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 2.3 - Ze X and Ze Y

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

Runeas led the two Veela across the sand towards the blanket. Shirou had barely caught his breath—Vivian still lay sprawled beside him, her abandoned book near her hip, and Avalon on her far side, humming with smug contentment.

Around them, the beach had not recovered. Moans drifted on the sea breeze from a dozen tangled groups.

" Maman ! Grandmaman ! What are you doing 'ere?"

The cry came from the cluster of young women Rin and Ayako had drifted towards earlier. A tall blonde rose from the group—no, two blondes, the second a touch shorter and visibly younger, scrambling up after the first with sand spraying from her knees.

Veela. Both of them. Shirou registered it as a pressure against his senses, a pull—except theirs barely tugged. The coven's territory pressed on him like a gale, every full-blooded Veela on the beach a separate gust battering at his control. These two were the softest patch in the whole storm. It helped that the cluster of witches near them carried not a trace of the aura; he could feel where the pressure dropped to nothing, and that absence marked the pair as clearly as the pull marked everything else.

'Diluted blood,'  he thought. 'Half? Quarter, maybe.'

The two women beside Runeas smiled at the approaching pair. The younger-looking of them—mature in bearing, though her face wouldn't have passed for thirty—opened her arms.

"Fleur and Gabby. I see you 'ave invited the friends you met from that dreaded tournament."

She gathered both girls into a hug, pressing a kiss to each crown of blonde hair. The older woman beside her—older only in how she held herself, in the patience of her smile, because her face was if anything smoother—watched with fond amusement, hands folded.

Shirou studied them despite himself. The maturity was unmistakable, layered into every gesture, yet neither could have been placed past her mid-twenties by face alone. Early thirties at a generous stretch for the elder. Beings of beauty aged slowly—more slowly even than wand-wizards or magi—and these two carried their decades lightly, letting them settle into bearing and voice rather than skin.

Runeas coughed into her fist.

"Shirou, Vivian, Avalon—this is Apolline Delacour, and her mother, Kalina Zoranova-Montclair."

Shirou stood. It was a nude beach. Everyone present was nude. He had spent the last several hours doing things on this blanket that made nudity the least of his sins, and the entire beach behind him was an active orgy of his own accidental making.

He still went scarlet, cupped both hands over himself—his erection refused to cooperate with the gesture, jutting past his fingers with stubborn prominence—and bowed at the waist.

"Pleased to meet you. I'm Shirou. Emiya Shirou."

Avalon made a strangled noise into the blanket that was definitely laughter.

Kalina's smile gentled further, crinkling nothing on her ageless face. "Such manners. 'Ow refreshing." The words carried a faint roll to the r , consonants landing a shade harder than French would place them—eastern European, worn smooth by years abroad.

Apolline gave him a curt nod, her gaze flicking down to his inadequate hands, then back up without comment.

" Maman ," the younger of the two girls—Gabby—tugged at Apolline's elbow, "where is Papa? You did not bring 'im?"

"Your father is quite busy with the Ministry, Gabby. And 'e 'as a date with your aunt later this evening."

"Ahh—AHHH!" Gabby clapped both hands over her ears. "I do not want to 'ear it!"

The elder sister—Fleur, then—pulled a sour face and looked out to sea, as though the horizon had personally offended her.

Shirou kept his expression carefully neutral. He was in no position to judge anyone's domestic arrangements. He was, at present, the least qualified person on the European continent to judge anyone's domestic arrangements. But the question must have shown on his face anyway, because Runeas leaned towards him.

"Keeping up with a Veela—even a half-Veela—is quite difficult for a regular wizard. So the Delacours maintain an open arrangement on both si—"

"AGAIN! Ahh—AHHH! I do not want to 'ear it!" Gabby's hands clamped tighter.

Kalina watched her granddaughter with open amusement. "Even you and Fleur, moyata chéri , will find it difficult to keep a man who can match your appetite—especially as you grow older. The fire only burns 'otter with the years. You also must extend the concession to your beau , as it will bring resentment."

"Still—I do not want to 'ear it." Gabby lowered her hands just enough to fold her arms, chin lifting. "My 'eart only 'as 'Arry in it."

Fleur and Kalina shook their heads in eerie unison. Apolline frowned.

"That sour-looking boy with ze green eyes and glasses?" Apolline's nose wrinkled. "Ze one who, alongside 'is twin sister, declared ze British dark wizard 'ad returned?"

"Harry has his moments."

A girl with bushy brown hair had approached with the rest of the group—she was still blushing—Rin and Ayako trailing behind. She winced as she spoke, right hand rubbing the top of her left arm in an absent, well-worn gesture.

"He grew up in a problematic house," she went on. "It... shaped him. He doesn't make the best first impressions."

"Or second," muttered a freckled redhead behind her, earning an elbow.

Fleur arched one elegant brow. "'Is twin sister is quite agreeable. I do not see 'ow ze two are brother and sister at all. Rose smiles. 'Arry glowers. Per'aps zey divided ze personality in ze womb and she took ze whole of it."

"'E is misunderstood!" Gabby rounded on her sister, accent thickening with every word. "You do not know what 'e carries! 'E 'as ze weight of ze whole country on 'is shoulders, and a sister to protect, and ze newspapers calling 'im a liar every week, and—"

"And ze manners of a kneazle dragged backwards through an 'edge," said Fleur.

"Fleur!"

"I am only saying what everyone thinks."

"You are saying what you think, which is not ze same—"

The debate spiralled. The bushy-haired girl attempted diplomacy and was talked over from both sides. Rin caught Shirou's eye across the group and mouthed something that might have been good luck , the corner of her mouth twitching.

Shirou took the opportunity to step closer to Runeas, lowering his voice beneath the sisters' escalating argument.

"You said something about hitting two birds with one stone."

"Ah. Yes." Runeas's crimson hair caught the sun as she turned, and her smile sharpened into the gleam that, in Shirou's eleven years of experience, preceded either a breakthrough or a disaster. Frequently both. "I would like to experiment with your semen."

Shirou choked on nothing.

"Everyone in our household is on a potent contraceptive," Runeas continued, unbothered. "My own brewing—quite a strong formulation. But your situation presents a fascinating variable. I was wondering: if you drank a contraceptive potion, would your seed retain its fertility-enhancing effects without actually impregnating the subject?"

She gestured, palm open, to Kalina and Apolline.

Both Veela regarded him with calm, evaluating interest. Kalina's expression held warmth. Apolline's was harder to parse—clinical, appraising, her gaze ticking between his face and his posture as though running down a mental checklist.

"Fertility-enhancing—" Shirou stopped. "Wait. Back up. What effects?"

"I ran some tests on Sakura after breakfast." Runeas raised a finger, slipping into the lecturing cadence she used when a subject genuinely delighted her. "Despite it not being her time of the month, her hormone profile showed marked elevation. Luteinising hormone, oestradiol—her body was preparing to ovulate entirely out of cycle. The contraceptive suppressed the actual release of the egg, of course. But had she not taken it, exposure to you would have triggered ovulation outright. Your aura, your seed, the energy you discharge—it doesn't merely arouse, Shirou. It primes. Her body recognised something in you and decided, quite independently of her calendar, that now was an excellent time to conceive."

Shirou's hands, still cupped over himself, tightened.

"That's—why would the curse do that?"

"Angra Mainyu is a curse of all the world's evils, and lust is merely its loudest voice in you. But underneath lust sits something older and simpler." Runeas's smile thinned into something scholarly. "Propagation. A curse wants to spread. Yours has apparently decided that the most efficient vector is the oldest one there is."

"That's horrifying."

"That's useful," Runeas corrected, "which brings me to the Veela."

She turned slightly, including Kalina and Apolline in the conversation with a tilt of her head, and her tone shifted into full lecture.

"You're aware, I assume, that full-blooded Veela cannot bear sons. Half-Veela likewise. It isn't superstition—it's biology, and rather elegant biology at that. A Veela egg's membrane carries unique magical glycoproteins woven through the zona pellucida. Magical receptors, in effect. When sperm approaches, those receptors read the chromosomal payload. Sperm carrying an X chromosome is permitted through. Sperm carrying a Y is actively repelled—chemically neutralised at the membrane. The egg itself enforces the selection. No son is ever conceived, because no Y-bearing sperm ever survives contact."

"A shield," Shirou said slowly, his smith's mind catching the shape of it. "Built into the egg itself."

"Precisely," Runeas said. "And because the shield is magical rather than purely chemical, it scales with the mother's power. A full-blooded Veela's egg is, for all practical purposes, impenetrable to Y-bearing sperm. Half-Veela—still extraordinarily robust. The shield never fails under normal circumstances."

Shirou let that settle, turning the concept over the way he turned metal in the forge—examining angles, stress points, potential fractures. "You said normal  circumstances."

"Clever boy." Runeas held up a second finger. "There is exactly one documented exception. When the Veela herself is severely ill—magically depleted, physically compromised—the egg's defensive energy falters. The glycoprotein shield weakens. Collapses, in extreme cases. And Y-bearing sperm, which tend towards higher motility, punch through the gap."

"So a sick mother produces a son."

"A sick mother produces a sickly son." Runeas's expression lost its scholarly brightness. "That's the cruelty of it. The very condition that permits the Y-sperm to reach the egg—maternal depletion—means the egg itself is compromised. Undernourished. Magically starved. The embryo develops without the double-X regulation that Veela magic requires. Genetic instability compounds. The foetus develops pyrokinetic episodes in utero —spontaneous combustion events within the womb. Most pregnancies end in miscarriage. Those that don't..."

She paused, and in that pause Shirou heard the weight of clinical data stripped of clinical detachment.

"The infants rarely survive their first year. Volatile magical surges—their bodies cannot regulate what they contain. The fire burns inward. The most recent documented male Veela of any maturity was born nearly a millennium ago. A king, by some accounts. He did not see forty."

The silence stretched. Waves filled it.

"A sickly egg makes a sickly son," Shirou repeated quietly. "The condition that allows the birth is the same condition that kills the child."

"Yes."

A flutter of blonde hair caught the edge of his vision—sharp footsteps crossing sand. Fleur had broken away from her argument with Gabby and was striding towards them, jaw set, cheeks still flushed from the debate.

"I 'eard what you are discussing." She stopped beside Apolline, chin raised. "You do not need to dance around it—I 'ave always known zis would fall to me. I am a quarter Veela. Ze dilution means my eggs carry a fraction of ze shield. Ze chances of a male child are still low, but zey exist, and without ze—" she gestured vaguely, searching for the English, "ze lethality. A quarter-Veela can bear a son and 'e will survive. It was always going to be my responsibility to continue ze Delacour name."

The certainty in her voice was brittle at its edges. Shirou could hear it—the hairline fracture running through a statement rehearsed too many times.

Apolline reached for her eldest daughter's hand, and the half-Veela's composure cracked in a different way.

" Ma chérie ..." Apolline's fingers tightened around Fleur's. "You know 'ow your father and I feel about zis. You know 'ow guilty I am—every day—zat I cannot give Jean a son of 'is own blood. Not because 'e asks. 'E 'as never asked. 'E never would." Her voice roughened. "But I see it in ze way 'e looks at Gabby and you sometimes. Ze pride, and underneath it, ze wondering. And I would not—I will not—place zat burden on your shoulders. On either of yours. Even for a quarter Veela, conception is already exceedingly difficult. Ze chance of a male child is still—" she shook her head. "You are young. You deserve to enjoy your life, and trying for baby number nine in ze 'ope zat you birth a son wouldn't be enjoyable to you or your potential beau ."

" Maman —"

" Non ." Apolline squeezed once and released her. The word was final.

Fleur pressed her lips together. Drew a breath through her nose. Then she turned to Shirou.

Her gaze started at his face and dropped—slowly, deliberately—down the length of him. Past his chest, past his abdomen, to where his hands still maintained their entirely inadequate attempt at modesty, and further, to where his erection jutted past his fingers with cheerful indifference to the gravity of the conversation. Her brow climbed.

"And what is so special about zis  one?"

Colour rose across her cheekbones even as her tone dripped scepticism. Her eyes flicked away from his groin a fraction too late for the dismissal to land properly.

Runeas smiled.

"Shirou's constitution is unique. He was reborn—converted through a process involving both Devil and Fae elements, the details of which are private. He exists as a convergence point: Devil, Fae, human. That constitution has granted him something not so different from your Veela allure—but with an additional effect."

She let that breathe for a moment before continuing. In broad strokes, she outlined for the Delacours what she had already told Shirou—the tests on Sakura, the hormone spikes, the out-of-cycle ovulation that the contraceptive had barely suppressed. Shirou watched their expressions shift as the implications landed. Kalina's warmth gained an edge of scholarly interest. Apolline's assessment sharpened into something closer to hunger—not for him, but for what he represented. Possibility. An answer to a question their bloodline had been asking for centuries.

"His aura doesn't merely arouse," Runeas concluded. "It primes. It rewrites the reproductive timeline of whomever he couples with—overrides the body's own calendar and forces fertility."

She paused, letting the implication settle.

"My hypothesis: if Shirou's priming effect is strong enough to override a normal woman's cycle, it may be strong enough to overwhelm a Veela egg's Y-rejection shield—without requiring the mother to be ill. A healthy egg, fully powered, but forced open by an external energy source that the shield was never designed to encounter. No sickly mother. No compromised embryo. A healthy son."

Fleur stared at Runeas. Then at Shirou. Then back at Runeas.

"Zat is a hypothesis."

"All great discoveries begin as hypotheses, dear."

Shirou's mind had already leapt three steps ahead—past the biology, past the implications, to the mechanics. "Hold on. Why does this need to be—" he gestured between himself and the Veela, "—direct? Can't I just... provide a sample? Into a cup? You could inseminate artificially and test the theory without anyone needing to—"

"I've already tested that." Runeas cut him off cleanly. "Back when your amulet still worked—remember? I was investigating whether your seed could serve as an alternative to the Evil Pieces for boosting Devil and phantasmal birth rates. Isolated in a sample, your sperm does elevate fertility, but no more than a standard fertility potion would. And standard fertility potions don't work on Devils or Veela."

"Then what makes it different when—"

"When it's delivered directly?" Runeas held up a finger. "Because it isn't solely the sperm that carries the priming effect. Your semen is a component, yes, but the mechanism is tripartite. First: the aura itself, which requires sustained proximity and builds over the course of intimate contact—skin to skin, breath to breath. Second: the seminal fluid, which carries a concentrated magical charge quite distinct from the sperm cells themselves. Third, and most critically: the energy your body releases at the moment of climax—delivered directly through tissue, into the reproductive tract."

She ticked each point off on her fingers.

"That pulse of energy—Devil, Fae, cursed—travels through the cervical wall and primes the egg at the point of contact. It's what I believe will overwhelm the shield from within. Once the egg is primed and the Y-rejection weakened, a healer inseminates with the father's seed. Jean's seed, in Apolline's case. The child would be theirs in every sense. You would simply be clearing the path."

She lowered her hand.

"Deposit your seed into a cup and you lose the aura buildup entirely. You lose the direct energy transfer. You're left with magically charged sperm in an inert container, degrading by the second." A thin smile. "I'm afraid there are no shortcuts here."

Shirou closed his eyes. Breathed. Opened them.

He turned to Apolline and Kalina. Both watched him—Kalina with that same gentle warmth, Apolline with something harder to read. Not hostile. Her gaze didn't waver or soften—it measured him, weighed him, filed him under a category she hadn't yet decided on.

"I should be honest," he said. "I'm not comfortable with this. Not—" he held up a hand before Runeas could interrupt, "—not with the science, or the goal. But you're married." He looked at Apolline directly. "And from everything I've heard in the last five minutes, happily so. I don't want to be the reason that changes."

Kalina spoke first.

"I 'ave been a widow for nineteen years, detseto ." Her voice carried no grief—only the settled quiet of something long mourned and laid to rest. "My 'usband was a wonderful man. A French wizard who loved a Veela and never once flinched from what zat meant. 'E died when Apolline was twenty. I 'ave lived in the coven since." She tilted her head, and the late sun caught the silver threading through her pale hair. "You do not need to worry about my 'onour. I gave it freely where I wished, and I took it back when I wished. Zat is the Veela way."

Shirou nodded—a short, respectful dip. Then he looked to Apolline.

The half-Veela met his gaze without flinching. Whatever calculation she had been running behind those blue eyes had reached its conclusion.

"My marriage to Jean is ze 'appiest zing in my life," she said. "Also ze most complicated. You must understand—a Veela's nature does not quiet with a wedding ring. It grows louder. Ze fire, ze need—it intensifies with every year. Jean knew zis when 'e married me. We discussed it before ze wedding, and again after Fleur was born, and again after Gabrielle." She crossed her arms beneath her bare breasts. "One man cannot satisfy a 'alf-Veela indefinitely. Ze coven exists for zis reason. I participate in coven activities regularly—it is not infidelity, it is management. And in return, I extend Jean ze same freedom. I 'ave asked 'im—more zan once—to take another wife. Someone who could give 'im a son, since I cannot."

Her jaw tightened.

"'E refused. Every time. 'E said 'e married me, and 'e would rather 'ave no son at all zan make me feel I 'ad failed 'im." A pause. "Ze stubborn fool. 'E does not realise zat 'is kindness makes ze guilt worse. But at least 'e 'as enjoyed several mistresses."

Gabby made another strangled noise at the mention of her father's arrangements, hands twitching towards her ears again before she caught herself.

The weight of it pressed against Shirou's chest—not the curse, not the aura, but something far more ordinary and far more painful. A family knotted around an absence. A husband who refused to seek an heir elsewhere out of love. A wife who couldn't provide one out of biology. Daughters who had shouldered the expectation without ever being asked to.

He recognised the shape of it. He had spent his life cataloguing the specific geometries of burdens people refused to set down—Kiritsugu's guilt, Sakura's patience, Rin's pride bracing against everything it concealed. This was the same architecture built from different materials. A load-bearing silence where a son should have been, and a whole family rearranging themselves around the gap.

Shirou exhaled. Long and slow, the way he did when he drew steel from the quench and knew the temper had either held or it hadn't.

"All right."

Runeas's grin threatened to split her face.

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End

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