The candles burned low, their light flickering against the walls, casting shadows that swayed like restless spirits. The scent of wisteria, sickness, and death filled the air, clinging to the walls—revolting and sweet, like an omen of the impending ruin.
Kagaya Ubuyashiki lay in the center of the room, his breath ragged and labored, though his expression remained serene. But inside, his heart ached with the kind of sorrow only a husband and father could know.
They should have left.
He had told Amane as much. Begged her, even. The Demon Slayer Corps could not afford unnecessary deaths, and Kiriya would need someone to guide him when the time came. She was more than capable. Their daughters were young—they had barely begun to live. Kagaya had made his peace with death long ago, but he had wanted more for them.
And yet, when he had spoken those words, his wife had only smiled.
"We are your family," she had said, her voice as steady as a mountain. "We will remain where we belong."
He had felt her hands in his. Warm. Steady. Resolute. They had decided long before he had spoken. No amount of pleading could change their minds.
That was the cruelest part.
They had chosen this.
Even now, as Muzan’s presence loomed closer, as the moment of their deaths crept toward them with inescapable certainty, his daughters did not waver. They played with their temari as if nothing were happening, their soft voices rising in song—light, unafraid. Their small hands passed the ball back and forth, undisturbed, in front of the father of all demons, in front of Kagaya himself—the man whose actions had sealed their fate. Always composed, strong as steel, like blades forged in fire—so much like their mother.
Tears stung at his unseeing eyes, but he did not let them fall. He could not even cry properly anymore. Not with Muzan standing before him.
"You should have gone." The words echoed in his mind. Even if he had spoken them aloud, his voice would have been lost in the quiet hum of the night.
But Amane knew. He could almost picture her turning to him, her gaze soft, knowing.
"And leave you alone?" she would have asked.
Yes. Yes. That was what he should have said. What he should have wanted. He wanted to insist, to beg again, even though it was far too late.
But he did not.
Because a part of him—a selfish, wretched part of him—was grateful they had stayed.
He had always thought himself prepared for death. He had watched generations of demon slayers march toward their end, had sent countless young warriors to their fate. But the waiting would have been unbearable without them. The fear, the pain, the inevitability of his demise—he could endure it all.
But not alone.
Their presence anchored him, softened the weight of what was to come.
And yet, as the night pressed in around him, as the firelight flickered against his daughters’ faces, Kagaya realized that it was not his own death that frightened him.
It was losing them.
As the fire crackled to life, Kagaya's mind, weakened by sickness but sharp as ever, grasped at memories, as if holding onto them tightly enough could keep them from slipping through his fingers. He thought of the wisteria trees swaying in the wind, the scent of spring thick in the air. He thought of the evening when they had first met, walking beneath their shade, a rare moment of intimacy granted before their arranged marriage.
She had been a beautiful young woman, barely reaching adulthood. A crown of fallen wisteria petals adorned her white hair as they walked, looking almost ethereal. He had been the mind of a man who had walked through the dark for far too long, trapped in the weak body of a sickly thirteen-year-old child.
"If you don’t want to, I will stop the marriage," he had told her, his voice quiet, meant to be reassuring. He had not wanted her to be forced into a life of suffering at his side.
He had expected hesitation, perhaps even relief.
Instead, she had knelt before him, taken his hands in her own, and smiled.
"Let me be the light that illuminates your path so you don’t falter. Let me be the pillar that holds you when you don't feel strong enough to stand. Let me be the voice that makes your feelings heard. Let me be yours, Oyakata-sama, and then let yourself be mine."
"When it's just the two of us, I am Kagaya, Amane. And you will be the Lady of this house for as long as you wish to be."
"This is where I belong. Right by your side."
And she never left. Not once.
Another memory surfaced, unbidden.
Nichika and Hinaki’s cries had filled their home that night, frail but insistent wails cutting through the stillness. He had felt too weak to hold them at first—perhaps it was his curse, or perhaps he had simply been a nervous thirteen-year-old, overwhelmed by the fragile weight of fatherhood. Amane had cradled their daughters, pressing a gentle kiss to each tiny forehead before turning to him.
"They are strong girls," she had whispered, placing the newborns against his chest, letting him feel the warmth of the lives they had created. His arms had trembled as he held them, terrified that he might drop them. But even in his fear, he had never wanted to let go.
Now, those same daughters played outside, their soft voices rising in song as they passed the temari ball between them. Clear. Unwavering. Unafraid.
Unaware—or perhaps knowing all too well—of what was to come.
And that was why the guilt clawed at him so violently.
Because he should have wanted them to flee.
Because he had let them stay.
A faint tremor ran through the earth, distant but growing.
It was time.
Kagaya exhaled slowly, reaching out to clasp his wife’s hand. He would have liked to hold all of his children close, but there were things far more important than himself. Amane’s fingers curled around his—gentle, unwavering. Unlike his, her hands did not tremble.
"Thank you," he murmured, the words far too small for the weight they carried.
"You are not alone."
Her voice settled into his bones, a balm to wounds too deep to heal.
And then, the flames rose, devouring the house, binding them together—forever.
The Infinity Castle twisted around them—walls shifting, staircases unraveling, gravity bending in impossible directions. The battlefield was alive, a breathing, warping labyrinth designed to disorient and destroy.
Senjuro's hands trembled around the hilt of his sword. His breath came fast, his heart hammering against his ribs as if it wanted to escape his chest. This was nothing like training.
This was death waiting to claim him.
They held as tight an arrow formation as they could—just as Mai had called it—Hiroshi at the front, screaming warnings over his shoulder and steering them toward what looked like the safest path. They cut down demons as they ran, moving as a single unit, reading each other’s movements as if they were written in front of them.
But the castle was alive, and it wanted to break them apart.
The ground yawned open beneath Mai. Her scream echoed as she fell. "Keep going!" she managed before she was swallowed by the shifting architecture. A sudden lurch in the walls slammed Senjuro sideways, ripping him from the others. He hit the ground hard, pain blooming in his shoulder.
A blurred figure crouched before him. A pair of glowing orange eyes locked onto him like a predator sizing up its prey. His hair was an unnatural shade of pink, his body heavily muscled.
Senjuro’s gaze landed on the kanji etched into his irises:
Upper Moon Three.
Senjuro could hear his own breathing—ragged, shallow, uneven. Every inhale burned his throat. Every exhale stuttered, as if his body refused to acknowledge what stood before him. His fingers clenched around the hilt of his sword, but his grip was all wrong—too tight, too stiff, like a child clutching a branch and pretending it was a blade. His heart pounded so hard it felt as though it might burst through his ribs. He could feel it everywhere: hammering at the base of his skull, thrumming in his trembling hands, clawing at his ribs like a caged animal trying to escape.
He was going to die.
The realization was an icy dagger lodged between his ribs, twisting deeper with every second. He had faced demons before, but none like this. None that carried such an overwhelming, suffocating presence.
Akaza wasn’t just strong—he was unnatural. A force beyond Senjuro’s comprehension. The Upper Moon’s very existence was an insult to reason—a monster wrapped in the shape of a man, strength condensed into a perfect weapon of death.
And he was smiling.
"Kyojuro!" Akaza’s voice rang out, almost joyful. But the weight behind it made Senjuro’s stomach churn. It wasn’t joy. It was hunger. Anticipation. The excitement of a predator that had found prey worth chasing.
Senjuro wanted to throw up. He wanted to run. He wanted to scream. His legs begged him to move, to turn and flee into the labyrinthine depths of the Infinity Castle and never look back. The terror pressed down on him like an iron weight, locking his muscles, stealing his breath, and messing with the beating of his heart.
His mind was screaming—Move! Move! Move!—but his body refused to obey.
This was the demon that had changed everything. The demon that had almost killed his brother.
It wasn’t fair. He wasn’t ready.
"Kyojuro is my brother," he said. Maybe to buy himself time. Maybe in the desperate hope that a more capable Demon Slayer would appear. Or maybe because, for the first time in his life, hatred pounded through his veins, and he had no idea what to do with it.
Akaza’s expression twisted, as if someone had forcefully poured lemon juice down his throat. He scrutinized Senjuro, and in that moment, Senjuro saw the judgment in his gaze. Not as a warrior. Not even as an opponent.
As a disappointment.
"I should have known," Akaza muttered, his voice sharpening, edged with fury. "Unlike your brother, you are weak. He was a bright flame… you are just flickering embers."
It was an insult. Senjuro knew that. But somehow, it gave him the strength he was missing.
Yes, he was a flickering ember.
And he was proud of it.
He would rather be flickering than dead and cold. He was still warm. And warmth could grow into a roaring fire. His potential lurked beneath the surface, dormant but always there. If the demon could not appreciate that, it was not Senjuro’s problem.
Before he could fully process the thought, Akaza vanished. He moved so fast that there was nothing but a blur of pink and muscle and killing intent in front of him. Too fast for Senjuro to fully register. He barely lifted his sword before the impact sent him flying.
Senjuro’s entire body convulsed in pain as Akaza’s fist collided with his guard. It was like being struck by a collapsing mountain, unavoidable and overpowering. His sandals scraped against the ever-changing floor, knees buckling as he struggled to stay upright. His arms screamed from the force of the blow, his bones vibrating as though they might shatter from the inside out.
He barely had a second to process it before—
"And you are too slow."
The voice was behind him.
A shadow swallowed his vision, and then—
CRACK
The next blow came like a thunderclap against his ribs.
Senjuro’s breath left him in a choked, strangled gasp. His sword slipped from his fingers as his body folded inward from the force of the attack. For a moment, he was weightless—his feet no longer touching the ground—before he crashed into the floor with a sickening thud.
Pain erupted through him in waves. White-hot. Blinding. His lungs spasmed, desperately trying to pull in air, but his ribs felt like they had collapsed in on themselves. His fingers twitched, scrambling for his blade, but his arms wouldn’t obey.
I can’t move.
The thought sent a new kind of fear spiraling through him. Colder. Deeper. Worse than anything he’d felt before. His vision blurred. Maybe from pain. Maybe from fear. Or maybe from the tears he was desperately trying to hold back.
For a moment, the only sound he could hear was the distant echo of shifting architecture and his own strangled breath.
Akaza didn’t even look winded. He rolled his shoulders, watching Senjuro with a mixture of disappointment and disinterest.
"Why are you wasting my time?" Akaza said, his voice now laced with boredom. "You are a waste of space," his features twisting with irritation.
Senjuro knew what that meant.
Akaza was going to kill him.
He was going to die alone.
His father would be waiting for him back at home, waiting and waiting, arms crossed, staring at the door as if Senjuro might step through at any moment. But he would not, he would never step through that door ever again, only the distant flutter of wings would make him look up—and then freeze.
The family crow would arrive, delivering the news with solemn finality:
"Senjuro-kun passed honorably on duty."
Tears would sting his father’s eyes, maybe he had changed enough he would let them fall with abandon. He would stand there, motionless, no letter to crush in his grip. Not even a goodbye. Not even a chance to say that he was forgiven—that Senjuro held no grudges. That he was loved.
That he did not want Shinjuro to lose himself again.
He wished his father would grieve, then move forward. Find happiness again. Take care of Kyojuro. Keep him from following the same self-destructive path their family always seemed doomed to take.
Would Kyojuro blame himself? Would he see Senjuro’s death as a failure—that he had not protected him well enough, not prepared him for this war? That he had sat idly and clap along as Senjuro engaged in a battle he could not win against a demon that was impossible to defeat for someone as weak as Senjuro.
No. He couldn’t let that happen.
Kyojuro had carried enough burdens already their whole lives, that had been the problem from the start. Senjuro refused to become another weight dragging him down. If his death meant anything, it had to mean that his brother would live a fulfilling life. That he would keep smiling, keep believing, keep moving forward the way he always had.
Because Kyojuro was the flame. And flames should never be snuffed out before their time.
And his haha… Giyuu… would he ever know?
Would a distant ache settle in his chest without knowing why? Would he feel the pain wherever the demons had him trapped, sensing the loss of something unspoken, a bond severed by fate?
Or would he find out only much later, in the hushed whisper of a passing crow, delivering the news with the same detached tone it always did? Would his death be another kind of torture that was not imparted by the demons but by Senjuro himself?
Would he grieve? Or would he, like always, shoulder the pain alone, another ghost added to the weight he already carried? Would he keep pushing forward despite everything, or would Senjuro be his downfall?
The thought made Senjuro’s throat tighten. He didn’t want to be a burden—not in life, not in death. He didn’t want to be another loss carved into those he loved.
But that choice had never been his to make.
The demon moved forward. And for the first time, Senjuro thought he saw death itself.
A blur of black and blue came between them.
A guttural growl rumbled through the castle, so deep it made the floor tremble beneath Senjuro. The sound wasn’t human—it was something raw, something primal, something born from agony and rage entwined. It reverberated through his bones, through the very air, rattling inside his chest, like a beast clawing its way back from the brink of death.
Before Senjuro could react, Akaza’s arm jerked back, his flesh splitting open as dark blood arced through the air. A fresh wound—a deep, brutal gash across his forearm.
The first injury he had sustained in this fight.
Senjuro gasped, vision swimming. Pain pulsed through his skull, his body screaming in protest as he struggled to lift his head, to make sense of what had just happened. The dim glow of the Infinity Castle cast long, distorted shadows across the blood-streaked floor. And standing between him and Akaza was a beast.
Clawed hands gleamed crimson in the shifting light, fresh blood dripping from razor-sharp nails. Each drop hit the ground with a quiet, sickening patter, almost deafening in the suffocating silence. The creature crouched low, its form tense, coiled like a bowstring about to snap. A creature ready to kill.
Akaza hesitated.
The flicker of surprise in his golden eyes was brief but unmistakable. He tilted his head, expression unreadable, as if re-evaluating this unexpected variable. And then, that flicker of surprise twisted into something sharper—something dangerous.
Excitement lit his features. A worthy opponent. A true fight to the death.
And yet—he remained effortlessly calm.
For a moment, beast and demon regarded each other in silence. A quiet, eerie pause before the inevitable. In a heartbeat and without warning, the beast lunged and Akaza was forced to move.
Senjuro barely registered the moment before it happened—a flash of movement too fast for his exhausted mind to process. Seven long tails snapped together like whips, the slender figure of the beast cutting through the air in a streak of black and blue, closing the distance in an instant.
Claws descended. A deadly arc aimed straight for Akaza’s throat.
A sharp hiss of breath escaped the demon’s mouth.
For the briefest second, Akaza’s eyes widened, golden irises flashing in the dim light. The beast’s claws raked across his chest, slashing deep, jagged wounds into his flesh. Blood spattered onto the ground, staining the endless shifting floors of the Infinity Castle. The sheer force of the attack sent Akaza skidding backward, his feet scrambling for purchase as he dug his heels in.
Senjuro sucked in a sharp breath.
Akaza had been hurt.
For the first time since the battle began, the demon was forced to acknowledge his opponent—not as a fleeting amusement, not as a momentary thrill, but as a threat.
But Senjuro’s stomach twisted as he took in the beast’s stance.
It was trembling.
It was slight—barely noticeable to anyone who wasn’t looking. But Senjuro saw it. The ragged, uneven rise and fall of its chest. The way its arms sagged for just a fraction of a second before tensing again. The barely-there tremor in its stance. The sharp, shallow gasps of breath. The blood dripping—not just from its claws, but from its own wounds. Seeping from its wrists, from its mouth, staining its once-soft fur in a dark, slick mess.
It was standing. It was fighting.
But it was breaking.
And Akaza saw it, too.
A slow grin stretched across the demon’s face.
"Not bad for someone half-dead."
His voice was rich with amusement, with something dark and delighted. He rolled his shoulders, flexing his wounded arm as if testing the damage. His muscles shifted, his stance loose, relaxed—completely unbothered.
The wounds on his chest had already begun to close. Skin knitting itself back together, muscle reforming in mere seconds. He didn’t look in pain. If anything, he looked exhilarated.
Then, he moved.
Senjuro didn’t even see it, one moment, Akaza stood still—the next, he was nothing but a blur. A streak of pink and muscle and death closing in like a storm. The beast tried to react, tried to raise its claws but Akaza was inside its guard before it could even process what had happened.
The first strike was brutal, a crushing blow to the ribs that sent a sickening crack through the air. The force of it was staggering, an impact so devastating that the beast skidded backward, claws scraping against the unstable floor, barely managing to keep its balance.
It was like watching a mountain crumble.
Senjuro saw the precise moment pain overtook instinct—the way the beast’s body lurched, how its breathing hitched, how its muscles fought to hold it upright despite the way its legs threatened to buckle beneath it.
Blood dripped from its lips, its breathing came in ragged, broken gasps. The wounds littering its body were still fresh, still screaming for relief, the attack had only made them worse.
And Akaza only smiled.
"You’re strong," he mused, tilting his head like a predator toying with its prey. "But he broke you already."
Senjuro’s hands clenched into fists.
He could see it.
The beast was powerful—there was no denying that. It had struck Akaza. It had hurt him. But power meant nothing when the body had reached its limit. The wounds. The exhaustion. Whatever had been done to that poor creature before this battle—it was finally catching up. It was breaking it down to pieces.
And Akaza knew.
And he exploited it.
"This isn’t enough," the demon murmured, tone laced with something almost… disappointed. "I expected more. He must not be important enough."
Then, he blurred forward again. A streak of yellow eyes and pink muscle, a raised fist. The next blow landed harder than the first one. A devastating, stomach-churning impact to the side of the beast’s skull.
Blood sprayed the air.
Senjuro’s heart slammed against his ribs.
No. It could not be happening, Senjuro could not be letting it happen, just laying there and watching as someone else got hurt trying to defend him.
Akaza exhaled, rolling his shoulders, utterly unfazed. His wounds had already vanished, as if they had never been there. His muscles coiled, preparing to strike again, prepared to kill.
This wasn’t a fight anymore.
This was a slow, calculated execution.
A blinding flash of blue sliced through the chaos, cutting through the dim light like a streak of lightning. Senjuro barely had time to process it before his breath caught in his throat. His vision swam, exhaustion dragging at his limbs, but then he saw it—that armguard.
No claws. No fur. Just a hand, firm and steady, gripping the hilt of a sword. And then—those eyes. Deep, empty blue, yet achingly familiar. The same ones that had looked at him with fleeting warmth, that had once treated him kindly when he felt most alone in the world.
A whisper of steel, a breath of motion.
"Water Breathing, Seventh Form: Drop Ripple Thrust."