"You are far too complacent!"
Muzan's voice cracked like thunder through Infinity Castle, his fury growing with every drop of blood that fell from his wounded face. The demon king's scarlet eyes narrowed to needlepoints as rage consumed what remained of his composure.
In that moment of absolute fury, Doma's seemingly magnificent blood demon art crumbled like sand before a hurricane.
The platform beneath Muzan's feet erupted into a writhing mass of organic horror. Dense tentacles shot out like countless whips, their phantom movements creating afterimages that sliced through the air with devastating precision. The towering Bodhisattva statue that had seemed so majestic moments before was torn apart piece by piece, its serene expression shattered into countless ice fragments. The Snow Princesses disintegrated under the relentless attack, their ethereal forms scattered like morning mist.
Doma's body was caught in the vortex of destruction, tossed through the air like a broken doll as the tentacles found their mark.
The truth was undeniable - while Doma's blood demon art possessed considerable power, the gulf between him and Muzan went far beyond mere spiritual strength. Their physical attributes were in completely different leagues. The terrifying supernatural cold that could freeze stone barely slowed Muzan's flesh-and-blood constructs. The raw power behind his cellular division could cut through the most fortified ice like paper.
Yet Doma did not seem surprised by this result. He'd never expected to hurt Muzan a second time - the Demon King's earlier wound had only occurred because he'd underestimated the ice demon's soul abilities. That single moment of damage had been enough to prove Doma's worth.
Even as his body flew through the air, spinning helplessly from the devastating impact, a satisfied smile played across Doma's bloodied features. His rainbow eyes glowed with an emotion that could have been pride.
That expression sent cold fear through Muzan's veins.
Whoosh!
A black shadow slipped past Muzan's heightened senses like smoke through fingers and shot upward with predatory grace. The figure approached from behind, moving with the fluid precision of a master assassin who had spent centuries perfecting his craft.
A massive demon sword wrapped in crackling soul power descended in a killing arc, its edge singing through the air with murderous intent.
Kokushibo had arrived.
"Hmm?"
High above the chaos, Oboro's eyes lit up with genuine interest as he watched the battle unfold.
Kokushibo's spirit ability bore the unmistakable shadow of techniques Oboro had witnessed during his time in the Hunter x Hunter world. Through his soul talent, the former Upper Moon One could completely suppress his aura and become undetectable to even the most advanced perception abilities.
It was part of the principle of "breath concealment" - a technique that carried more than a hint of the legendary "Zetsu" from that other reality.
However, this method worked directly on the soul rather than on mere physical energy, making its effects far more profound and difficult to counteract. Even Muzan, with all his enhanced senses and supernatural awareness, had failed to detect Kokushibo's approach.
Beyond stealth, Kokushibo had another devastating advantage. He could channel soul power directly into external objects, imbuing his weapons with spiritual energy that could inflict damage on a metaphysical level. His approach lacked the elaborate tricks and complex strategies that characterized other fighting styles, but its simplicity made it terrifyingly effective. A single scratch from his demon sword, the Hollow Crying God, would wound the target's very soul, causing injuries that couldn't be healed by normal regeneration.
Oboro's heightened perception swept over the entire Infinity Castle, taking in the broader scope of the battle. Most of the Inheritors were already in serious trouble, trapped in the elaborate "cages" Muzan had prepared in advance.
The Demon King had successfully separated his enemies and dispersed them into various chambers within his dimensional stronghold. Each chamber contained blood samples he'd left behind specifically for this purpose. The moment an Inheritor materialized in one of these rooms, the dormant cells would activate and transform into writhing masses of flesh and blood that would attack with relentless hunger.
While these constructs weren't as powerful as the ones Doma was currently battling, they were still formidable enough to challenge even seasoned demon slayers.
The reason Doma had managed to catch Muzan off guard earlier was because the Demon King's attention was constantly divided. He was simultaneously controlling multiple battlefields, managing the spatial functions of Infinity Castle, and directing countless cellular constructs throughout dimensional space. His mind was operating on several levels at once, processing information from dozens of different conflicts.
Both Doma and Kokushibo understood the nature of Infinity Castle and Muzan's limitations within it. During the original timeline, Nakime had helped shoulder much of the burden of spatial manipulation, allowing Muzan to focus on direct combat. Without her assistance, maintaining control of this alien dimension had consumed a significant portion of his mental capacity.
This distraction had created the opening Kokushibo needed to launch his devastating sneak attack.
Swoosh.
Moon Breath: Piercing Face, Falling Moon.
Kokushibo knew better than to underestimate Muzan's power. He threw everything into this strike, holding nothing back as brilliant blade light erupted around his weapon. Two oval arcs of cutting energy tilted through the air before exploding into countless moonwheel projections that surrounded Muzan from every angle.
The attack was absolute and inescapable.
The stone platform beneath Muzan's feet immediately began to transform, organic material flowing upward to form a protective cocoon of flesh and blood. The barrier enveloped him completely, creating a living shell designed to absorb the incoming attack.
But Kokushibo's attack had been calculated for this very reaction.
There was a subtle shift in his swordsmanship, a modification of the breathing technique that increased the cutting power of his weapon. While the fleshy cocoon managed to deflect most of the moon-shaped blows, Kokushibo used the serrated edge of his demon sword to slice through the surface of the barrier. His blade penetrated deep and stabbed viciously into Muzan's shoulder with enough force to pierce bone.
He'd originally intended to cut across the throat, but chose the quicker stabbing motion to prevent his target from dodging at the last moment.
The moment his soul-enhanced blade made contact, Muzan's entire spiritual essence convulsed in agony. The pain of a wounded soul transcended physical suffering, causing his consciousness to reel as if his very existence were being torn apart. His mind felt like it was exploding from within, every thought becoming a spike of agony radiating through his being.
Looking at Kokushibo's changed features - the three pairs of crimson eyes that marked his demonic evolution, the cold calculation in his ancient gaze - Muzan's face twisted with blue veins of rage. He could understand Doma's betrayal; that creature had never possessed true human emotions, his thought patterns remaining alien and unpredictable even after centuries of observation.
But Kokushibo was different. This man had once knelt before him in submission, serving faithfully as Upper Moon One for over four hundred years. Muzan had never expected that this former failure, someone who had lived his human life in the shadow of his younger brother's greatness, would one day find the courage to raise a blade against the Demon King himself.
Breathe.
High above, Oboro's eyes narrowed in recognition as he analyzed Kokushibo's movements.
Breathing techniques.
As he'd expected, Kokushibo's identity as a swordsman remained fundamentally unchanged despite his transformation into a demon. Breathing had become an integral part of this world's evolved power system, and someone of his caliber would naturally study and refine these techniques to their absolute limits.
The integration was seamless - demonic enhancement combined with traditional breathing methods to create something entirely new.
"You and he..." Muzan spoke through gritted teeth, his voice dripping with cold contempt, "...are not comparable."
The flesh cocoon suddenly exploded outward in a shower of crimson rain, each droplet carrying concentrated cellular corruption designed to destroy everything it touched.
Kokushibo tried to dodge, but Muzan's grip on the demon sword slowed his movements by crucial milliseconds. Blood cascaded over him like acid rain, and wherever it touched his skin, festering began immediately. His limbs began to writhe and twist as Muzan's enhanced cells began their work of systematic destruction.
But Kokushibo's will proved stronger than the corruption. Through sheer determination, he suppressed the cellular invasion, his spiritual energy burning away the foreign material before it could take root. His physical strength far exceeded Doma's, making him far more resistant to Muzan's cellular attacks. Still, time was against him - he couldn't maintain that resistance indefinitely.
"Ah!"
A scream echoed from somewhere deep within the Castle of Infinity, the sound of absolute terror and pain reverberating through dimensional space.
Another Heir had fallen, overwhelmed by Muzan's soul spores and consumed by the flesh and blood constructs that had lain in wait. The Demon King's mind operated on multiple levels simultaneously, his consciousness divided between the intense battle with Kokushibo and Doma and the management of dozens of smaller conflicts throughout his domain.
This mental split worked to the advantage of his two primary opponents - had Muzan been able to focus his full attention on either of them, the battle would have ended in moments.
The space-warping abilities of Infinity Castle alone weren't enough to handle fighters of this caliber. Once this group of enemies managed to unite their forces, dealing with them would become exponentially more difficult than the current strategy of divide and conquer.
But the uneven performance of the Heirs was becoming painfully apparent. They couldn't even defeat Muzan's "clones" - mere extensions of his will rather than his actual body.
"Alas."
Even though Oboro had known from the beginning that his cultivation of the Inherited Will hadn't reached perfection in this world, he still felt a pang of disappointment when he saw the results unfolding before his eyes.
These were only conflicts with Muzan. The Heirs hadn't even faced the most refined example of will in the world - the members of the Demon Slayer Corps, who embodied centuries of accumulated wisdom and determination - and they were already falling.
The gap between theory and practice was becoming painfully apparent.
"No!"
Another heir's death cry echoed through the castle, confirming Oboro's assessment.
...
His attention shifted back to the primary battle between Kokushibo and Muzan.
Kokushibo's soul ability allowed for perfect concealment, making him effectively "non-existent" as long as Muzan couldn't physically see him. Without being able to determine his opponent's location, the Demon King couldn't use the spatial functions of the Infinity Castle for targeted attacks.
He had no target to focus on.
Kokushibo moved like a phantom assassin, using his intimate knowledge of the castle's layout to launch hit-and-run attacks from unexpected angles. Emerging from floating platforms and ruined buildings in all directions, he struck with surgical precision before disappearing back into cover.
For the first time in centuries, Muzan's expression showed genuine fear. He whirled constantly, scanning his surroundings as he expanded the range of his soul spores to create eye-like sensors on every available surface. The organic growths sprouted from walls, platforms, and debris, each desperately searching for some trace of his elusive enemy.
This was a massive drain on his soul energy. If he couldn't eliminate Kokushibo quickly and decisively, the prolonged battle would exhaust his spiritual reserves and leave him vulnerable to finishing attacks from multiple opponents.
In Oboro's analytical view, Doma's combination of soul power and blood demon arts resembled a "mage" archetype - powerful in versatility, range, and elemental control through ice and snow manipulation.
Kokushibo, on the other hand, functioned as an "assassin" whose skills were a near-perfect counter to Muzan's fighting style.
Even Oboro hadn't expected Kokushibo to be able to fight the Demon King on equal terms. This level of combat far exceeded anything that would have been possible before his intervention in the development of this world.
Consider the Heirs who had already fallen - in the original timeline, any one of them would have posed a serious threat to Muzan. Now they were being systematically eliminated by what amounted to autonomous extensions of his will.
The improvement in the power level of this world had been extensive and dramatic. The leap from the lower to the higher reality brought changes that altered every aspect of combat and conflict.
"Interesting."
Suddenly, Oboro's eyebrows raised as his attention shifted to another part of the battlefield.
Doma, despite his severely weakened condition from the earlier beating, had used the chaos of Kokushibo's attack to launch a surprise attack on one of the struggling Inheritors. Instead of trying to escape or recover by conventional means, the ice demon had devoured his supposed ally in order to regain his own strength.
Doma wasn't just using the Inheritors to put pressure on Muzan - he was treating them as disposable "blood bags" to fuel his own regeneration.
The plan revealed a level of calculated ruthlessness that impressed even Oboro.
Truthfully, in the entire Demon Slayer world, only the Ubuyashiki clan truly deserved to be called "wise" in Oboro's estimation. Their intelligence wasn't just cunning or manipulation - it represented true strategic brilliance combined with a deep understanding of human nature.
Compared to figures like Hisoka, Pariston, or Chrollo of the Hunter x Hunter world, the wisdom of the Ubuyashiki clan focused more on intellectual prowess than psychological complexity. If they were transplanted to this reality, they might be able to compete with deeper schemers through sheer brainpower, but victory would remain uncertain.
Aside from the Ubuyashiki family, most of the characters in this world remained relatively simple in Oboro's estimation. After all, that simplicity reflected the fundamental spirit of the Demon Slayer - a reality that celebrated pure emotion and unwavering will over elaborate deception. Even Muzan could only be described as cautious rather than ingenious in his planning.
Doma could be described as clever rather than stupid, but his actions rarely rose to the level of true strategy. However, his ability to manipulate the situation to his advantage while fighting for his life surprised even Oboro.
The Ice Demon proved that even in the most desperate of circumstances, survival can breed innovation.