Fate/Charm of the Devil Fae

This fic is inspired by Sticky Situation by  Professor Quill,   In Bloom by Flight of Fancy,  and to a certain extent Benefits of Saving a Veela by WD_ONeill . Please check them out.

Story Starts

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Ch 2.1 - The Ladies'

Remedy

Disclaimer: Everyone here is at least 18 years of age.

Shirou found himself back in the forge before he quite registered how he'd got there.

Lady Avalon had seized his wrist the instant the others began to scatter, tugging him along with the gleeful determination of someone who had waited all morning for exactly this moment. Taiga had muttered something about overdue lesson plans and faculty meetings, already half out the door in a whirlwind of mismatched shoes and jangling keys. Rin had loudly claimed she had a date with Ayako; the glance she threw over her shoulder as she left—sharp, knowing, and just a little too long—still burned quietly behind Shirou's eyes. Runeas, meanwhile, had asked Sakura to help with paperwork in that calm, commanding tone of hers, and Sakura had simply inclined her head with a small, composed smile before following her out.

And so here he was, alone in the forge with Lady Vivian and Lady Avalon.

Not the worst possible company. He'd spent countless hours in this room with the pair, after all. Vivian watched him with the cool, dissecting precision of a master smith inspecting flawed steel; Avalon, on the other hand, radiated the restless impatience of a child promised sweets and then told to wait another hour.

The past twenty-four hours refused to settle in his mind. One moment he'd been retreating into the familiar rhythm of hammer and heat and clean, honest steel; the next he was tangled in sheets with Rin and Sakura as though some invisible wall between them had simply dissolved.

There had always been something between the three of them—an unspoken charge that shimmered in the air like heat rising from a quenched blade. And despite what Rin, Rias, Ayako, Taiga—practically everyone—liked to say, he wasn't that dense. He'd noticed. Of course he'd noticed.

Knowing, though, was one thing. Acting on it was another entirely.

Especially when the alternative was dragging the people he cared about into the slow, grinding decay that had already hollowed out so much of him.

His gaze had drifted somewhere distant when—

Flick.

Shirou rubbed his forehead, wincing as the sharp sting spread across the bridge of his nose. He turned to find Lady Avalon pouting at him, finger still poised from where she'd flicked him right between the eyes. Her lips were pursed, brows drawn down in an expression that somehow managed to be both regal and petulant at the same time.

"You're brooding again."

She leaned in without warning, draping one long leg across his thighs. The scent of her hit him like cool morning mist—dew-soaked grass, crushed wildflowers, berries, and citrus. Something low and involuntary tightened in his chest; his pulse kicked hard against his ribs.

"Was this morning's little romp with your former homeroom teacher not enough to keep that mind of yours occupied?" Her voice had dropped to a sultry murmur, her breath ghosting across his ear, and an involuntary shiver raced down his spine.

Lady Vivian cleared her throat from the far side of the low workbench, the sound crisp and deliberate.

"Avalon," she said, tone even but edged with the faintest trace of long-suffering patience, "please. Let's not get distracted right now. We can… set that aside for later. There's a more important task at hand."

Lady Avalon huffed dramatically, withdrawing her leg with exaggerated reluctance. Shirou felt the sudden absence of her warmth more keenly than he cared to admit. She settled back against the low couch tucked against the forge wall, arms crossed, chin lifted in perfect, theatrical indignation.

"Hmph."

Lady Vivian pinched the bridge of her nose and let the silence do the rest of the scolding.

Shirou's gaze drifted to the far wall of the forge, where a row of unfinished blades hung from iron hooks like sleeping birds. The metal caught the low amber glow of the banked coals, and for a moment the light shifted—warped—became something older and crueller.

Fire.

Not the clean, purposeful fire of the forge. The other one. The one that had eaten Fuyuki alive.

He'd been small. So terribly, impossibly small. The world had become a corridor of flame and falling timber, and every direction led deeper into the inferno. Bodies lay where they'd fallen—some still moving, fingers twitching, mouths open in silent pleas he couldn't answer. He stepped over them. Past them. Through them. Not because he was brave but because stopping meant dying, and something animal and desperate inside his ruined chest refused to let him stop.

The smoke had long since scoured his lungs raw. His skin cracked and wept. His hair had burned away on one side of his head. And still he walked, barefoot over rubble that glowed cherry-red beneath a sky choked black.

Then the mud.

He remembered falling face-first into it—cool, blessed mud at the edge of a drainage ditch—and thinking with the peculiar clarity of a child who had already accepted death that the ground felt nice. That was the thought. Not help  or please  or someone . Just: the ground felt nice.

He'd lain there, half-submerged, watching the fire paint the clouds orange, and waited.

Kiritsugu Emiya found him like that.

Shirou didn't understand the relief in the man's eyes, or why he was smiling.

The man's face was a ruin of grief. Hollow eyes, cracked lips, skin grey with ash and something worse—something that went deeper than soot. He'd fallen to his knees beside Shirou and pressed both hands to the boy's chest as though trying to hold the life inside him by force alone. The golden scabbard—Avalon—had already been placed within Shirou, its light pulsing faintly beneath charred skin like a heartbeat learning a new rhythm.

But even Kiritsugu could see it was failing.

Avalon had been severed from its true owner. Without a connection to Arturia Pendragon, the Noble Phantasm was running on fumes—a lamp with a cracked reservoir, wick sputtering while the oil pooled uselessly below. It could heal, yes. It was healing, knitting together tissue that should have been beyond salvation. But the curse of Angra Mainyu was already inside Shirou too, threaded through his circuits like black wire through copper, and every inch of flesh Avalon repaired, the curse corroded anew. The two forces ground against each other in an endless, agonising stalemate, and the boy caught between them was simply being torn apart.

Kiritsugu had watched the golden glow flicker. Watched it dim. Watched it rally, then dim again, each cycle weaker than the last.

And he'd made a choice.

Shirou's memories of that night were a disjointed mess—shattered glass held up to firelight, each shard showing a different angle of the same catastrophe. But he'd pieced together what happened from what Kiritsugu, Runeas, and others had told him over the years that followed.

Kiritsugu had made a choice. He'd produced a sheet of paper from the lining of his overcoat, something prepared as a last resort if all hope were lost. A summoning circle, keyed to a specific devil.

It had been a gift from Natalia Kaminski.

Later—much later—Shirou learned the full story. Natalia, the woman who'd raised the Magus Killer from a feral child into the deadliest freelancer the moonlit world had ever produced. She'd given Kiritsugu several of these summoning circles before her death—or rather, before Kiritsugu had killed her, a mercy that was no mercy at all, aboard a plane full of the Dead. The circle was keyed to a specific devil, she'd told him. A contact. An old debt. Use it only when there is no other choice.

No other choice.

Kiritsugu's hands had trembled as he fed his dwindling od into the circle. His magic circuits—already degraded, already failing from the backlash of the Grail's destruction—screamed in protest. Blood trickled from his nose, his ears, the corners of his eyes. The circle blazed crimson.

And Runeas Gremory stepped through.

Through the haze of Shirou's unending cycle of destruction and creation—the curse burning, Avalon rebuilding, neither willing to yield—the figure who emerged had looked… wrong, in that place. Too clean. Too vivid. Too present  against the hellscape of burning Fuyuki. Her crimson hair fell past her shoulders in a cascade that caught the firelight and threw it back richer, deeper—arterial red against the crude orange of mundane flame. Her eyes swept the scene with the detached precision of someone cataloguing assets: the dying man, the dying boy, the fading golden light beneath the boy's skin.

Kiritsugu had spoken three words. "Save this child."

Runeas had been irritated, at first. Someone had dared to disturb her long sleep.

But then she'd studied the boy—for what felt like hours to Shirou, though it was probably no more than seconds, perceived through the delirium of a body tearing itself apart—and something in her expression shifted. He could just make out the gleam of her smile through the smoke and the agony.

"Hmmm… interesting."

She'd produced a chess piece from thin air—a queen, carved from crimson crystal that pulsed with demonic energy so dense Shirou could taste it even through the haze of pain and smoke. The Evil Piece. A fragment of the original system designed by Ajuka Beelzebub, capable of reincarnating any being into a devil, rewriting their fundamental nature at the spiritual level.

The theory was sound. Devil physiology was inherently resistant to external curses, and the curse of sin was something devils naturally basked in. If the Evil Piece could take hold, it would give Shirou's body a framework to coexist with both Avalon's fae energy and Angra Mainyu's corruption—not purging either, but metabolising the conflict into something survivable.

Runeas placed the queen piece against his sternum. The crystal sank into his flesh like a stone into still water.

For three seconds, nothing happened.

Then Shirou screamed. That was most of what he remembered—screaming—as pain became his only world.

Later, Runeas told him most of what happened next.

His body spat the chess piece out.

It erupted from his chest in a shower of light—not violently, not painfully, but with the quiet finality of a door closing. The crimson crystal tumbled across the mud, its glow guttering, dimming, going dark. Inert. A piece of pretty glass and nothing more.

That much, Runeas told him years later, her voice carrying the particular edge it always took on when recounting something that had genuinely surprised her. She'd caught the piece before it hit the ground, turning it over between her fingers with an expression Shirou would eventually learn to recognise as genuine bewilderment—an expression vanishingly rare on the face of a woman who had lived for centuries and seen the rise and fall of entire civilisations.

"Fascinating," she'd murmured.

Because the transformation had worked. Shirou's body had absorbed the demonic conversion completely, fundamentally altering his nature from human to devil. His senses had expanded. The dark energy hummed through his circuits alongside his od, a second current running parallel to the first.

But the piece itself had been rejected. Expelled. As though his body had taken what it needed and discarded the vessel, the way a butterfly sheds a chrysalis.

An Evil Piece that had been used successfully and yet remained. Runeas had never seen anything like it. No one had. The piece should have bonded permanently, should have become part of his spiritual architecture, an anchor tying him to her peerage. Instead, it sat dead in her palm, its purpose fulfilled and its connection severed.

The reincarnation, however, hadn't solved the core problem—that part, Shirou understood firsthand, even if the technical explanation came later. Avalon and the curse still warred inside him. The devil transformation had strengthened his body enough to survive the conflict, but it hadn't tipped the balance. If anything, the three-way collision of energies—divine sheath, corrupting curse, demonic nature—had made the internal landscape more volatile. More unstable.

Runeas made another decision. According to Kiritsugu—who recounted this part in halting, fragmented sentences over the course of many years—she drew a transportation circle in the air with one finger, a casual gesture, almost lazy, and the world folded around them.

They emerged in Hell.

Not the fire-and-brimstone caricature of human mythology. The Underworld—a territory within the reverse side of the world, the shadow cast by the reality humans knew. When the Age of Gods had waned and Mystery had begun its long retreat, when the ether thinned and the texture of reality hardened into something inhospitable to the mythical and the phantasmal, this was where they'd gone. Dragons. Faeries. Elementals. Beings of legend that could no longer sustain themselves in a world increasingly defined by human cognition and human disbelief. They'd crossed over, slipped through the thinning membrane between sides, and made their home here.

Shirou had seen the Gremory estate many times since, of course, but the description Kiritsugu gave of that first arrival never quite matched what Shirou later came to know. His father had spoken of rolling hills of deep violet grass beneath a sky that held three moons—two small and silver, one vast and copper-red—and architecture that was old European in style but scaled for beings that thought in centuries rather than decades: soaring buttresses, windows of stained crystal that depicted wars Shirou would later learn had shaped the course of supernatural history.

Kiritsugu had barely registered any of it. He'd carried Shirou through corridors of dark marble, following Runeas's brisk stride, his deteriorating body running on nothing but stubborn refusal to set the boy down.

They'd placed Shirou in a room. High-ceilinged, white-walled, filled with soft light from sources he couldn't identify. Medical equipment that blended the arcane and the modern lined the walls—monitoring crystals alongside heart-rate machines, enchanted salves beside sterile gauze. Kiritsugu had stood beside the bed like a sentinel, grey-faced and swaying, until Runeas ordered him to sit before he collapsed.

It was in that room, sometime during the first night, that the others came.

This part Shirou remembered himself—or thought he did. The memories had the slippery, oversaturated quality of fever dreams, and he'd never been entirely certain where his own perception ended and what others had filled in for him began.

He'd been drifting—not sleeping, not waking, suspended in the grey space between—when the air in the room changed. A pressure built behind his eyes, in his teeth, in the marrow of his bones. Something ancient and impossibly powerful had taken notice of Avalon's presence on this side of the world.

Two figures appeared in the doorway.

The first was a woman. Even through the delirium, her presence registered like a stone dropped into still water—ripples of something vast and quiet spreading outward from where she stood. Pale skin, features carved with the severe elegance of old Celtic statuary—high cheekbones, a jaw that could cut glass, eyes the colour of deep water under moonlight. Her hair fell past her shoulders in waves of pale silver-white, and she wore robes of muted grey-blue trimmed in silver thread that caught the light and held it prisoner. There was something about her that reminded Shirou of a lake—still on the surface, fathomless beneath.

Lady Vivian. The Lady of the Lake. Keeper of sacred blades, warden of Avalon's mysteries, a being whose name was woven into the very fabric of Arthurian legend.

He learned her titles later. In that moment, all he knew was that the woman with the pale hair felt like drowning—not violently, but the way deep water simply closes over you without malice or hurry.

The second figure was harder to hold in focus. Tall—he thought. White-haired—maybe. There was a cloak that seemed to be made of flowers, or perhaps the flowers were growing from nothing, blooming and dying and blooming again in a cycle that made his fever-addled eyes ache to follow. The details wouldn't stay fixed. Every time Shirou tried to look directly at the man, the image slid sideways, like trying to read words written on moving water.

But the smile. That, he remembered with absolute clarity.

It was the most irritating thing Shirou had ever seen, even through the haze of agony. It was the smile of someone who found existence itself mildly amusing.

Merlin. The Magus of Flowers. Half-incubus, full nuisance, architect of kingdoms and their ruin in equal measure. Those descriptors, too, came later—from Vivian, mostly, delivered in a tone that suggested she was reciting a criminal record rather than an introduction.

They had both felt Avalon's presence the instant Runeas brought it across the threshold into the reverse side. A Noble Phantasm of that magnitude, disconnected from its wielder, flickering between function and failure—it sang across the supernatural frequencies like a distress beacon, and these two, bound as they were to the legends surrounding the scabbard, had answered.

Merlin moved to the bedside first—or at least, that was how Vivian described it later, because Shirou's own memory of the next few minutes was a smear of light and pressure and the overwhelming sense that something fundamental was being rearranged.

He placed one hand over Shirou's chest—palm flat, fingers spread—and closed his eyes. The smile didn't waver. If anything, it deepened.

Then he did something.

To this day, Shirou couldn't describe what happened next. Neither could Vivian, and she'd been standing right there. Merlin's magic didn't operate on principles that could be observed, catalogued, or replicated. It simply was . One moment, the room held four people and a dying boy with a golden scabbard guttering inside him. The next moment, the room held four people, a dying boy—

—and a woman.

She stood at the foot of the bed, blinking in apparent confusion, as though she'd just been woken from a deep sleep. Silver-white hair that pooled around her bare feet. Eyes like morning sky reflected in still water. A dress of white samite that seemed woven from light itself. She looked at Shirou, then at Merlin, then back at Shirou.

Lady Avalon.

Not Arturia. Not the King of Knights. Something else—something adjacent, something born from Avalon's essence given form and will and breath. An emanation. A spirit. A consciousness that had always existed within the scabbard, dormant, dreaming, and which Merlin had somehow—through means none of them understood—drawn out into independent existence.

All of this, Shirou learned in pieces. Runeas supplied the clinical framework—what the monitoring equipment had recorded, what her wards had detected, which was almost nothing. Vivian provided the emotional texture—the impossible pressure in the room, the way reality had seemed to hold its breath. Lady Avalon herself remembered only waking, and the first face she saw, and the inexplicable certainty that she had always been meant to be here.

But one detail every witness agreed on.

The Magus of Flowers had looked at the woman he'd conjured—or freed, or awakened; no one could agree on the right word—and then past her, his gaze fixing on something none of them could see. The eternal amusement cracked, just enough to show something underneath. Something old and tired and fond.

"That stubborn girl," he'd said softly. But he wasn't looking at Lady Avalon when he said it. He was looking at nothing. Or perhaps at something very far away.

And then he vanished.

Gone. Not teleported, not astrally projected, not hidden behind an illusion. Simply gone, as though he'd never been there at all. No trace of his magical signature, no residual energy, nothing. Runeas's detection wards hadn't even registered his departure.

They hadn't seen the Magus of Flowers since. Eleven years, and not a whisper. No sightings among the supernatural factions. No rumours from the Mage's Association. No dreams, no messages, no cryptic poetry left on windowsills. Merlin had walked out of their lives as completely as a stone sinking into deep water.

No one knew why he'd done it—what purpose Lady Avalon's existence served in whatever grand design the Magus of Flowers kept behind that insufferable smile. No one knew who the stubborn girl was. No one knew where he'd gone, or what had called him away with such urgency that he'd abandoned an unexplained miracle on their doorstep without a word of explanation. Runeas found it irritating. Kiritsugu found it irrelevant. Lady Avalon herself simply accepted her existence with the serene pragmatism of someone who had no framework for questioning it.

Though sometimes, when Shirou caught Lady Vivian staring at a particular point in the middle distance—her gaze unfocused, her lips pressed into a thin, knowing line—he suspected she had her theories.

She simply chose not to share.

What followed was recovery. Slow, grinding, agonising recovery.

It was Vivian who proposed the solution. Not out of charity—she was frank about that, when Shirou eventually asked. Avalon was hers . Not in the way it had been Arturia's, not as a wielded Noble Phantasm, but as a sacred trust. She was the Lady of the Lake. The scabbard had passed through her hands before it ever reached the King of Knights, and the fact that it had been reduced to a mere equal of the curse festering inside some half-dead child was, in her words, "an offence she was not prepared to tolerate." That the boy housing it might also be saved was, she implied, a convenient secondary outcome.

Shirou had never been entirely sure he believed that. Vivian's dispassion had always struck him as something she maintained with effort rather than something that came naturally. But he'd learned not to press. She offered partial ownership of the scabbard—not to restore the severed bond with Arturia; that connection was gone, whatever had happened at the Grail War's end had seen to that—but to forge new ones. If new anchors could be established, new connections made, the Noble Phantasm might stabilise enough to win its war against Angra Mainyu's curse.

Two anchors. Two new partial owners.

Shirou himself was the first—obvious, given that Avalon already resided within him. The ritual that bound him to it was painful in ways that had nothing to do with the body. It required him to open his spiritual core completely, to bare every circuit and every pathway to Vivian's exacting examination, and to accept the scabbard not as a foreign object but as part of himself. An organ. A limb. His.

Lady Avalon was the second. She was, in a sense, already Avalon—or Avalon was already her. The binding formalised what Merlin's impossible act had begun, giving her a claim on the sheath's power that ran parallel to Shirou's own. Two owners. Two anchors. A bridge of shared purpose spanning the gap that Arturia's absence had left.

The days that followed blurred together. Five, six—Shirou lost count. What he remembered was fragmentary: the curse fighting back with everything it had, sensing the tide turning. Fevers that left his sheets soaked. Seizures that took three people to hold him through. Stretches of delirium during which he apparently spoke in languages he didn't know—Runeas kept clinical notes of these episodes, and showed them to him once, years later, in a tone that made clear she found the data fascinating.

Kiritsugu never left the room. That much, everyone confirmed. Runeas monitored his vital signs with clinical detachment that fooled nobody.

And Lady Avalon—he learned this from Vivian, because he hadn't been conscious enough to know it himself—sat at the head of his bed for the entire ordeal, both hands clasped around his, her eyes closed, pouring her nascent existence into the scabbard like water into a cracked vessel. Willing it to hold.

And on the last night, Avalon won.

The golden light blazed so brightly it turned the windows white. The curse didn't die—couldn't die, not truly—but it finally stopped warring with everything within him. His circuits stabilised. The three competing forces—divine, cursed, demonic—reached an uneasy homeostasis, each checking the others in a balance that held through tension rather than harmony.

But the body that emerged from that crucible was no longer the one that had entered it. Eleven years old and already something unprecedented: a devil who housed a divine Noble Phantasm and carried a portion of All the World's Evil in his spiritual core. His magical circuits had been reforged by the competing energies into configurations that shouldn't have been possible—dense, multifaceted, capable of processing od and demonic power and traces of divine energy simultaneously. His physical capabilities exceeded anything a boy his age should have possessed. And his body had drifted—subtly but irrevocably—from the template of what Shirou Emiya should have been, towards something no classification system in the moonlit world or the Underworld had a name for.

He was still Shirou. He was still—

"Tea."

The word cut through the memory like a blade through silk. Shirou blinked, the forge snapping back into focus—the hanging blades, the banked coals, the workbench cluttered with projects in various stages of completion. The scent of jasmine and something greener, herbaceous, rose to meet him.

Lady Vivian stood before him, a ceramic cup held in both hands, steam curling from its surface in delicate spirals. Her expression was unreadable—that lake-surface calm she wore like armour—but her eyes held the faintest trace of warmth. She'd brewed it whilst he was lost in remembering.

Of course she had. She always knew.

"You were gone for rather a long time," she said, extending the cup.

Shirou took it. The ceramic was warm against his palms, grounding him in the present, in the solid reality of fired clay and fragrant tea and the woman watching him with ancient, patient eyes.

"Sorry," he managed. "I was—"

"Brooding," Lady Avalon supplied from the couch, her tone laced with vindication. "I told  you."

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End

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