Crossover Shipgirls: Building a Dimensional Empire from Scratch

On the shore.

The army of demons swept across the landscape in a feral frenzy. Muzan, thoroughly convinced by the web of lies spun by Setsuna, believed wholeheartedly that someone had finally found the elusive Blue Spider Lily he had pursued for centuries. Based on his covert investigations, today was supposed to be the day to ambush and intercept it here.

Yet the area was eerily quiet. Not a single person was in sight, and even the few local fishermen had been evacuated. No members of the Demon Slayer Corps had appeared either, despite the large-scale demon movement that should have drawn their attention.

"Could it be…?"

"Was this… a trap? Or did something go wrong?"

Muzan maintained his composure and stayed concealed in the rear, letting his subordinates scout ahead while he mulled over the situation.

Suddenly—

Buzz—

A deafening roar echoed from above.

"Air-Strike—"

"Airstrike ready."

Accompanied by Perseus' impeccably posh London accent, over 300 dive bombers and torpedo bombers emerged from the horizon, blanketing the sky.

SBD Dauntless, SB2C Helldivers, Stukas—classic aircraft from World War II, enhanced by the Wisdom Cube, soared with extraordinary maneuverability at low altitudes. With the shipgirls' unrestricted access to mass production, aircraft models were sourced freely across factions.

The piercing shriek of the diving planes sent waves of panic rippling through the demon ranks. These creatures had never encountered such flying monstrosities before, and the sheer unfamiliarity triggered an instinctual terror.

Boom! Boom!

Before the Upper Moons could react, high-yield aerial bombs rained down directly onto them.

Kokushibo, whose body could endure the Sun Breathing techniques and shotgun blasts, was obliterated in an instant. Though demons possessed formidable regenerative powers, capable of recovering from near-fatal injuries, the sheer intensity of the carpet bombing overwhelmed their limits.

Boom! Boom! Boom!

One after another, bombs exploded at close range. The shockwaves alone vaporized lesser demons, while sturdier ones were reduced to shreds. Any surviving fragments were further annihilated in subsequent blasts, their remains scattered into molecular oblivion amidst the inferno.

The shipgirls spared no expense when dealing with enemies, opting for oversaturation attacks to obliterate any resistance. With their port's industrial prowess, ammunition was essentially limitless.

Boom! Boom!

The first wave of dive bombers transformed several kilometers of the landscape into a raging inferno.

Following closely behind, torpedo bombers locked onto any stragglers. The torpedoes, designed to pierce battleship armor hundreds of millimeters thick, effortlessly turned even the most durable demons into dust.

Swoosh—

After several rounds of airstrikes, the blazing light of explosions illuminated the night sky. The fiery reflections on the water painted a surreal and hauntingly beautiful scene.

What had been a cacophonous battlefield now fell silent, save for the crackling of flames and the distant drone of planes.

Nearly the entire demon horde had been reduced to ashes by the relentless bombardment.

"Where's Muzan? Don't let him escape," Setsuna reminded.

"Commander, rest assured."

"If he had remained hidden, tracking him down might have taken some effort…"

"But since he dared to show himself, once locked on, there's no chance—absolutely no chance—of escape," Taihou declared as reconnaissance planes fed Muzan's coordinates to the radar display.

On the radar, a large red dot raced away from the battleground. Several high-altitude reconnaissance planes had already locked onto Muzan's trajectory as soon as he was identified.

"Oh, he's fast—worthy of being called the Demon King."

Setsuna calculated the red dot's speed using the radar scale. Muzan was moving at nearly subsonic speeds, far outpacing a bullet train.

"In terms of raw speed, it's impressive…"

"But, well, still slower than our shells and missiles," Richelieu remarked as she summoned her rigging.

With the radar displaying Muzan's trajectory, her massive main battery slowly adjusted its firing angle.

Boom! Boom! Boom!

The salvo of shells lit up the forest in the distance, raising several mushroom clouds.

Terror. Regret. Confusion.

It was impossible to describe the complexity of Muzan's emotions after surviving the carpet bombing.

Not even when he had been sliced into a thousand pieces by Yoriichi centuries ago had he felt this level of terror.

The Sun Breathing techniques were merely a counter to his kind, wielded by an extraordinary swordsman. Those abilities, though formidable, were still within the realm of comprehension.

But the devastation wrought by these planes was beyond belief. Muzan couldn't grasp what the dive bombers and torpedo planes even were.

Their bombs possessed explosive power dozens of times greater than anything humanity had ever devised.

The Upper Moons who had accompanied him were reduced to ash in mere moments.

Had Muzan not been swift and reactive, his fate would have mirrored theirs.

"Humans… Could this be a new weapon from the Demon Slayer Corps?"

"No, they couldn't possibly have such power…"

Gritting his teeth, Muzan fled, abandoning his remaining forces and vanishing into the distance. He resolved not to show himself again until he could investigate the attackers and ensure his safety.

The psychological shadow left by the dive bombers far surpassed even Yoriichi's Sun Breathing techniques.

Whoosh—

In the blink of an eye, Muzan had traveled dozens of kilometers, fleeing far from the burning battlefield.

Using his Blood Demon Art to conceal himself, Muzan was confident that no one could find him. If he focused on escaping, no living being in this world could match his speed.

"Am I… safe?"

He scanned his surroundings. Silence. No planes in sight. No signs of the Demon Slayer Corps or any pursuers.

Suddenly—

Whoosh!

A sharp whistling sound pierced the air behind him.

"What?! They caught up already?!"

Instinctively, Muzan turned to look.

What greeted his eyes was a small, sleek object hurtling toward him at high speed.

Due to the relatively low speed difference and Muzan's exceptional reflexes, he had time to study the object in detail.

On its nose was a symbol he didn't recognize—a yellow background, a black circle, three radiating lines, and a large black exclamation mark below.

"?"

"What is this thing?"

Before he could finish the thought—

BOOM!!!

A brilliant light engulfed the area as a miniature sun erupted before his eyes.

"Tsk, tsk, tsk. As expected of the progenitor of all demons," Setsuna muttered.

"Took a small-yield hydrogen bomb to the face, and you're still alive."

On the deck of Prinz Eugen, Belfast casually dumped Muzan at Setsuna's feet.

"So, you're considered a magical creature, huh? Does that mean only the real sun can kill you?"

Setsuna asked curiously.

Muzan lay sprawled on the ground, barely clinging to life. After enduring a catastrophic bombardment, even his five brains and seven hearts, or even three heads and six arms, wouldn't have been enough to survive a hydrogen bomb.

"I think it's a combination of the yield being too low and the fact that he ran so fast," Richelieu said dismissively.

"I deliberately reduced the bomb's yield to less than a tenth of its normal power. Otherwise, he'd already be dust," she added.

Muzan, utterly incapable of using his Blood Demon Arts or even standing, could only survive thanks to the sheer vitality and regeneration of a demon. Weakly, he lifted his head, staring at Setsuna with a faint glimmer of recognition.

"You are…"

"We've met before. Don't tell me you forgot," Setsuna said, kicking him lightly.

"Last time, you tried to kill me. Admittedly, I was butt-naked back then and couldn't put up much of a fight."

"!!!"

"It's you?!" Muzan's gaze sharpened, memories flooding back of this peculiar man who had inexplicably appeared on a holographic screen. The girls accompanying him now—those strikingly beautiful faces—were unmistakably the same ones from that time, even if they were dressed differently.

"Don't kill me," Muzan whispered weakly.

As the Demon Progenitor, his pride was immense, but his fear of death made him abandon all dignity to plead for mercy.

"Why shouldn't I?" Setsuna asked.

"I can share my blood with you, grant you unimaginable power and eternal life," Muzan offered.

"..."

"Yeah, I'll pass," Setsuna replied indifferently.

He casually pulled out a syringe, extracting a small sample of Muzan's blood and sealing it in a sterile container.

"You're pretty pathetic, but hey, you're still a boss character. Might as well keep a sample for research or something," Setsuna remarked casually.

Muzan's face twisted with boundless resentment. He could never have imagined his downfall would come in such a ridiculous fashion.

"Oh, you were looking for this, weren't you?"

Setsuna pulled a Blue Spider Lily from his pocket and tossed it onto Muzan's prone body.

"This… this… this!!"

Muzan's eyes widened, a burst of vitality coursing through him as he clutched the flower.

The object of his centuries-long obsession—the key to overcoming his weakness to sunlight—was now in his trembling hands. Reverently, he cupped it, brushing his fingers gently across its petals.

A thought flickered in his mind: if he devoured it now, could he restore his strength and escape this predicament?

Then—

Plop!

The spider lily's blue color smeared away, staining Muzan's fingers with blue paint.

"???"

Muzan glared at Setsuna, his eyes silently screaming: Why are you like this?

"Don't blame others for your stupidity," Setsuna replied with a shrug.

"Well, you've got your spider lily. Time to hit the road, satisfied at last."

As the sun slowly rose over the horizon, its rays touched Muzan's skin.

Sizzle—

Under the sunlight, Muzan dissolved into a wisp of black ash. A gust of sea breeze swept his remains away, scattering them into the Pacific Ocean.

[Current World: Demon Slayer]

Mission Objectives:

 1. Eliminate the Twelve Kizuki.

[Reward has been issued. Host may now return to the original world.]

As Muzan dissolved into ash under the sunlight, the system notification echoed.

Setsuna examined the two rewards he received.

One was the Breathing Techniques of Sun and Moon, the same ones used by Yoriichi Tsugikuni and Kokushibo.

They enhanced swordsmanship—Sun Breathing imitated fire enchantments, while Moon Breathing allowed the user to unleash sword energy resembling razor-sharp blades.

The other reward was Muzan Kibutsuji's Blood Demon Art, a collection of bizarre techniques, including self-division, Infant's Form, and spiked whips.

"..."

"Commander, are you planning to learn any of these?" Belfast asked. "I thought the Demon Slayer Corps had similar moves."

"I'll skim through them when I'm bored and maybe pick up a trick or two, but going deep into them? Nah. These are beginner-level village techniques—not really worth much," Setsuna replied casually.

"Honestly, I'd rather learn Excalibur or Taixu Sword Art."

He thought for a moment.

"These Breathing Techniques could be modified into something useful though. Maybe I'll turn them into a required part of our physical education curriculum. Help the people back home strengthen their bodies."

"Ah…"

The shipgirls didn't need to learn such techniques themselves, but imagining a future where the Starsea Alliance taught Breathing Techniques in schools amused them.

"What's our next step, Commander?" Prinz Eugen asked thoughtfully.

"Should we head back now? I wonder how far Akashi has gotten with the Gundam project, and what's happening at the port…"

"Return? Not yet."

"We still have one very important matter to handle," Setsuna declared, waving his hand.

"Everyone, manifest your ship forms. Advance 32 degrees northeast. Next stop: Tokyo Bay."

"Let's give the Shogunate a proper education."

"Open the gates—bring free trade!"

The fleet sailed into the open sea. The shipgirls jumped off the deck, manifesting their ship forms one after another.

On the boundless ocean, a vast fleet suddenly materialized.

Leading the formation was the gold-and-white Richelieu-class battleship, followed by two 30,000-ton armored aircraft carriers.

On the flanks were a heavy cruiser and a light cruiser providing escort.

The fleet sliced through the waves, its momentum unstoppable.

Along the way, numerous patrol ships trembled in fear, hastily retreating to the side to avoid confrontation.

The sight of this fleet left the observers stunned and silent.

The timeline of Demon Slayer placed it in the early 20th century. At this time, the Imperial Japanese Navy had a combined fleet tonnage of only 70,000 tons, including torpedo boats.

In the face of battleships and aircraft carriers, their vessels were nothing more than insignificant ants.

"Commander, we're receiving communications from the Yokosuka Coastal Defense Fleet."

"They request that we state our purpose. Repeating: please identify your fleet and declare your intentions."

Ships stationed in nearby military ports continually sent hails to the mysterious fleet.

By convention, an unauthorized fleet entering territorial waters could be fired upon without warning.

However, none of the defenders dared attack this overwhelming force. Instead, they desperately tried to contact the flagship to ascertain its identity.

"This is the Starsea Alliance Expeditionary Fleet," Belfast responded.

"The fleet is scheduled to land in Tokyo Bay in two days. Please have the Shogunate prepare to receive us."