Building a Dimensional Empire from Scratch

To be honest, Setsuna wasn't particularly interested in the various Breathing Techniques or Blood Demon Arts of the Demon Slayer world.

The power ceiling in this world wasn't exactly high.

Physical enhancement?

Could mere mortal flesh compare to his body, now on par with shipgirls, after absorbing a Wisdom Cube?

Combat power?

A joke. Even if someone managed to build a legendary Nichirin Blade-sized Gundam, they couldn't scratch his armor.

While Blood Demon Arts might hold some minor utility, it wasn't worth the cost—especially since he had no intention of becoming a demon. Future worlds would undoubtedly offer far more powerful abilities.

At best, the quest rewards could serve as a collectible. Once his settlement grew and the population increased, anyone interested could practice them.

What he had in mind was something entirely different:

Resources.

They were in short supply. Whether for constructing Gundams or building the city, the consumption rates were staggering.

Agriculture and light industry? Practically non-existent.

But as luck would have it, the Demon Slayer world presented an opportunity.

The overall combat strength wasn't impressive, but their production capabilities were passable.

Perfect for a tributary state.

It was also a great chance to give the shipgirls a glimpse of another world.

With that thought, Setsuna snapped his fingers, calling for Richelieu.

"Everyone, assemble!!!"

"Prepare for an expedition!!!"

Half an hour later, every shipgirl in the base had gathered, with a few Bulins tagging along to listen in.

This would mark their first official contact with another world.

Previously, due to Setsuna being the only person on Earth, the Vanguard Fleet hadn't interacted with any otherworldly individuals.

Let alone a place filled with a unique worldview and supernatural beings like demons.

"Let me briefly introduce this world," Setsuna began, clearing his throat.

In simple terms, he outlined the world of Demon Slayer:

Set during the Taisho era, the world was plagued by demons, born from the progenitor Muzan Kibutsuji.

The story revolved around the Demon Slayer Corps and their long-standing struggle against these supernatural foes.

He didn't go into too much detail about the plot—after all, bringing an entire fleet of shipgirls was bound to derail it entirely.

Instead, he focused on explaining the technology level, the concept of demons, and the era's setting.

"Taisho era, huh... Doesn't the Sakura Empire fleet have a senior ship from that time?"

Taihou suddenly recalled someone.

"Mikasa? Well, in a way, they are from the same era," Setsuna replied.

Apart from the sailing ships, Mikasa was arguably the most senior shipgirl in the harbor.

It was a shame she wasn't part of the Vanguard Fleet. Seeing her reaction to an era teeming with "demons" would've been interesting.

"Let's stay on track," Richelieu interjected, standing beside Setsuna and signaling for silence.

"This expedition has two objectives," she announced.

"First, complete the two missions: eliminate the Twelve Kizuki and resolve the threat of demons in that world."

"Second, the Commander wishes to establish diplomatic relations with that world."

"Like our alliance with the harbor?" Yukikaze raised her hand to ask.

"Not quite. Given their limited capabilities, they're better suited as a tributary state," Setsuna replied.

While demons were a significant presence in the Demon Slayer world, the Shogunate still held administrative control.

Having a unified governing body made things simpler.

Like the Black Ships incident of old, all it took was a display of overwhelming power to bring those so-called Shoguns and Emperors to their knees.

"Oh..."

The shipgirls half-understood, half-didn't.

Diplomatic affairs were typically the Commander's responsibility. Their focus was on more tangible concerns.

"And the first mission? Eliminating the Twelve Kizuki and the demon threat?"

"What's the plan?" Essex asked directly.

"Hmm..." Setsuna thought for a moment.

"We'll take out Muzan."

Eliminating the demon threat essentially meant eliminating its leader, Muzan Kibutsuji.

As the progenitor of all demons, defeating him would resolve the issue at its root.

The fleet had seen Muzan before during their brief encounter in the Azur Lane world.

Who would've thought he was the big boss of another world?

"So, it's that guy?"

The shipgirls recalled the gloomy-looking middle-aged man.

"He didn't seem that strong, right? At least he doesn't have the Commander's ability to travel between worlds," Richelieu remarked thoughtfully.

"If he did, he would've been too busy trying to fight us instead of throwing a tantrum," she concluded.

"..." Setsuna was momentarily at a loss for words.

"Well, you're not wrong. From an interdimensional perspective, he's not particularly powerful."

He then explained the demons' unique abilities:

Enhanced physical traits, vulnerability to sunlight, bizarre Blood Demon Arts, and so on.

To the ordinary, human-led Demon Slayer Corps, each of the Upper Rank Twelve Kizuki was an overwhelming challenge, requiring strategy and sacrifices to defeat.

For the shipgirls, however, the approach was... unconventional.

"If Muzan has five brains and seven hearts, do we just blast him directly? Should I use high-explosive rounds or armor-piercing shells?"

Prinz Eugen pondered over the most effective ammunition loadout.

"It seems like demons have no air defense capabilities. We could deploy carrier-based aircraft for large-scale bombing. Thermobaric and fuel-air explosives are an option if necessary," Essex proposed, leaning into the White Eagle's firepower superiority.

"Battleships and carriers can provide integrated air-sea strikes. After the air raid, I'll offer artillery support," Richelieu nodded in agreement.

"We can also launch underwater torpedo attacks! They don't seem to have anti-submarine capabilities!"

Dace and Albacore were practically bouncing with excitement.

Given the starkly different worldviews and power systems, the shipgirls' strategies against otherworldly enemies were decidedly... unique.

They instinctively relied on their familiar tactics and methods to tackle the problem.

"..."

Setsuna nearly laughed aloud.

"If Muzan knew how highly you all think of him, he'd probably bow in gratitude."

"Just follow my lead when the time comes. And keep your white phosphorus and napalm bombs in check! We can't have any collateral damage!"

"Also, Richelieu, Taihou, head to the naval base and bring extra weaponry. It'll come in handy."

After the meeting concluded, Setsuna finalized the roster for the first interdimensional expedition.

"Richelieu, Taihou, Belfast, Illustrious, and Prinz Eugen—you'll accompany me."

"The rest will remain stationed at the base. Yat Sen, you'll oversee operations while I'm away."

Managing the Demon Slayer world wouldn't require a large number of people, and the ongoing construction at Constantinople demanded personnel to remain behind.

"Understood. I wish you a safe journey."

Yat Sen accepted the responsibility with her characteristic calm, making her a natural choice as acting base commander.

"Only five slots? Why can't I come along?"

"I wanted to see another world too, Commander!"

The shipgirls not chosen for the mission wore expressions of disappointment.

The Vanguard Fleet had 30 members. Subtracting Akashi, who was preoccupied with Gundam construction, left over 20 shipgirls to linger idly on Earth.

"Be patient. Once this mission is complete, I'll reopen the dimensional gate."

"My plan is to establish long-term, stable relations between the two worlds. Someday, you'll be able to visit as casually as shopping trips."

Setsuna reassured them with a smile.

"Yay!"

A cheer erupted among the shipgirls.

"Suit up in your uniforms, bring our flag, and prepare for departure."

Setsuna clapped his hands.

"Let's give those natives a small dose of technological awe."

Two Days Later

Setsuna stood on the naval base's plaza, flanked by five shipgirls dressed in sleek black-and-red military uniforms.

The uniforms carried a sense of responsibility and pride.

At this moment, they represented the Starsea Alliance, venturing into another world as emissaries of their Commander's will.

"I will personally dedicate this world to you, Commander. No matter if it's the Demon King or anyone else, anyone standing in your way shall be crushed by me!"

Taihou's entire body trembled with excitement. Being selected for the first expedition left her feeling both honored and overwhelmed.

"No matter the world, I'll ensure you are well cared for," Belfast remarked with her usual poise.

"Everyone, to your positions."

Setsuna calmed the group and activated the hyperdimensional map, locking onto the Demon Slayer world.

[Demon Slayer: Steam Age]

• A world set in the Taisho-Era Japan, with a technological level in the mid-Steam Age. Steam engines and firearms are in moderate use.

• Features supernatural entities known as "Demons" that possess combat abilities far surpassing humans, but they also have notable weaknesses.

Comparison of Strengths:

• Technological Level: Like a Ant.

• Military Strength: Like a Ant.

• Economic Power: Like a God.

Upon first contact, the balance of power between the two worlds had drastically shifted.

The military and technological prowess introduced by the Azur Lane fleet utterly dwarfed the Demon Slayer world.

That said, the Starsea Alliance lacked basic agricultural and light industrial capabilities—fields where the steam-powered natives actually outperformed them.

"Such a vulnerable neighbor. If this were Stellaris, they'd be easy pickings..."

Setsuna mused, opening the portal to the Demon Slayer world.

[Opening Dimensional Portal: Starsea Alliance → Demon Slayer World]

With a swirl of distorted space, a kaleidoscope of colors appeared before them, the portal slowly expanding into a stable passage.

"Follow me!" Setsuna beckoned his team and stepped through, followed closely by the five shipgirls.

"Commander, may victory be yours."

The shipgirls left behind waved farewell from the plaza.

Moments Later

A disorienting sensation of weightlessness and vertigo lasted for what felt like ten minutes.

After the dazzling display subsided, the team found themselves standing on solid ground once again.

Before them lay a desolate, run-down village.

The thatched rooftops and wooden structures displayed a distinctly Japanese aesthetic.

"So, this is the Demon Slayer world…"

"It feels so backward."

The accompanying shipgirls rubbed their temples, adjusting to the new environment as they surveyed the surroundings.

It was deep into the night, the eerie silence amplified by the fog drifting through the dimly lit streets.

There wasn't a single light in the houses along the road, lending the scene an almost horror movie-like atmosphere.

"There's a faint scent of blood in the air," Belfast remarked, her sharp senses picking up on something subtle.

Shipgirls possessed heightened perception compared to ordinary humans.

"Illustrious, Taihou—deploy reconnaissance planes and survey the area," Setsuna ordered.

New to the Demon Slayer world, they needed to determine their exact timeline and location.

While defeating the Twelve Kizuki and Muzan wasn't challenging for the expedition team, finding these hidden adversaries would be the real hurdle.

"Understood."

The two carrier shipgirls partially materialized their flight decks.

Toy-sized aircraft launched into the air, buzzing around in circles before expanding to their full size and dispersing into the night sky.

Shortly after, Taihou's reconnaissance plane relayed information.

"Northwest, 320 meters—there's a fight underway."

"One side appears to be a small team of humans wielding katanas… and the other…"

"A young man, roughly twenty years old, holding a fan?"