Infinity Castle.
An endless labyrinth of rooms, halls, and corridors decorated in a traditional Japanese Castle.
Dim lights, sliding doors, and tatami-style rooms.
But that was as far as common sense could go.
This endless architecture stretched beyond the horizon, defying gravity itself and ever-shifting in formation and arrangement following the melody of a biwa.
Constructed by the Blood Demon Art of an Upper-Moon Demon, this was the very stronghold of demonkind.
The Homebase of the Demonic Progenitor, Kibutsuji Muzan.
However, the usual, trepid, and ominous silence no longer accompanied its prestigious halls...
The sounds of widespread combat cried out to the infinite.
Wailing cries of swordsmen clad in recognizable uniform and demons alike, the spraying of blood and flesh, the clanging of steel with bone and tough skin...
Crows with special tags on their necks flew across the endless battlefield...
It was all-out war, it was...
The Final Struggle between The Demon Slayer Corps and Demonkind.
A battle on multiple fronts, as the Hashira united against each of the present Upper Moons, aiming to reach and finally slay the bane of their existence.
Right now, at this very moment, the battle raged on.
Demons, inches from having their heads sliced cleanly off.
The Breath style-users are bleeding and aching from every pore in their bodies.
A close battle with no immediate discernible victor.
And right now...
At this very moment...
Another joined the fray.
Neither Human.
Nor Demon.
...
Something Divine.
-----------------------------
Within an open hall among the nigh-infinite within the castle, a battle raged on between an Upper Moon and a reluctant, marked man.
Fists met katana, as a visible water dragon seemed to emanate from the Hashira's blade, their fight sending wood and debris across the entire room.
Blood splattered across from both combatants, but with Upper Moon Three holding the clear physiological advantage.
He closed in on the furious and shocked Hashira. "YOU'VE RUN OUT OF BREATHING STYLES! GIYU!" He sneered with a grin.
In response, Giyu swung his katana at the Demon's face, only to be met with a ghastly shattering noise.
His eyes widened as his katana met Akaza's fist, broken from the side.
Shock crossed his features as he processed the situation.
A process too slow, as before he knew it, Akaza's deadly hand was primed for his chest - a death sentence.
Unbeknownst to both of them, the young Demon Slayer who stood at the sidelines had reached an epiphany and had already been on the move.
A quiet, flame-exuding hiss left the boy's lips as he moved smoothly, his eyes seeing beyond the world, and his katana followed.
Yet, before Kamado Tanjiro's katana could bisect the Demon's hand, his senses roared out at him.
And not only to him, but it seemed.
Akaza had backed away immediately, ceasing his final fist with an almost inexplicable bewilderness.
Whilst before Giyu could feel it, he found himself being shoved aside by Tanjiro, his shock still present.
The very thing that had startled both hyper-aware combatants had begun to make itself clear.
Distant protest from the infinite roof started getting closer and closer, as if something large and heavy were crashing down at insane speeds.
Both Akaza and Tanjiro's eyes were almost fixated on what was bulldozing through the roof, sweat trickling down both their injured faces.
Giyu, his expression a mixture of shock and dismay, had his head shifting from both Akaza and the roof above them, not understanding what was going on.
Tanjiro's sudden feat, sudden awakening, was something to question later, he mused.
With a quick turn of his head, he inquired with a deathly serious countenance, his shattered blade gripped without issue. "Is it another Upper Moon? Muzan?"
Logic hadn't followed him at the heat of the moment; even so, he voiced his only guess at what would cause such... distress.
No words managed to leave Tanjiro's lips before the very thing that had drawn their attention finally crashed down from the roof with a resonating explosion of dust and debris.
And the most distressing part was the heavy, audible thud with which the thing landed below them.
Slowly, the dust cleared to reveal the man, demon... whatever this was.
A looming muscular humanoid being with white skin and strange ornaments on its chest.
The abomination towered over each of them twice over, but that wasn't the strangest part.
Indeed, the uncanniness began above the neckline.
Four wings protruded where its eyes should have been, and an eager, anticipating, almost divine golden dharmachakra protruded above its head.
And a chilling, enthusiastic grin split its face.
Giyu's voice rang out amid the silence, his tone pragmatic. "No eyes... It's not an Upper Moon." Yet... he found himself trembling.
One of his hands loosened its hold on the hilt of his katana, trying to physically suppress the shaking, to no avail.
His eyes, questioning, turned to look at Tanjiro, "Tanji-" His breath hitched, as he could only see the young boy he had saved back in that forest biting his lips in frustration, blood drawing from his face and shaking far more than Giyu.
Was he more afraid? Giyu wondered.
'Or does he sense more?' Was his almost fearful revelation.
Swiftly, his head turned to Akaza to be met with an even more sorry sight.
The gleeful, battle-crazed Upper Moon Three was staring at the abomination before them as if it were Muzan himself, nay, Death itself.
His voice was barely a whisper, yet in the deathly silent room it was as if he had screamed with the full might of his lungs. "Overwhelming malice...Every cell in my body... is screaming at me to... Run."
He had never felt this way.
Not in recent memory.
Not in fractured memory.
Not even in the face of Muzan himself.
This was...
"Weak." He spat out with inconceivable amounts of venom and disdain.
His muscles tightened with an almost disturbing click as visible disgust crossed his features, striking a pose, an invisible compass manifested below him. "FINAL FORM!" His tone was a panicked, infuriated mess as his fists began their initial swing.
"BLUE SILVER-" He never got to finish that sentence.
Tanjiro and Giyu's eyes widened as the noise only reached them after the fact.
The noise of a disturbing, loud squelch and splatter.
Blood, flesh and organs launched across the room and decorated their already bloodied visages with further taint.
Once more, their breach hitched as they heard a noise they had never come across before, known as a sonic boom.
They could only register the long-tail appendage that was at the back of the abomination's head when the noise came, and the feeling of unfamiliar flesh and blood sliding from their faces and clothing.
A hitched, bewildered exhale left Tanjiro's lips as he regripped his blade and turned to the disturbed Giyu with a swift nudge. "GIYU-SAN! WE HAVE TO RUN-"
Their entire being screamed.
The abomination turned its head towards them, its bloodied chest and front before them.
The disturbing, eager grin persisted.
How could it see?
How was it looking at them?
How did it move so fast?
Is Akaza truly dead?
Those questions died a quick, horrible death in their minds as both Giyu and Tanjiro, with reckless, foolish hope, turned their back on the abomination and took a step forward, wanting to run, even if their bodies screamed at the pain, even if their instincts screamed at the hopelessness of it.
For a moment, Tanjiro felt like death was an inch away.
However...
"GRAAAAAAAAAAAAH!" Akaza's defiant roar was on par with the ear-shattering noise of the abomination's movement.
With a light turn around, the abomination was met with Akaza's attack, a rapid flurry of punches.
It was only rapid in the eyes of the two Demon Slayers, however.
Each blow had been easily deflected or met with a punch far more powerful than it, pummeling it at its root, forcing Akaza's pained and swift regeneration.
All the while, a wheel above trembled.
In the span of seconds, nay, milliseconds, Akaza found himself being turned into paste an agonising, innumerable number of times.
The Abomination's grin persisted.
"GIYU-SAN!"
"I KNOW!"
Their legs moved before they could even answer one another, as they bolted out of there, the squelching getting quieter and quieter as they managed to cross a few rooms.
But then, in the distance, exactly where that Abomination was... a disturbing, inevitable noise reverberated.
GACHINK.
"WHAT IS THAT THING?!" Tanjiro cried out.
"I DON'T KNOW!" Giyu spat out with the same vehemence.
They crossed a few more rooms, managing to come across a few members of the Demon Slayer Corps fighting Demons.
With quick slashes of their blades, being of much superior skill and not against Upper Moons, they relieved the others of their struggle.
"Thank you! Hashira-sama-"
"GET OUT OF HERE! EVERYONE!" Tanjiro warned with a commanding shout, a panic unlike any other on his features.
In that exact moment, a crow flew overhead, roaring out a disturbing announcement:
"CAW! CAW! UPPER MOON THREE HAS BEEN SLAIN BY... ANOTHER DEMON? CAW! ABANDON ALL FIGHTS! SEARCH FOR MUZAN!"
The announcement echoed across the entire castle, coming from many of the crows that surveyed the dimension.
Man?
Demon?
Abomination?
Nothing was quite clear, but it was best to steer clear.
But, truthfully, it was none of those.
Tanjiro, Giyu, and the rest of the small ensemble they managed to come across ran for their lives, searching for Muzan and an exit.
Tanjiro inquired, "Giyu-san, what are the chances that thing is on our side?"
Giyu's expression was a tight frown, and he turned to meet Tanjiro's own as they ran.
From the boy's expression, it was clear that he didn't believe that possibility.
Not from the sheer malice that thing exuded.
Giyu stated coldly, almost to himself. "...It doesn't matter if we die here. As long as we manage to slay Muzan."
One of those pesky, infuriating noises resonated once more - the noise of the instrument that rearranges the rooms of the Castle.
He murmured quietly to Tanjiro. "Sorry, Tanjiro-"
His eyes didn't even manage to widen in time, as a large palm, large enough to engulf their whole figure, fell upon Tanjiro.
With a disturbing splat, as Giyu's legs carried him just a bit further as he couldn't stop his momentum, his head froze at the visage of the disturbing abomination, its hand having crushed Tanjiro to paste.
No, this was no Demon.
He did not discriminate against either man nor demon.
One way or another, the wish of the Demon Slayer Corps will be fulfilled, but a heavy price will have to be exacted.
For the Divine General had been unleashed upon this dimension.
All within it shall perish, and he shall enjoy it.
To Be Continued!
-Author Note Start-
Nakime noticed Mahoraga's random targeting of both sides, so she is manipulating rooms so he goes to kill Slayers first, in case you didn't understand the instrument sound.
But yeah, it's a bloodbath, and it will start out with the Slayers side, unfortunately, later on it'll be the Demons as they try to defeat/remove Mahoraga only for him to ADAPT and fk em up.
Hope you enjoyed lol.