There was no trace of chakra, no sign of senjutsu—only an unfamiliar energy, something entirely different. It was obvious this belonged to a system he had never encountered before.
The shinobi world had always been unique in that way, filled with countless methods of cultivation, many of them hidden from the public eye.
The most widespread was the chakra system. And it wasn't just shinobi who used chakra—samurai did as well.
Beyond that, there were systems that harnessed natural energy, like senjutsu, and others practiced by shrine maidens, among many more.
So the sudden appearance of a system he didn't recognize didn't trouble him much. If anything, he accepted it readily.
"So these Hashira… to truly bring out their strength, they need to wield their swords? Only when they execute their sword forms does their aura peak?"
Uchiha Sasuke watched closely as each Hashira in the footage unleashed their techniques, their presence surging dramatically the moment their forms were completed.
Clearly, this was the true essence of the Breathing Styles.
So the method Kaede Kitahara had taught him today… was this very same Breathing technique?
As everyone continued to fall, members of the Demon Slayer Corps began appearing one after another—most of them young faces.
The last to descend was Kamado Tanjiro. As he landed on a platform, Kibutsuji Muzan stood on a distant elevated stage, his expression twisted with fury. Gone was the calm composure from earlier—now he stared down at Tanjiro with a ferocious glare.
"You think this is enough to corner me?" Muzan roared. "You're the ones headed for hell! You filthy demon slayers… tonight, I'll wipe you all out!"
"The one going to hell is you, Muzan! We'll never let you go!"
Tanjiro shouted back, his voice raw with fury.
"If you can do it, then try!" Muzan bellowed.
"We will destroy you, Kibutsuji Muzan!"
Tanjiro said as he descended.
Seeing this, everyone recalled what Kaede had written in his diary before—the battle within the Infinity Castle.
So this mysterious space… was the Infinity Castle?
The name suited it perfectly. The place stretched endlessly, vast beyond measure.
Before they could dwell on it further, the scene shifted.
A blond swordsman sprinted through the depths of the Infinity Castle at incredible speed.
They recognized him immediately—one of the Demon Slayer Corps members who had made quite an impression earlier when he fell into the castle.
His expression carried a dark edge as he dashed forward.
"I can hear it clearly… that guy's definitely nearby."
Demons attempted to ambush him along the way, but each one was cut down in an instant.
"Unforgivable… I'll never forgive you!"
Cold resolve burned in his eyes.
"Thunder Breathing, First Form—Thunderclap and Flash."
Uchiha Sasuke recognized the technique immediately. It would have been impossible not to—after all, he had spent the entire day practicing Thunderclap and Flash.
But there was a key difference. Agatsuma Zenitsu's Thunder Breathing carried no chakra, while the version Sasuke practiced was infused with it.
"So it's been specifically modified?"
Sasuke didn't bother questioning where Kaede Kitahara had obtained Thunder Breathing from. That wasn't what mattered.
Soon enough, Zenitsu located Kaigaku's hiding place.
"I'm sure of it. He's nearby."
"That guy… he has to face me."
In the next instant, he stood before a room and spoke coldly, "You're inside, aren't you? Come out. I know you're there."
"That's not how you talk to your senior, you know."
A mocking, teasing voice drifted from within.
"Since we last met, you might've gotten a little stronger… but you're still as sloppy as ever."
The door opened, and a figure stepped out dressed much like a swordsman. But his fingernails had become claw-like, marking him clearly as something no longer human.
A chilling, sinister aura clung to him—the unmistakable presence of a demon.
"Long time no see, Zenitsu."
Now everyone could finally see his face clearly. Though largely human in appearance, his ears were pointed, and his eyes glowed green. Within them, one bore the character "Upper," the other "Six."
It didn't take long for everyone to realize—this had to be the Upper Rank Six mentioned in Kaede Kitahara's diary.
"Upper Rank Six… which means there might also be Lower Ranks. And if they're numbered, then there should be at least five above him," Uchiha Itachi murmured as he carefully recorded it all on a sheet of paper.
What Kaede had once mentioned offhand—that Sasuke had also entered the Infinity Castle—was no trivial matter in Itachi's eyes. If it were true, then this information could determine Sasuke's survival in the future.
At the right moment, he would pass this along as preparation—so Sasuke wouldn't be helpless when facing the demons within the Infinity Castle.
"These demons were once human. The oldest of them, the Demon King Kibutsuji Muzan, has lived for over a thousand years. His regenerative ability is extraordinary, likely due to the immense vitality of his cells," Itachi continued noting everything down. "These demons also consume humans—perhaps that's necessary for their survival."
"Other demons are probably weaker than the Demon King, so they must have weaknesses. Otherwise, the Demon Slayer Corps couldn't possibly fight them. They fear sunlight—even the Demon King."
"Perhaps… Amaterasu, known as the flames of the sun, could be effective."
Itachi didn't know the limits of Muzan's regeneration, but Amaterasu was said to burn endlessly unless it reduced its target to nothing.
And it was called the fire of the sun.
Perhaps it was the perfect counter.
"They have no chakra, but they possess their own Blood Demon Arts. Each demon's ability seems unique. In contrast, the Demon Slayer Corps has developed various Breathing Styles—sword techniques that counter them. There must be some method behind it, otherwise it wouldn't make sense for them to fight without chakra."
"The strongest user of each Breathing Style… must be what they call a Hashira."
While Itachi continued jotting things down, the scene carried on.
"Kaigaku, since you've become a demon, I no longer recognize you as my senior."
Zenitsu spoke coldly.
"You really haven't changed at all," Kaigaku sneered, arrogance clear on his face. "Still a pathetic little runt, huh?"
"What, did you become a Hashira? Come on, Zenitsu—besides the First Form, what else have you learned?"
"You're awfully arrogant for someone at the bottom of the Upper Ranks," Zenitsu replied, his expression icy.
"So you've at least learned to talk back," Kaigaku said dismissively.
"Why did you become a demon?" Zenitsu demanded.
Before Kaigaku could respond, Zenitsu exploded.
"Why would a successor of Thunder Breathing become a demon? If you hadn't, Grandpa… he wouldn't have had to commit seppuku! He could've died with dignity—cutting his throat or piercing his heart! But because a demon emerged from the Thunder Breathing lineage—because of you—he had no choice!"
"And what does that have to do with me?" Kaigaku smirked. "What, you want me to break down in tears and repent?"
"I've never respected those who can't properly judge my worth. I serve only those who recognize my value. That old man died in agony, and I couldn't be happier. I worked so hard, yet he refused to name me the sole successor. That old fool actually made me share it with trash like you."
"Maybe he used to be a Hashira, but in his old age, he must've gone senile."
"What a beast."
Uchiha Sasuke clicked his tongue in disgust.
Now he finally understood. These two were exactly as Kaede had written—Agatsuma Zenitsu, who could only use the First Form of Thunder Breathing, and Kaigaku, who couldn't even master that.
And Kaigaku had chosen to become a demon for power, without the slightest remorse toward the master who had taught him.
Because of a traitor like him, their teacher had been driven to take his own life.
Though Sasuke himself hadn't been taught by that former Thunder Hashira, Kaede had already said he was the best successor to Thunder Breathing. If the chance ever came, Sasuke wouldn't hesitate to eliminate such a disgrace.
"Grandpa wasn't senile!" Zenitsu roared. "Maybe I'm trash, but you're worse than garbage! I might only know the First Form, but you can't even use it! Grandpa died like that because of a successor like you!"
At that, Kaigaku finally lost his patience and reached for the blade at his back.
"Thunder Breathing, Fourth Form—Distant Thunder!"
But before he could even fully draw his sword, Zenitsu struck.
A flash.
His blade sliced clean across Kaigaku's neck, blood spraying into the air.
"You're too slow, garbage," Zenitsu said without hesitation.
"He cut me… so fast… This guy… it's like he's a completely different person."
Kaigaku stared in shock.
"It's hard to believe. Compared to the fear I once felt… this bastard in front of me is nothing."
"That strike hit his neck. If it were a shinobi, they'd already be dead—or at least gravely injured. But he only bled once… just like we thought. These demons really do have unbelievable regenerative abilities."
Uchiha Itachi observed closely as Kaigaku's neck wound, after bleeding briefly, began to heal.
It was beyond comprehension. Any normal shinobi would have been finished by that blow.
Even he wouldn't have fared better.
Though these people lacked chakra, their movements were astonishingly fast, much like taijutsu specialists. Without even realizing it, Itachi had activated his Sharingan to observe the battle more closely.