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Tides of Dominion

Summary:

In the far future, the world is no longer divided by nations but by realms—sky, land, and sea. Each believes it stands apart, yet the ocean connects them all. The tides see everything. They remember everything.

Deep below the surface lies a civilization that breathes water and bleeds bioluminescence. Where warriors wear living armor, instincts are coded in their DNA, and love is both a bond and a biological command.

Once, they fought beside humankind against the Hajō plague. Now, they fight beneath them—guarding a world that has long forgotten who keeps it alive.

But even the sea has its limits.
When hearts clash and instincts awaken, what was meant to be survival becomes something far more dangerous.

After all, the tides do not forget—they record every touch, every war, every promise whispered between breaths.

Welcome to their world an underwater empire where there are no Kaiju on land but something far more powerful beneath the waves.

And maybe, one day, we’ll see what the other realms are hiding too.

Chapter 1: The Betrayal

Summary:

Even the strongest currents can break, and even the fiercest loyalty can falter when love and ambition collide. In the ruins of betrayal, the tides remember all, every promise, every loss, every heart that dared to defy the ocean itself.

Notes:

Hiii! This chapter is dedicated to that one fesshyy scholar of mine who triggered the prophecy for this AU of our Narumi Gen and Hoshina Soushirou! 💅✨ @deadswordmaster

And to those who’ll soon become my fishy scholars, get ready, you’re about to learn a lot about tides, tails, and the beautiful chaos under the sea~ 🌊🐠

(See the end of the chapter for more notes.)

Chapter Text

Long before the pink-tailed Merfolk cut through abyssal currents, before the bioluminescent spires of the Sea Realm glimmered with orders and dominance, peace was tenuous.

The Narumi Clan and Hoshina Clan had long stood as the bedrock of the Merfolk Defense Force. For centuries, they were warriors of unmatched skill, the vanguard against Hajō invasions, and the keepers of secrets older than coral mountains. Every flick of tail, every pulse of bioluminescence, every flickering scale conveyed rank, prowess, and authority.

Yet even the strongest currents can betray the unprepared.

The Kasai Clan, cunning and ambitious, had grown envious of the Narumi and Hoshina legacy. Their amber tails glimmered with ambition, a color that warned: power-hungry, restless, and dangerously innovative. The Kasai heirs had cast their eyes skyward, seeking forbidden knowledge, artifacts, and most dangerously… love.

Ryoma Kasai’s heart was stolen by Aeris, a winged noble of the Sky Realm. Their love was secretive but fierce, a current that could not be contained. Hidden rendezvous, whispered promises, and clandestine plans unfolded under the veil of ocean darkness.

“You know the rules, Ryoma!” Kael Hoshina snapped, violet fins quivering with contained fury. “You swore loyalty to the Sea Realm. Your heart cannot belong to them!”

Amber eyes glimmered with defiance. “My heart belongs to no realm, Kael. Not even yours. And certainly not the ocean that claims to rule my every action!”

Kael’s voice trembled with raw power and betrayal. “Care for? Or betray us all? The Hajō won’t wait for your romance to blossom. Your desire will drown us!”

The tension throbbed through the coral halls. Hajō, ever-sensing weakness, circled in the shadows, their bioluminescent eyes flickering like dying stars. The patrols of the M.D.F. felt it—a pulse of impending doom.

Yutaka Narumi, patriarch of the Narumi Clan, barked orders from the observation trench. “Hold the lines! Protect the children and host candidates! Kasai treachery must not fracture our defense!”

Ryoma’s bitter laughter echoed through the trench. “I warned you! I warned you about the Sky Realm, about what we could gain! And still, you cling to rules as if loyalty can replace evolution!”

Kael lunged, water rippling violently around them. “I trusted you!”

“I trusted you, Kael, to see that love is not a chain!” Ryoma’s amber fins struck like daggers, but his gaze lingered on Aeris, who hovered above the currents, wings slicing through water like blades.

Aeris cried out, her voice quivering: “You promised, Ryoma! We would be free, together!”

Ryoma’s tail flickered with guilt, amber scales darkening under pressure. “I… I never wanted this… I never…”

“Then why?” Kael’s violet eyes glowed. “Why betray your home? Why endanger the Sea Realm?”

The ocean held its breath. Hajō, sensing hesitation, surged forward. Their monstrous jaws and spiked tails shredded through barricades, cutting down junior warriors who had trusted the Kasai line.

Currents clashed with armor. Bioluminescent spires illuminated the chaos: pink and violet tails of Narumi and Hoshina veterans flashed, amber of Kasai rebels glinted like embers. Harpoons, kinetic energy bursts from Bio-Skin suits, and sonar pulses crisscrossed the battlefield.

Kiyomi Narumi, an elder and tactician, shouted over the chaos. “Flank the northern trenches! Seal the breach! Do not let them reach the host chambers!”

Ryoma’s eyes, torn between rebellion and love, flicked to Aeris. “I must go, or all of this… all of it is meaningless.”

“No!” Kael’s spear flashed. “If you leave, the Sea Realm falls!”

A moment of hesitation. One wrong flick of a tail, and a Hajō lunged. Ryoma deflected with his amber-scaled forearm, but the cost was high: a fellow Kasai fell, a dear friend lost to the ocean’s relentless hunger.

The clash escalated. Currents churned with the screams of warriors, the shrieks of Hajō, and the swish of tails—a symphony of carnage and grief.

Elder Kasai voices pleaded, hesitant: “Ryoma… reconsider. The Sea Realm is not yours to gamble!”

Ryoma’s amber eyes hardened. “Love is worth the cost.”

Aeris dove into the current, wings spreading to shield him. “We will face this together.”

And in that instant, the battlefield fractured further. Allies turned on each other, fury and grief melding into chaos. Hajō tore through weakened ranks. Bio-Skin suits flared, tails pulsed, and screams of betrayal echoed for miles.

By the time the tide calmed, the battlefield was a graveyard of coral, broken Bio-Skins, and lost blood. The Sea Realm had survived, but trust had cracked like shattered glass.

Kael Hoshina hovered over the fallen, violet tail dimmed with exhaustion and rage. “We will rebuild… but never forget. Never forgive. The Kasai have shown what love blinded by ambition can destroy.”

Yutaka Narumi, patriarch, whispered: “The Sea Realm endures, but new blood must rise. One that embodies the lessons of betrayal, survival, and duty.”

Among the bioluminescent ruins, two merfolk stirred.

Gen Narumi, descendant of the Narumi line, tail shimmering in pink—strong, dominant, born of the bloodline forged in conflict.

A rare host male from the Hoshina Clan, destined to carry and preserve the line, ensuring the legacy of warriors and survival.

Above, in exile, Ryoma and Aeris vanished to the Sky Realm, their love victorious but hollow, leaving behind devastation and a prophecy: the next generation would rise from the ruins of betrayal.

The tides remembered. Every whisper, promise, and betrayal recorded in eternal memory.

Notes:

Sooo, how was it?! 😭💅
Did the tides drag you in?

Thank you for swimming this far, my fishy scholars. Your curiosity keeps this AU alive (and me dangerously close to researching marine biology again 😭).

🐚 INFO DUMBS FOR MY FISHY SCHOLARS 🫡✨

(Because y’all are nosy and I respect that kind of energy)

🌊 1. The Realms

There are three realms — Sky, Land, and Sea.
Each thinks they’re the main character. Spoiler: the Sea actually is. 👁️

🪸 2. The Sea Realm (Mizukane)

A whole civilization under the waves — bioluminescent coral cities, sonic gates, and tech so advanced it’s alive.
Their armor? Bio-Skin — a living second skin that breathes, adapts, and sometimes rolls its eyes at your choices.
It reacts to emotion, readiness, and frustration. 😤

💗 3. Tail Colors = Power Level

Tails are like mood rings but deadlier.
The rarer the hue, the stronger the merfolk.
Pink? That’s top-tier apex energy.
Narumi’s tail is top-tier, glowing just enough to intimidate. 💅

⚔️ 4. The Hajō

Think underwater Kaiju. Big. Ancient. Angry.
They nearly ended all three realms until the Defense Force rose.
Now, their remains fuel Bio-Tech — sustainable chaos ♻️🐉

💔 5. The Kasai Incident

Once upon a high tide—
Ryoma Kasai fell in love with someone from the Sky Realm.
He tried to bridge both worlds.
He lost his tail... and gained wings.
The Sea called it betrayal.
The Sky called it romance.
Historians called it: “A hot mess with feathers.” 😭

🏛️ 6. The Noble Clans

🐉 Narumi Clan — Warriors born from chaos. Rarely are males born as hosts — too much apex in one genome.

🌊 Hoshina Clan — Calm but sharp as coral blades. One of the rare clans capable of producing a male host. 🦈🧘‍♂️

🔥 Kasai Clan — Once noble, now cursed. Coral palaces swallowed by the sea after Ryoma’s betrayal. 🌩️

🎮 7. Modern Times

Even underwater, tech rules.
Narumi Gen discovered gaming consoles and hasn’t slept since.
Hasegawa files 47 “Captain, please rest” reports daily. 😭

🪶 Bonus Fact

Ryoma Kasai didn’t “grow wings.” His Bio-Skin mutated.
He traded scales for feathers.
Scientists cried. The Sea raged.
And somewhere, a fish whispered:
“Bro really chose ✨airplane mode✨.” 🐟✈️