As the door opened, the nine Hashira saw Muzan's ugly form.
Behind him, they could glimpse the fierce battle raging deep in Infinity Castle—a storm of black and gold lightning streaked through, demons falling in droves before Gusion.
For a brief instant, the Hashira and Gusion met eyes across the distance, sharing mutual trust.
A bead of sweat trickled down Muzan's brow. He was now trapped between a rock and a hard place but judged Nezuko to be the easier meal—attacking Gusion was too costly.
So, mustering his courage, Muzan charged at the Hashira.
He was the Demon King, after all—immortal, with unmatched regeneration. With ten hours before dawn, he figured he could wear the Hashira down.
"Muzan!"
A golden flash zipped by, and Muzan instinctively dodged, darting far to the side.
Only after dodging did he realize it was just a kid using Thunder Breathing—not very strong. He'd flinched out of sheer fear, mistaking it for Gusion.
"Boar rush! Boar rush!"
From the flank, Inosuke charged in—just as Stone Hashira's attack struck. Muzan had to deal with him first.
This time, prepared, Muzan reinforced his body with Blood Demon Art to block the blow.
Boom—
But even in this ugly, defensive form, Stone Hashira's strike sent him flying, the shock almost shattering his organs.
He healed instantly, but Stone Hashira's threat level soared in his mind.
He noted the earthy-yellow markings on Stone Hashira's face, recalling the purple-black marks on Gusion—both unlike the Demon Marks he'd seen before, and unaffected by age.
Had the Demon Slayer Corps evolved again—mastered a technique greater than Demon Marks!?
Muzan felt heavy but also relieved at his decisive move. He had to become sunlight-immune as soon as possible, or future clashes with the Corps would only get harder.
He'd already decided: once he got Nezuko, he'd flee—becoming a sunlight-immune demon, he'd leave this country and start over elsewhere.
As for Kokushibo and the others?
Let them die. Once he was perfect, he'd need no subordinates—there was room for only one immortal king.
The Hashira engaged Muzan in fierce battle. Tanjiro climbed the wall to watch; if the Hashira hadn't ordered someone to stay with Nezuko against surprise attacks, he'd be fighting too.
Lady Tamayo emerged from the house, watching the battle with grim hatred.
"Why is my regeneration slowing down?"
After a while, Muzan realized he couldn't quickly finish off the Hashira—not only that, but his regeneration was slowing.
He sensed Lady Tamayo's gaze and demanded:
"Tamayo, what did you do to me!?"
He felt not only slower regeneration but some force sapping his demon cells.
Tamayo smiled coldly.
"Oh? Shouldn't you ask the Hashira what they did to you, instead?"
Her and Shinobu's two months of research had paid off. Following Gusion's advice, the Hashira hadn't just relied on Shinobu's poison-injecting sword. All their blades were coated in potent toxins.
Even the righteous Flame Hashira agreed—against a fiend like Muzan, there was no need for chivalry.
So, all the Hashira used a mix of poisons—including the one that could turn demons back to humans and one that rapidly depleted Muzan's lifespan.
Immortal Demon King? Immortality was relative. Tamayo calculated that even well-fed, a demon might only live ten thousand years; Muzan, perhaps twenty thousand at best. Her poison made Muzan age fifty years a minute.
There was also Shinobu's wisteria poison, further corroding Muzan's cells.
So, facing nine fully-powered, Demon God-enhanced Hashira, Muzan was suffering a beating—and the longer it went, the weaker he felt.
…
While the Hashira kept Muzan from Nezuko, another battle raged in Infinity Castle.
"Hahaha! Amazing! I've never met a man as strong as you—come, fight me more!"
Akaza, freshly regenerated, laughed as he charged Gusion, punching at his head.
At this point, he'd stopped worrying about being ganged up on—Gusion's strength let him easily kill other demons even while fighting both Akaza and Kokushibo.
"How pathetic."
Gusion deflected Kokushibo's blade, then swept a leg back to kick Akaza's head off, armament haki reinforcing the blow.
He caught his breath—Thunder Breathing empowered him for brutal attacks. Standing firm, he struck at Kokushibo again.
With his title's bonus, he overpowered Kokushibo, knocking him away, then rushed at Akaza as he reformed.
Akaza truly loved fighting, but he'd forgotten his original reason for becoming a demon.
His life was tragic—he trained to protect, but his master and fiancée were poisoned by jealous rivals. Akaza killed them all, but couldn't bring back his loved ones.
Gusion empathized. He too once had a beloved master and senior, both killed by villains. He avenged them but became a wandering soul with no purpose.
He was grateful to the End Space for giving him a second chance—to find meaning in living again.
He would defeat all the extraordinary, and reach the pinnacle of martial arts.
He would make his name echo across worlds, so his master and senior in heaven would know he hadn't let them down.
Maybe, with the End Space's power, some tragedies could even be reversed.
Akaza wasn't so lucky. After losing his loved ones, he'd lost all reason to live, drifting until Muzan turned him into a demon, erasing his memories and turning him into a battle-obsessed warrior.
But Akaza just wanted to die—to see his master and fiancée again. Muzan kept him chained to the world as a demon for centuries.
"Let me send you on your way."
Gusion reached Akaza and swung his Zanpakutō, the purple-gold blade severing Akaza's head.
As his head and body separated, Akaza's eyes first looked lost, then sorrowful, and finally grateful as he looked at Gusion.
"May you meet the ones you long for…"
Gusion said softly, then turned and blocked Kokushibo's sword.
"How rude…"
Gusion slid back to dissipate the force.
"I hate it when people interrupt me."
It had been over ten minutes since the fight started. Gusion, battling both Akaza and Kokushibo, had traversed Infinity Castle, slaying countless demons.
Now, only he and Kokushibo remained—maybe ten demons left alive.
"Just the two of us left? Good. Now you can't escape."
Kokushibo, blade at the ready, said:
"Come, let us settle this between us."
Gusion pulled a black pill from his inventory and swallowed it—a soul-restoring item he'd bought in the End Space. Sustaining Demon God mode and high-level binding spells had drained him.
As he gulped the pill, he nearly choked on Kokushibo's words.
He tilted his head, puzzled:
"When did you get the idea I was running away?"
He had indeed used Thunder Breathing and Shunpo to move constantly—but that was to hunt demons, not to escape Kokushibo.
"Weren't you?"
Kokushibo switched to a two-handed grip.
"Let me see your true strength—face me head-on."
From his view, Gusion had never fought him continuously, always breaking off to kill other demons—clearly, a sign of avoiding the duel.
He admitted Gusion was incredibly fast, but that was movement speed—true sword speed was another matter.
He didn't think he'd lose in a direct duel—he'd mastered Moon Breathing for centuries and was confident.
Compared to the past, he was stronger—even if his brother Yoriichi returned, he believed he'd win.
"You're Yoriichi's brother, aren't you?"
Gusion felt his energy recovering.
"Don't say that name!"
Like a cat with its tail stepped on, Kokushibo's voice was furious.
"Whoa, touchy? Don't tell me you still couldn't beat your little brother, even after becoming a demon? Not even when he was old and weak?"
Gusion laughed. He didn't usually taunt his foes, but Kokushibo's reaction was too entertaining.
Sure enough, Kokushibo's face turned pitch-black.
"What are you, exactly?"
He now seriously suspected Gusion wasn't human—or rather, was sure he was a primordial demon like Muzan.
Otherwise, there was no way to explain how Gusion knew so much, or could use Blood Demon Arts.
"Me?"
Gusion steadied his breathing, lightning crackling around him.
"Of course, I'm human!"
As he spoke, he launched Shunpo—electricity and sonic booms crackling as he dashed like a raging thunderbolt, Mazuko Sword slashing at Kokushibo's neck.