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My Sister Summoned The Demon King

Summary:

Legends Say The Demon King would be slain by the Hero once their sword had been awakened. This tale is one known high and low but one certain blue-haired gremlin does not believe in it's credibility and her carelessness leads to unforeseen consequences.

Chapter 1: You Stand Before The Overlord

Chapter Text

The first words came not as sound, but as something older—etched into the fabric of the world itself, whispered through ages long forgotten:

When shadow devours the heavens and the Demon King rises once more,

a single soul shall answer the call of the blade.

The sword, dormant in sacred stone, shall awaken in the grasp of the chosen—

Phaethon, bearer of balance,

whose light shall cut through ruin

and restore the harmony of all that is.

The prophecy flowed like a tide, its cadence both solemn and unyielding, painting a vision of cosmic ruin and fragile hope. Stars blinking out. Kingdoms trembling. A lone figure standing before an ancient weapon that pulsed with a quiet, sleeping power.

Then the voice became flesh.

“…and thus, when the hour of despair comes again, it is not armies nor kings who shall save us—but the one who can awaken the blade!”

The murmur of the prophecy dissolved into the lively noise of a crowded town square. Sunlight warmed the cobblestones, banners swayed gently in the breeze, and a large gathering of townsfolk craned their necks toward the raised platform at the center.

Upon it stood a robed prophet, voice ringing with dramatic weight, hands raised as if grasping at unseen truths beyond the sky.

“The sword rests still within the Church—silent, watchful, waiting!” the prophet declared. “It awaits its master… its hero… its Phaethon!”

The crowd responded with a ripple of excitement—some in awe, others amused, and many simply entertained by the spectacle.

Amid the sea of faces stood two siblings.

Belle leaned slightly closer to her brother, her voice low enough to dodge the rising clamor around them. “Do you actually think any of that is real?”

Wise didn’t answer immediately. His gaze remained fixed on the platform, though his expression was more thoughtful than captivated.

“…The sword in the Church doesn’t look like it’s just for show,” he said after a moment. “And I’ve heard some of the mages talking—something about strange ether fluctuations near it. Happens every now and then.”

Belle raised an eyebrow, unimpressed. “Rumors and spooky talk? Come on, Wise. It’s just another town folktale. Something to keep people entertained.”

Wise exhaled softly, scratching the side of his head. “Maybe. Maybe not. I just… can’t say for sure.”

Before Belle could reply, the prophet’s voice rose one final time before fading into a concluding flourish. The crowd clapped and murmured, shifting with anticipation as another figure stepped forward.

The bishop.

Draped in rich ceremonial robes, he carried himself with gravity that silenced much of the lingering chatter.

“People of the Kingdom,” he began, his voice calm yet commanding, “the Day of Pompey draws ever closer.”

A hush fell.

“With it comes uncertainty… and danger. The Church can no longer wait in silence. The prophecy must be answered.”

The bishop raised a hand toward the towering cathedral behind him.

“Starting tomorrow, we will begin testing the people of this land. We will search for the one who can awaken the sword—the one named in legend as Phaethon.”

A wave of energy surged through the crowd.

Voices rose in excitement. Young men straightened, exchanging determined looks. Some boasted in half-jokes about their chances. Nearby, a group of young women whispered eagerly, their laughter threaded with curiosity and gossip.

“It’s finally happening…”

“Do you think anyone can actually do it?”

“I heard Sir Alric is going to try—”

Belle, however, turned back to Wise.

He wasn’t looking at the bishop anymore.

He seemed elsewhere entirely, lost in a quiet storm of thought.

She nudged him. “Hey. You’re not actually thinking about trying, are you?”

Wise blinked, snapping out of it. “What? Why would I—weren’t you just saying the whole thing’s nonsense?”

Belle grinned, folding her arms. “Oh, I still think it is. But that doesn’t mean I don’t want to watch. Imagine all those macho idiots trying to prove they’re some chosen hero—it’s going to be hilarious.”

Wise rolled his eyes. “You’re a different kind of person, you know that?”

“If I wasn’t your sister,” Belle shot back with a playful smirk, “your life would be unbearably dull.”

Wise snorted. “Please. Eous would take your place just fine.”

Belle gasped in mock offense before lightly punching his arm. “That’s mean!”

“You’re the one who said it first,” Wise replied, a faint smile slipping through.

Their banter blended into the excited hum of the square—but somewhere beneath it all, beneath the laughter and doubt and bravado, the quiet weight of the prophecy lingered.

Waiting.

 

--------- THE NEXT DAY ----------

 

The following day arrived with far less mystery and far more noise.

The town square had doubled in energy overnight. Where yesterday there had been curiosity, today there was excitement—loud, restless, contagious. A wooden platform had been erected before the Church’s grand entrance, and at its center stood the sword, still embedded in its stone pedestal. It gleamed faintly under the sun, as if aware of the attention.

The crowd surged and shifted, bodies pressing shoulder to shoulder as people craned their necks for a better look.

Somewhere within that mass stood Wise and Belle—alongside a third figure.

Shepherd.

He looked exactly as one might expect from someone named Shepherd yet dressed like he had wandered out of three completely different professions at once. Layers of mismatched fabrics, a crooked hat, and an ever-present grin gave him an air of cheerful unpredictability.

“…And this one?” Belle said, squinting past the moving heads in front of them. “He looks like he eats iron for breakfast.”

“Would explain the confidence,” Wise muttered.

On the platform, a broad-shouldered man stepped forward, cracking his knuckles as if he were about to wrestle the sword into submission.

Belle nudged both of them. “Alright, let’s make this interesting. I say he fails.”

Wise didn’t even look at her. “I’m not a gambling man.”

Belle groaned. “Oh, come on. Then bet that nobody succeeds. You literally said this is probably all nonsense.”

“I said I don’t know,” Wise replied calmly. “That’s not the same thing.”

Before Belle could argue further, Shepherd leaned in between them, lowering his voice conspiratorially.

“Well, I’m with Belle on one thing,” he said, eyes gleaming. “Events like this? Perfect for making money. Whether it’s real or not doesn’t matter—only what people believe about it does.”

Belle pointed at him in triumph. “Exactly! That’s what I’ve been trying to tell him since last night!”

Wise raised an eyebrow. “You weren’t talking about economics. You were calling it a bedtime story.”

“Same difference,” Belle shot back lazily.

“It’s not even remotely the same—”

“Shh!”

Shepherd suddenly held up a hand, cutting them off.

All three turned their attention forward.

The burly man had reached the sword.

The square seemed to inhale all at once.

He planted his feet firmly, wrapped both hands around the hilt, and pulled.

Nothing.

A flicker of tension passed through the crowd.

He grunted, muscles tightening as he pulled harder. The veins in his arms strained, his jaw clenched—

Still nothing.

After a few seconds, he let out a frustrated growl, readjusted his grip, and this time yanked with all his strength.

The sword didn’t budge.

A ripple of murmurs broke out.

Determined to recover, the man suddenly lifted the sword just enough to loosen it slightly—more a wobble than a draw—and swung it dramatically through the air.

Or at least, he tried to.

The blade barely moved, dragging awkwardly before slipping from his grip entirely.

He stumbled forward, nearly falling off the platform.

For a brief moment, there was silence.

Then—

Laughter.

It spread quickly, rolling across the crowd like a wave. Some chuckled politely. Others laughed openly. A few clapped mockingly.

The man’s face burned red as he straightened himself, muttering something under his breath before storming off the platform.

Belle snorted, barely containing herself. “Oh wow. That was worse than I expected.”

Wise folded his arms, watching quietly. “At least he tried.”

Shepherd tilted his head, grin widening. “And just like that, the odds shift. Confidence goes down… doubt goes up…”

Belle shot Wise a look, still amused. “Told you. This is going to be fun.”

Wise didn’t respond immediately.

His eyes lingered on the sword.

Just for a second longer than necessary.

“…Yeah,” he said at last. “Fun.”

The bishop clasped his hands together, offering a polite nod toward the departing man.

“Your effort is commendable,” he said warmly, ensuring the failed challenger left with some dignity. Then, without losing rhythm, he raised his voice. “Next!”

Before Wise or Belle could react—

“That would be me.”

Both siblings turned.

“Shepherd?” Belle blinked in disbelief. “You? Since when are you consistent with anything you say?”

Shepherd adjusted his crooked hat with a proud smirk. “Oh, I’m very consistent… when there’s profit involved.”

“…You’re not serious.”

“I am,” he replied cheerfully. “Made a little wager earlier. Group of fellows betting on whether some oddly dressed individual would step up. Naturally, I volunteered to fulfill that role.”

Wise sighed quietly, already understanding. He’s scamming them again…

But he said nothing.

Shepherd was already weaving his way through the crowd, slipping onto the platform with a theatrical flourish. A few amused chuckles followed him up.

“Observe,” Shepherd announced dramatically, placing a hand over his chest. “A man of destiny.”

He grabbed the sword.

Nothing happened.

He pulled again—less impressive.

Then he tried lifting it with both hands.

It barely shifted… if at all.

The crowd erupted almost immediately into laughter, louder than before. Some pointed, others clapped mockingly.

Shepherd froze for a second, then gave a deep, exaggerated bow. “Ah… it appears destiny is running late today.”

He hopped off the platform, rejoining Wise and Belle with a grin far too satisfied for someone who had just failed so spectacularly.

Belle crossed her arms. “You’re way too happy about that.”

“Of course I am,” Shepherd said smugly. “I just made money.”

Wise shook his head. “You’re unbelievable.”

Before either of them could continue, the bishop’s voice rang out again.

“Next!”

Belle’s eyes lit up.

She leaned forward just enough and cupped her hands around her mouth.

“WISE!”

Time slowed.

Wise’s head snapped toward her in horror. “Don’t you—”

“THIS ONE WILL TRY!”

The bishop turned, spotting him almost immediately. “Ah! Step forward, young man!”

The crowd’s attention shifted as one.

Wise froze.

“…You’re joking,” he whispered.

He leaned toward Belle, voice tight. “You are officially the worst sister of all time.”

Behind them, a small cluster of village girls stirred.

“Isn’t that Wise?”

“It is…”

Astra giggled quietly. “Oh, this I have to see.”

Evelyn, arms neatly folded, simply nodded. “He’ll either surprise us… or embarrass himself.”

Wise rubbed his face. “I’m not doing this.”

The bishop called again, more expectant. “Young man, please come forward.”

Dozens—no, hundreds—of eyes were now on him.

Wise shifted uncomfortably. “Yeah, no. Definitely not happening.”

Belle rolled her eyes. “Oh, for the love of—”

Before he could step back—

She grabbed his arm.

“BELLE—!”

And just like that, she dragged him forward through the crowd.

“HEY—let go—!”

Too late.

They reached the platform, climbing up as murmurs spread like wildfire.

Wise leaned in, half whispering, half hissing. “When we get home, I’m actually going to kill you.”

“Oh relax,” Belle said cheerfully. “It’s not a big deal.”

The bishop gestured respectfully. “Come, young man.”

Wise stayed rooted. “I can’t do this.”

A few voices from the crowd began to chant.

“DO IT!”

“TRY IT!”

“COME ON!”

“I really don’t want to,” Wise insisted.

Belle groaned loudly. “It’s simple! You just grab it!”

Wise shot her a glare. “You say that like it’s lifting a broom.”

“Because it is!” she snapped back. “Here—like this—”

And before thinking—

Belle reached out.

Her fingers wrapped around the handle of the sword.

The world changed.

A sudden pulse—subtle at first—spread outward.

Then the air tightened.

A soft hum began to resonate, low and unfamiliar, like something awakening from deep beneath reality itself. Light flickered along the blade, faint streams of ether curling into existence around it.

The crowd fell silent.

Every voice. Every whisper.

Gone.

The wind itself seemed to hesitate.

Wise stared.

Belle froze.

The sword… was responding.

The hum became a roar.

Ether surged violently around the blade, no longer a faint glow but a spiraling storm of light and energy that twisted the air itself. The ground trembled—no, shook—as a sudden earthquake rippled beneath the town.

Cracks snaked through the stone plaza.

Screams broke out across the crowd.

“W-what is happening?!” Belle gasped, her voice trembling as she tried to yank her hand away.

It wouldn’t move.

The sword held her.

“No—no, no, no—!” she panicked, pulling harder. “Wise—I can’t let go!”

A violent shockwave burst outward.

Wise was thrown back, landing hard on the ground. “—Ghk!” He winced, scrambling to push himself up, one hand bracing against the shaking stone. “Belle, stop it!”

“I can’t!” she shouted, fear flooding her voice.

The air grew heavier. Brighter.

And then—

It took control.

Belle’s arm stiffened unnaturally. Her body straightened, as if caught in invisible strings. Her eyes widened in terror as she felt herself moving—not by her will.

The sword rose.

Slowly at first.

Then sharply, her arm lifted high into the sky, the blade pointing upward as the ether gathered—condensed into something blinding, roaring, impossible to contain.

“WISE—!” she cried—

A beam erupted.

A colossal pillar of light shot skyward, tearing through the heavens with a thunderous force that echoed far beyond the limits of sight. Clouds split. The very sky seemed to ripple as the energy pierced upward, vanishing into infinity.

Then—

Silence.

The quake stopped.

The light faded.

The wind stilled.

What remained was a town frozen in awe.

No cheers. No laughter.

Only stunned, breathless quiet.

Belle staggered, her body trembling violently as control returned to her. “I—… I… what… what was that…?” Her breath came in sharp, uneven bursts, fear gripping her completely now.

Without thinking, she threw the sword away.

It clattered across the platform and slid to a stop near Wise.

He stared at it.

Then at her.

Just as shaken.

Nearby, the bishop slowly pushed himself back to his feet, hands trembling, his composure shattered. His eyes were fixed on Belle as if seeing something no one else could fully understand.

Other clergy rushed toward the platform, equally stunned, whispering urgently among themselves.

“…It responded…”

“The ether—did you see—”

“It’s her—”

The bishop stepped forward, voice unsteady but rising with disbelief and dawning certainty.

“Th-this… this can only mean…”

The air grew tight again—but this time with anticipation.

He raised his voice.

“The prophecy…”

All eyes locked onto Belle.

“…has been fulfilled.”

A beat.

Then, with reverence that carried across the stunned crowd:

“She is Phaethon.”

The words fell like thunder.

Gasps rippled outward. Shock. Confusion. Awe. Fear. Every emotion collided as the reality sank in.

Belle stood frozen.

“…No,” she whispered, shaking her head faintly. “No, that’s… that’s not—”

The sword.

The light.

The power she couldn’t control.

Her hands trembled as she looked at them, as if expecting them to still be glowing.

“This…” Her voice cracked. “…this was supposed to be a joke.”

Wise slowly rose to his feet, still trying to process it himself. His gaze moved from the sword… to his sister.

To Phaethon.

Neither of them laughed this time.

Because the story—

Was real.

Belle’s head snapped up, eyes wide with panic.

“I’m not Phaethon!” she shouted, her voice trembling as she pointed back toward the sword. “That—whatever that was—wasn’t me! The story’s fake! This is all fake!”

The bishops recoiled slightly at her outburst, but the lead bishop quickly stepped forward, face still pale but resolute.

“You cannot deny what we all witnessed,” he said, voice shaking but insistent. “The sword answered only to you. The ether itself bent to your will—”

“My will?!” Belle snapped, almost hysterical. “I couldn’t even let go of it! That wasn’t me controlling anything! This is a setup—mages, tricks—something!”

The crowd murmured uneasily, shifting between doubt and fear.

“She’s scared…”

“But that light—”

“There’s no way that was fake…”

“I’m telling you, I’m not your hero!” Belle yelled again, more desperate this time, her hands trembling at her sides. “I don’t want anything to do with this—!”

The bishop opened his mouth to push back again—

And then—

The sky tore open.

A deep, violent red spread across the heavens like spilled blood, swallowing the blue in an instant. A massive, swirling portal ripped through the air above the town, its edges crackling with dark energy.

The light of day vanished.

Everything turned crimson.

A bone-deep dread fell over the entire township.

Screams erupted.

“RUN!”

“What is that?!”

“It’s happening—!”

People scattered in every direction, panic overtaking reason as stalls overturned and bodies pushed against each other in desperate escape.

Wise snapped out of his shock, scrambling to his feet. “Belle!”

He grabbed her arm, trying to pull her away. “We need to go, now!”

But she didn’t move.

Her eyes were locked on the sky, horror frozen across her face.

“…I… did this…” she whispered faintly. “This is because of me…”

Wise tightened his grip. “It’s not! Belle, move—!”

Before she could respond—

Something descended.

From the depths of the crimson portal, a small figure drifted downward, slow and effortless, as if gravity itself had no claim on her.

A young woman.

Blonde.

A massive axe rested casually over her shoulder, as though it weighed nothing. Her posture was loose—lazy, even—as she descended with a bored expression.

She landed softly on the platform.

Right in front of them.

The wood creaked under her boots.

She smacked her lips faintly…

“…Tch.”

Then yawned.

A long, unbothered stretch as chaos unfolded all around her.

Her eyes lazily scanned the fleeing crowd, unimpressed. People running, screaming, disappearing into alleys—it didn’t interest her.

Her gaze drifted.

To the sword.

Resting on the platform.

Then to the bishops—

Who immediately cowered, dropping to their knees, faces covered, too afraid to even look at her.

Finally—

Her eyes settled on Wise and Belle.

Still there.

Still frozen.

Watching.

Another small yawn escaped her.

And then—

She moved.

There was no warning.

No buildup.

One moment she stood there—

The next—

Wise was gone.

A single blur of motion, faster than thought itself, and suddenly he was in her grasp, lifted effortlessly as she launched skyward again.

“WISE?!”

Belle’s scream tore through the air as she lunged forward—but she was far too late.

“LET HIM GO! WISE—!”

The woman didn’t even look back.

She rose straight into the portal, carrying him as if he weighed nothing at all.

Then—

The red sky collapsed inward.

The portal snapped shut.

The light returned.

The wind resumed.

The town square fell into stunned silence once more.

Belle stood there, arm outstretched into empty air, her voice breaking into nothing.

“…Wise…”

Gone.

“……”

“It was supposed to be a joke...”

 

Wise’s scream was ripped from his throat as the world shattered around him.

“LET GO OF ME—!”

The red portal swallowed everything—light, sound, air—turning his surroundings into a violent tunnel of twisting crimson and black. The force dragged him forward relentlessly, his body weightless yet painfully strained as if reality itself was tearing at him.

Wind howled past his ears.

Or maybe it wasn’t wind.

Maybe it was something worse.

“STOP—THIS—!” he choked, reaching out desperately toward nothing, as if he could grab onto the world he had just lost.

There was no ground.

No sky.

Only the endless pull.

Seconds stretched into something unbearable.

Then—

Impact.

“—GHHK!”

Wise slammed hard onto solid ground, pain shooting through his shoulder as he rolled across rough, uneven stone. He groaned, clutching his arm as he struggled to breathe again.

“…agh… damn it…”

The air here was thick. Heavy.

Wrong.

He forced himself up slightly, wincing, and looked down.

Black bricks.

An endless pathway of dark stone stretched beneath him, polished yet ancient, disappearing into a horizon swallowed by shadow. Everything around him felt… still. Oppressively still.

A soft thud echoed behind him.

Wise looked up.

The blonde woman stood there, axe resting casually over her shoulder, as if she had just dropped by for a stroll. Her expression was the same as before—half-lidded eyes, bored, faintly annoyed by existence itself.

She stared down at him.

Silent.

Watching.

Wise’s chest tightened.

He pushed himself backward instinctively, scrambling across the cold bricks, panic finally catching up to him. “S-stay back—!”

His body didn’t listen well. His shoulder screamed in protest, his movements clumsy and desperate.

He tried anyway.

He had to.

But before he could get far—

“Now…”

Her voice cut through the stillness.

Low. Lazy.

And unmistakably carrying a slow drawl.

“…you ain’t goin’ nowhere.”

Wise froze.

The weight in her voice wasn’t loud.

It didn’t need to be.

It settled into the air like a fact.

Before Wise could even think to run again—

She was already there.

Her hand clamped around the back of his collar, lifting him clean off the ground like he weighed nothing.

“HEY—!” he choked, legs kicking uselessly. “LET—GO—!”

“Mm…” she muttered lazily, barely even looking at him as she adjusted the axe on her shoulder. “Quit yer squirmin’.”

With a small flick of her wrist, she steadied him like an object rather than a person.

“The Overlord’s been lookin’ for ya.”

Wise froze.

“…The what?” His voice cracked despite himself.

That word—Overlord—echoed in his chest like a death sentence.

His entire body began to tremble.

“Y-you’ve got the wrong person,” he said quickly, panic spilling out. “I’m not anybody important—I don’t even know what’s going—”

“Sure,” she replied flatly. “That’s what they all say.”

And just like that, she started walking.

No—dragging him.

Wise’s shoes scraped against the black bricks as she hauled him along deeper into that dark, endless landscape. The air grew colder the farther they went, the silence heavier, suffocating him with every step.

His mind raced.

Overlord? Why me? Did this have to do with Belle? The sword? That beam?

“…Belle…” he muttered under his breath, dread tightening his chest.

After what felt like minutes—too long for comfort, too short for sanity—the woman finally stopped.

Before them stood a massive door.

Towering. Black. Carved with faint, jagged patterns that seemed to shift if you looked too long at them.

She raised a hand and knocked.

Knock. Knock.

Casual. Like she was announcing a delivery.

“Got ‘im,” she called lazily.

There was a deep rumble.

The doors creaked open, slowly revealing darkness within.

Without hesitation—

She tossed Wise inside.

“—HEY—!”

He hit the ground awkwardly, flipping forward and landing partly on his head.

“Ghk—!” Pain shot through his skull as he groaned, clutching it. “Agh… my… head…”

Before he could gather himself—

A sharp voice cut through the darkness.

“Hey!”

It was another woman.

Younger. Much more energetic but way more aggressive

“Get up already!” she snapped. “What are you, some kind of lazy bones?!”

Wise blinked, still dazed, barely managing to push himself onto his elbows.

“…I just got thrown through a portal…” he groaned weakly.

The response?

Footsteps.

Closing in.

Another voice cut in—wild, sharp, far too eager.

“Oooh, should I burn ‘im?” it said, almost gleeful. “Bet he’d move real quick then~”

“NOPE!”

Wise shot upright instantly, scrambling to his feet despite the lingering dizziness, heart hammering in pure survival instinct.

“I’m up! I’m up—okay?!” he blurted, raising his hands slightly as if that might somehow help.

“Good,” the first voice snapped. “Took you long enough.”

His vision wavered for a moment… then slowly focused.

And what he saw made his blood run cold.

It was a throne room.

Vast. Oppressive. Built from the same black stone as the path outside, its towering walls stretching high into shadow. Dark banners hung like silent watchers, and at the center of it all—

A throne.

Wise swallowed hard.

“Face forward,” the first woman ordered. “Show some respect. Ain’t you the ‘hero’ from the legend?”

The word hit him like a weight.

Hero.

His heart sank.

Slowly—hesitantly—he lifted his gaze.

And saw them.

Four figures.

To the left stood a short blonde woman with sharp red eyes, her arms crossed tightly. Her expression was a permanent scowl—pure hostility carved into her face as she looked down at him like he was something unpleasant stuck to her boot.

Next to her—

Another blonde.

Also red-eyed.

But where the first radiated anger, this one radiated chaos.

Her smile was wide. Far too wide.

Unhinged.

Excited.

Her eyes gleamed like she was watching her favorite show unfold right in front of her.

“Hehehe… this is him?” she said softly, almost vibrating with anticipation.

Wise’s stomach dropped.

To the other side stood something… different.

A woman, yes—but not entirely.

Her form was lithe, almost feline. Sand-colored features, a long tail curling lazily behind her, and a mask obscuring her face. What little expression she showed—or didn’t show—was pure boredom.

She looked at Wise like he wasn’t even worth the effort of curiosity.

And then—

At the center.

Seated high upon the throne.

The final presence.

A towering woman with green hair and deep yellow eyes. She wore heavy armor, every plate scarred as if it had seen countless battles. A crown rested upon her head—not ornamental, but earned. Her posture was relaxed, yet her very existence pressed down on the room like gravity itself.

Power.

Unquestionable.

Absolute.

Wise felt himself shrink under her gaze.

His legs nearly gave out.

…I shouldn’t be here.

The woman on the throne leaned forward slightly, resting one armored elbow against her thigh.

Her eyes locked onto him.

And when she spoke—

The room seemed to listen.

“You stand,” she said, her voice low and commanding, carrying the weight of authority that did not need to be proven, “before the Overlord.”

A pause.

Just long enough for it to settle into his bones.

“Caesar King.”