The battle raged on within the irregular space.
Yet something had gradually changed.
Kaigaku had initially dominated Emiya utterly, but now Emiya began to hold his own as time went by.
The rate at which Emiya acquired new wounds steadily decreased.
It was as if Emiya had grown familiar with Kaigaku's fighting style, his Nichirin Blade continuously pressing closer to Kaigaku's neck.
Emiya had learned Thunder Breathing through [Projection]. However, since the original blade's wielder knew only the First Form, that was the only form Emiya had mastered.
Yet, Thunder Breathing's First Form was foundational, with nearly every other form evolving from it.
Aside from Zenitsu, virtually anyone who mastered the First Form could deduce how the other techniques operated.
And Emiya was certainly no exception.
During repeated clashes with Kaigaku, Emiya gradually grasped the release methods of Thunder Breathing's other forms, swiftly analyzing each technique's strengths and weaknesses.
If this continues, I'm confident I can sever his head.
Kaigaku was a rare demon who relied primarily on swordsmanship rather than his Blood Demon Art. Compared to those bizarre demon abilities, Emiya greatly preferred battling such an opponent!
Impossible! He's practically read all my moves!
The longer Kaigaku tangled with Emiya, the more shocked he became. Despite his Blood Demon Art continuously taking effect—despite Emiya being merely human, with wounds mounting—he remained as formidable as when they first engaged. Golden lightning still danced furiously around Emiya, as if none of Kaigaku's strikes had landed.
Talent and disposition...
Kaigaku suddenly recalled his worthless teacher, his face twisting once again in fury.
Disposition?
He, who had betrayed countless others and lived solely for himself, possessed no such lofty ideals.
Talent?
He couldn't even master Thunder Breathing's most basic form.
Hatred consumed him.
He loathed these gifted individuals.
Why...why couldn't that talent have been mine?
If he'd possessed such talent, he wouldn't have betrayed the Demon Slayer Corps and become a pathetic demon unable to witness sunlight.
All of it—everything—was their fault!
At this thought, Kaigaku's black-lightning figure abruptly vanished.
"—!"
Emiya reacted instantly, retreating backward without hesitation.
Clang!
Almost simultaneously, a ruthless slash of lightning carved through the space Emiya had just occupied, slicing the very air itself.
His speed increased again!
A cold dread settled in Emiya's heart.
Kaigaku reemerged slowly from a cloud of dust, greenish eyes blazing furiously as he lunged forward.
Clang!
The sharp clash rang clearly as Emiya blocked the descending dark-golden Nichirin Blade.
Clang!
A second clash immediately followed, Emiya's counterattack similarly thwarted by Kaigaku's blade.
"How infuriating! A mere Mizunoto-Ranked Demon Slayer actually pushed me this far!"
Kaigaku had aspirations of becoming an Upper Moon. Yet now he struggled evenly against someone of the lowest Demon Slayer rank.
How could he ever join the ranks of the Upper Moons at this rate?!
BOOM!
From the gap between the massive rooms, violent waves of black lightning surged outward, shaking the entire space and shoving surrounding buildings aside.
"[Thunder Breathing—Fourth Form: Distant Thunder]!"
Black lightning once again warped space as Kaigaku lunged toward Emiya—this time aiming specifically for Emiya's Nichirin Blade.
Kaigaku's own weapon wasn't even a true Nichirin Blade, merely a facsimile created by his Blood Demon Art. Despite this, it was sharper and sturdier than ordinary blades.
Yet, bafflingly, Emiya's blade remained undamaged after countless fierce collisions, while Kaigaku's own blade bore tiny cracks.
Naturally, this was thanks to Emiya's [Reinforcement], greatly enhancing his crimson Nichirin Blade's durability, allowing it to survive such relentless impacts.
As for Emiya's other blade—he'd lost track of it during the battle with Mukago.
Kaigaku shared Mukago's intention: A Demon Slayer without a Nichirin Blade posed no threat.
No matter how powerful, or how explosive one's potential, a petty heart never changed.
Just like Muzan, who despite his immense power remained cowardly.
Leopards never changed their spots.
However, Kaigaku clearly hadn't anticipated that Emiya wasn't merely a swordsman. Emiya's battles weren't decided solely by his Nichirin Blade.
The instant Emiya's blade flew from his grasp, Emiya used Kaigaku's own force against him, tumbling downward.
Golden lightning covered Emiya's foot, driving it squarely into Kaigaku's head.
"[Thunder Breathing—First Form: Thunderclap and Flash]!"
With a sound like a massive bell tolling, Kaigaku was violently propelled backward, blasting through the air like a cannonball.
Yet Emiya's figure became a swift blur, streaking through the sky like a meteor to swiftly catch Kaigaku.
A golden Nichirin Blade pressed firmly against Kaigaku's neck, ready to sever it in an instant.
BOOM!
Before Emiya could decapitate Kaigaku, the rooms themselves seemingly sprang to life, slamming fiercely into Emiya from all sides and hurling him aside.
"[Thunder Breathing—Third Form: Thunder Swarm]!"
Seeing Emiya thrown violently into the air and tumbling like a kite with severed strings, Kaigaku's lips curled into a contemptuous smile.
This was the Infinity Castle.
Aside from himself, who was here specifically to train with Upper Moon One, another entity ruled this domain.
If Lord Muzan had been present, how could a mere human have caused such trouble?!
Kaigaku, cloaked in rippling black lightning, appeared instantly beside Emiya.
"DIE!!!"
He roared madly.
"[Thunder Breathing—Seventh Form: Honoikazuchi no Kami]!"
In that instant, Emiya's golden Nichirin Blade erupted with terrifying brilliance, conjuring a golden thunder dragon beside him.
A flash of lightning, too swift for human eyes, appeared directly in front of Kaigaku.
Shhhhk—!
Flesh tore apart.
Kaigaku saw only spinning rooms around him.
"[Thunder Breathing—Sixth Form: Rumble and Flash—]!"
"No," Emiya's icy voice cut through, "you're no longer capable of that!"
Emiya hung silently from a nearby wall, blood gushing from countless wounds, watching impassively as Kaigaku's body tumbled downward.
Thud!
A heavy impact echoed from below as Kaigaku's lifeless body slammed into a window. He stood motionless for a moment, then crumpled to the ground, devoid of life.
Yet Emiya's battle was far from over.