Peach Mountain, beneath the old tree.
Ever since he withdrew from the frontlines of the Demon Slayer Corps, Kuwajima Jigorō had settled here. After losing a leg, he chose to become a cultivator, devoting himself to training the next generation of Demon Slayers.
Kaigaku and Zenitsu...
He often wondered how those two children were faring now.
His first disciple, Kaigaku, had already joined the Demon Slayer Corps. Zenitsu had been kicked off the mountain to train on his own. Now, all that remained atop Peach Mountain was a solitary old man.
"I just hope... they stay true to their hearts,"
Jigorō murmured as the wind stirred his white hair. Leaning on his cane, the old master's heart was restless.
"Master…"
Not far from Peach Mountain, Kaigaku had already arrived. He clenched his fists. The thought that he would soon have to face that elderly man—as a demon—darkened his expression.
But then he recalled that chilling encounter with Muzan Kibutsuji.
Kaigaku's face grew even grimmer.
"Master, for the sake of my future... please die."
With his mind made up, Kaigaku turned to head toward the nearby Demon Slayer Corps base.
This was a task given by Lord Muzan himself.
An orphan since birth, Kaigaku had fully accepted his new identity as a demon. Once upon a time, he had trained in slaying demons to grow stronger. So now, if becoming one could give him even more power—then so be it.
As Kaigaku's figure disappeared into the distance, heading toward the Demon Slayer base, three new figures emerged in the place he'd just left.
"What a good child. So quick to act,"
Gyokko cackled, peeking a part of his grotesque body out from the mouth of his pot.
"With this, Lord Muzan's plan should come to fruition."
"Follow him. No matter who comes, they must die,"
Upper Moon One—Kokushibō—cast a cold glance at Gyokko and Hantengu.
With the highest-ranking demon's command, the two didn't dare say another word. Quietly, they followed Kaigaku.
After all, they were the true assassins Muzan had dispatched. Until the target appeared, they would not expose themselves.
….
"No! Quick—send a message to headquarters!"
"Traitor! Kaigaku, why would you do this?!"
As a former Demon Slayer, Kaigaku knew exactly where the nearby bases were.
He didn't even break a sweat—most of the members there were slaughtered almost effortlessly.
And this… was him going easy.
Back in the Infinity Castle, after receiving Muzan Kibutsuji's blood, Kaigaku finally understood what true power felt like.
"Strong… so strong… unbelievably strong…"
He watched the horrified expressions of his former comrades as they fell beneath his blade, and a surge of exhilaration flooded his chest.
Yes.
Becoming a demon had been the right decision.
This path… could truly make him strong.
Seeing those who once mocked him for not mastering the First Form of Thunder Breathing—now die helplessly before him—Kaigaku laughed like a madman.
"Mock me, will you? Call me worthless, will you? Well, now you're dead!"
"Hahaha!"
From an ordinary, mediocre Demon Slayer… to a terrifying demon—
Watching those who once stood above him collapse under his feet—Kaigaku was completely drunk on his newfound power.
Only he knew how much strength Muzan's blood had given him.
If it weren't necessary to draw attention and stir chaos to alert the Corps…
None of these weaklings would've had a chance to send any messages at all.
All of it… he allowed.
"You... you'll die a miserable death, Kaigaku…"
Kaigaku sneered at the dying man beneath his blade, completely unmoved.
A miserable death?
If he failed his mission for Lord Muzan, he would indeed die.
So if killing you lets me live—
Then die.
Kaigaku had always been selfish and arrogant. Now that he was a demon, his worst traits had only amplified.
He had become a full-blown egotist.
….
Morning.
As the thick stench of blood rose into the air, someone finally noticed.
"What the hell—!"
"This… how could this happen?!"
At the Demon Slayer Corps headquarters, Kagaya Ubuyashiki had already received the devastating news.
The moment he read it, his body trembled. Nearly fainting from shock and fury, he was quickly caught by his wife, Amane.
"Kagaya, breathe… stay calm."
Under her gentle presence, Kagaya slowly calmed down.
But his eyes still burned with fury.
"How could this be…?"
A member of the Demon Slayer Corps—Kaigaku—had not only defected and turned into a demon, he had also slaughtered an entire base of his comrades.
If this had been caused by a normal demon's assault, Kagaya would have mourned the loss deeply, but he wouldn't have burned with rage.
But betrayal—by one of their own—was something that pierced deeper than any wound.
He couldn't understand.
Why?
Why would one of their own commit such atrocity?
As the leader of the Demon Slayer Corps, Kagaya knew he was not a swordsman.
He was the strategist, the heart behind the warriors who risked their lives each day.
He remembered every single person who died under his watch.
Each death, a weight on his soul.
And this death… this betrayal…
Was unforgivable.
"Someone, gather all the available Hashira. And summon Yohan from the Butterfly Mansion."
"This... this crime must be avenged."
Kagaya's voice was full of cold resolve.
Beside him, Amane whispered with worry,
"Kagaya... I fear another person may be in danger."
Kagaya's eyes widened with realization.
"Send no message to Kuwajima Jigorō. Don't let him hear of this."
"And—dispatch a Hashira to Peach Mountain immediately."
To slaughter an entire Demon Slayer base—this wasn't something ordinary slayers could handle.
It would take a Hashira.
Maybe even more than one.
"Muzan Kibutsuji…"
"I will end you."
"You are the filth that stains the Ubuyashiki name!"
Having calmed down, Kagaya's mind quickly pieced everything together.
He knew—this had all been orchestrated by Muzan.
That very day, messenger crows took to the skies from the Demon Slayer headquarters.
The news of the destroyed base spread like wildfire.
And after being briefed by Kagaya himself, Yohan also made his decision—
He would head for Peach Mountain.
"So this is the kind of trickery you resort to now?"
"Muzan Kibutsuji…"
In his past life, Yohan had already despised Kaigaku's type—
Arrogant, selfish, easily swayed by power.
He never understood why the former Thunder Hashira had chosen to train someone like him—much less consider him a successor.
But now wasn't the time to judge Jigorō's past decisions.
Whatever mistakes had been made, Jigorō's efforts to nurture new blood for the Demon Slayer Corps could not be denied.
So Yohan would go.
To settle this.