him and Kael.
One after another, doors slammed shut as the space was continuously elongated.
However, Kibutsuji Muzan's collapsing mood showed no sign of improving. What made him extremely broken and terrified was that in Kael's eyes, which burned like torches, he saw no trace of killing intent—only the appetite that emerges from hunger.
I've become a dish?!