Prologue

The cavern beneath Ryuudou Temple was a place forsaken by light, a domain where the foul stench of corruption thickened the air, and the oppressive darkness seemed alive, writhing with malevolent intent. The cavern walls, slick with an eerie, pulsating ichor, radiated a sickly glow, casting long, twisting shadows that seemed to reach for him with skeletal fingers. The oppressive heat and the unnatural hum in the air bore witness to the eldritch power swelling from the pulsating core of the Holy Grail, now blackened and malformed—a grotesque, pulsating heart of pure malevolence, twisted by the curses of Angra Mainyu.

Shirou Emiya stood before it, his breath ragged, his body barely holding together. His flesh had ceased to be entirely human. Jagged, twisted blades jutted out from beneath his torn skin, their metallic gleam catching the unholy glow of the cavern. Each movement sent searing pain coursing through his veins, his body a prison of steel, warping and distorting under the weight of his own existence. His left arm—Archer's cursed limb—was the epicenter of his suffering, a conduit for power his body was never meant to wield. The flesh had long since withered away, replaced by something unnatural, something divine and damning in equal measure. Each nerve, each muscle fiber screamed in agony, a chorus of suffering that no human mind could bear for long.

But he endured. He had no other choice.

His mind was fracturing, unraveling with every second as the cursed arm continued to siphon what little remained of his sanity. His sense of self was slipping away, drowned in the infinite sea of swords that existed within him. Unlimited Blade Works—his reality, his identity—was eating away at him from the inside out. He had long since lost count of how many times he had deployed it. Too many. Far too many. Each projection, each reforging of an ideal not his own had chipped away at his humanity, leaving behind something far closer to the weapons he wielded than the boy he once was.

And yet, that pain was nothing compared to the agony in his heart.

Sakura was gone. He had failed her. Failed to save the one person who had needed him most. He had tried—God, he had tried—but in the end, he had been too weak, too slow, too hesitant. She had been consumed by the very Grail he now sought to destroy, her body nothing more than a vessel for the filth that had once been her tormentor.

Illyasviel was gone. His sister, the one who had stood by him despite knowing her fate. She had chosen to save him, sacrificing herself to close the Lesser Grail, knowing full well she would not survive the process. He should have protected her. He should have done something—anything—to ensure she lived. But she had given her life for him, and he had not been able to stop her.

And Saber—Artoria—was dead by his hand. He had seen no other choice. Corrupted by the Grail's miasma, she had become a shadow of the knight he had once admired, her once-pristine armor blackened, her golden hair tainted by the darkness that had taken root within her. He had slain her. Not as a hero, not as a savior, but as a man who had no choice. The sound of her final breath, the way her cursed sword shattered in her grasp—those memories would haunt him until the end of time.

There was no salvation for him. There never had been. But he could still end this.

He could still destroy the Grail.

"Trace... On."

His grip tightened around the hilt of Excalibur Morgan, the dark counterpart of the sword of promised victory as it materializes on his hands in a spark of blue energy. A blade of corrupted legend, the same weapon he had torn from Saber's grasp in a desperate, hollow struggle. Its power thrummed through his body, resonating with the endless field of swords within him. A mockery of the true ideal, but one powerful enough to sever the festering malignance before him.

Shirou staggered forward, every step a battle against his own failing body. The cavern trembled as the Grail pulsed, tendrils of darkness writhing, reaching for him, desperate to claim him as it had claimed so many before. He raised the sword high, the cursed blade drinking in the malice that flooded the air. His vision blurred. His breath hitched.

This was it. His end.

With one final push, he brought the blade down, its edge cleaving through the sickly, pulsating core of the Grail. A deafening roar erupted as a tidal wave of curses exploded outward, black ichor spraying in all directions. The world itself seemed to fracture, space twisting and convulsing as reality buckled under the sheer force of destruction.

And then—

A blinding light. A rift, impossibly vast, yawning open within the collapsing cavern. For a moment, Shirou felt nothing. No pain. No regret. Just the endless expanse before him, a void that defied comprehension. The power of the Second Magic, something he had no right to even begin to understand, had been embedded in the Grail's core, a failsafe beyond human reckoning. And now, with the core shattered, it had activated, tearing open the boundaries between worlds.

Then, the pull came. An overwhelming force, an undeniable gravity that seized his very soul, yanking him forward into the abyss. He could not resist. He did not have the strength to fight it.

The last thing he saw was the collapsing cavern, the remnants of the Holy Grail vanishing into the darkness. And then—nothing.

A void. Endless, shapeless, and utterly consuming.

Shirou did not feel pain anymore. He did not feel the weight of his broken body, nor the searing agony of his decaying nerves. There was only silence—a deep, suffocating silence that swallowed every thought, every trace of his existence. Was this death? Was this the price he had to pay for wielding a power beyond his limits? For failing those he swore to protect?

Memories flickered before him, floating like shattered glass in the abyss. Sakura's smile, tinged with sorrow and longing. Illya's small, fragile hands reaching for him, only to slip through his fingers. Saber's golden radiance, now tainted by the darkness he had been forced to strike down. Rin, her fierce determination crumbling as she lost everything.

He had failed them all.

A bitter wind—or was it just his mind conjuring sensation in the nothingness?—brushed against him, carrying whispers that clawed at the edges of his consciousness. He could not make out the words, but they filled him with an unbearable sense of unease. Shadows stirred, shifting and warping around him, forming ephemeral shapes that dissolved the moment he tried to focus on them.

Then, a presence. Ancient, unfathomable, watching him from the depths of the void. It was not hostile, yet it was not kind. It was merely... observing.

Shirou wanted to move, to reach out, to struggle against the weightless abyss, but his body did not exist here. He was only a soul, stripped bare of flesh and bone. And then, as if in response to his silent rebellion, the void trembled.

A crack of blinding light split the darkness apart. Not light. Something deeper, something older. A force that should not have existed in the void. It wrapped around him, pulling, twisting, warping. He felt himself being dragged, spiraling through the unseen currents of this impossible space. His thoughts scattered, unraveling like threads from a fraying tapestry.

This is not where you belong.

The words were not spoken, yet they echoed in the depths of his mind, resonating with something beyond his understanding. The force coiled around him, relentless and unyielding, as if rejecting his very existence. And then, with a final, overwhelming surge, it hurled him forward.

There was no impact, no sensation of breaking through a barrier, but suddenly, he was falling—or was he rising?

A gasp tore through his lips. Air filled his lungs. His body convulsed, overwhelmed by sensation. The feeling of skin, of muscle, of warmth. His heartbeat, rapid and disoriented, thundered in his ears.

Shirou opened his eyes.

A dimly lit room greeted him. He was lying on a bed, tangled in sheets damp with sweat. The air smelled of something faintly familiar—wood, ink, and a trace of metal. His breath was ragged, his mind struggling to grasp the dissonance between what he had just experienced and what he was now seeing.

Something was wrong.

His left arm. He lifted it hesitantly, staring at the limb that should not be his. The unnatural steel and scarred flesh of Archer's arm were gone. In its place was a slender, pale arm—one that did not belong to him. His fingers trembled as he reached for his face, feeling unfamiliar contours, a jawline too refined, hair too silky. His pulse quickened as he forced himself upright, legs unsteady beneath him.

The mirror. There had to be a mirror.

His feet moved before his mind could catch up. He staggered toward the dresser at the corner of the room, gripping its edge for support. And then he saw it—his reflection staring back at him.

A young European man with golden-blonde hair and steel-gray eyes. A face he did not recognize, yet one that was seared into his mind from memories that were not his own.

Isaiah - no, Kiba Yuuto.

Panic surged through him, a cold realization settling in his bones. He was no longer Shirou Emiya. He was someone else. Someone whose body he had no right to inhabit.

His mind reeled, fragments of another life flooding in—memories of laughter, of blades, of a Holy Sword Project, of the dead of many orphans Made as a test subject, of guilt and Anger for surviving. Of a master named Rias Gremory. Of comrades he had never met, yet now somehow knew. It was overwhelming, suffocating.

He gritted his teeth, gripping the edge of the dresser as he steadied himself. No. He could not afford to lose himself. He needed to think. To understand.

How had this happened? Why was he here? And more importantly...

What was he supposed to do now?

Shirou stared at his reflection, his golden hair and steel-grey eyes an unfamiliar sight staring back at him. The face was not his own, but the body responded to his every gesture. He raised a hand, flexing the fingers experimentally, watching as the movements carried out with perfect synchronization. The weight of his limbs, the way his muscles shifted—everything was subtly different from what he remembered. This body was younger, more refined in its movements, yet it lacked the sheer resilience and battle-worn toughness of his own.

His mind was still reeling from the realization. He was no longer Emiya Shirou. At least, not in body. Somehow, his soul had found itself anchored into another existence—that of Kiba Yuuto. He exhaled shakily, his breath catching slightly. His magic circuits had always been a fundamental part of him, but when he reached inward to assess them, he found something unnerving. Unlimited Blade Works was there, its presence as overwhelming as ever, but he also senses something else. Something different, but also similar to his Reality Marble. And a certain name suddenly appears in the back of his mind.

Sword Birth...

If the memory of this Kiba Yuuto guy is correct, Sword Birth is the name of his power, the name of the Sacred Gear that's been bestowed upon him. Sacred Gears are items with powerful abilities bestowed upon humans by God of the Bible. Each of them possession different form and powers.

Sacred Gears are like the equivalent of Noble Phantasms to his world. Each possessing different form and powers. Some even have Conceptual ability.

Shirou internally chuckles at this piece of knowledge. If he didn't have Kiba Yuuto's memory, he probably won't notice about the existence of this so called Sword Birth in this body.

He took a step back from the mirror, his eyes scanning the room. The bedroom he is currently in was not ones that he used to. It was grand, a stark contrast to the modest bedroom he once knew back at the Emiya's residence. The walls were adorned with elegant, traditional Japanese and European designs, seamlessly blended together to create an air of aristocracy. A chandelier hung from the ceiling, casting a warm golden glow across the polished wooden floor. A large four-poster bed rested against one side of the room, its silk sheets untouched and pristine. To the side, a mahogany desk was neatly arranged with books, documents, and a few personal trinkets. There was a fireplace in the corner, though it was currently unlit, its ornate carvings adding to the regal atmosphere.

He walked towards the window, pulling aside the heavy curtains. The mansion grounds stretched beyond what he could see, with immaculately maintained gardens, cobblestone paths, and high iron gates in the distance. The night sky loomed overhead, the moon casting a silver glow over the estate. He let out a slow breath. This was Kiba Yuuto's life—one of wealth and refinement, completely unlike his own. But why had he ended up here?

Shirou turned back, rubbing his temples. His thoughts were scattered, raw from the transition. His last memory was of the collapsing cavern, the Holy Grail's core reduced to nothingness by his own hand. Then, the void. A sea of infinite nothingness, where even time and space had lost their meaning. He remembered floating, unable to move, unable to think beyond the pain of existence itself. The Dimensional Gap. A realm beyond worlds. And yet, instead of fading away, he had been brought here, bound to a body that was never his.

He clenched his fists. What was he supposed to do now?

His fingers dug into his palms as he struggled to steady his thoughts. He needed answers. There was no point in panicking—he had survived worse. He had fought through death, through suffering, through despair. This was just another battle, another obstacle he had to overcome.

A knock at the door jolted him out of his thoughts.

"Yuuto?" A soft, feminine voice called from the other side. "Are you awake? It's getting late."

Shirou hesitated. He knew that voice. It was Rias Gremory.

He swallowed, his mind racing. If he was truly in Kiba Yuuto's body, that meant he was part of Rias's peerage. He was a Devil now. The implications sent a chill down his spine. He had basically fought against the supernatural during the Fifth Holy Grail War, cutting down beings that preyed on humans. And now he is one of them?

Taking a steadying breath, he moved to the door, his hand hovering over the handle for a moment before he turned it. As the door swung open, he was met with Rias's curious gaze. She stood there in a silk nightgown, her crimson hair cascading down her shoulders, an expression of mild concern softening her features.

"Are you alright? You seemed distracted at dinner earlier," she asked, tilting her head slightly. "You know you can talk to me if something's bothering you."

Shirou forced a small smile, masking the turmoil inside. "I'm fine, Buchou. Just... thinking about some things."

She studied him for a moment before nodding. "Alright. Just don't push yourself too hard. You know we're all here for you."

With that, she turned and left, the soft click of her footsteps fading down the hall. Shirou exhaled slowly, leaning against the doorframe. That interaction had been more difficult than he anticipated. He wasn't used to people showing concern for him—at least, not in this way.

He stepped back into his room, his mind settling. He needed a plan. If he was stuck in this world, he couldn't afford to act recklessly. He had to learn everything he could about Kiba Yuuto's life, about this world's rules, and about what it meant to be a Devil. Only then could he figure out his next move.

For now, though, he would play along. Until he understood the full picture, he would be Kiba Yuuto.

But deep down, he knew—he was still Emiya Shirou somewhat. And he would find his own path, no matter where fate had thrown him.

A/N: Hello everyone, welcome to my story. First story to be precise. I have read many good stories in this site, mainly Fate series crossover with either Shirou or EMIYA as the Protagonist. Recently I have been reading some good fic where Shirou died in his old world and reincarnated as another character from other series, and I must say eventually I got interested in writing one myself, even though I'm not sure if my writing is as good as the veteran authors in this site. Also, hopefully I don't make any mistakes in my Vocabulary or grammar since English isn't my mother language. Anyway, for the story I intend to have Shirou reincarnated as Kiba Yuuto. Why him? Let's just say him and Kiba are quite similar. So, this story starts around a year or a few months before the main story of DxD begins, simply because I want to write an arc where Shirou adapts with his new identity and new life. Well, then. That's all that I wanted to say. Please leave me some reviews, I'd like to know if you think this story has potential.