Crossover Shipgirls: Building a Dimensional Empire from Scratch

”From now on, Universe No.26 will be your Underworld. Its entrance will be the Gate of the Underworld.”

”As for how you want to build your netherworld, that’s entirely up to you.”

Setsuna smiled as he looked at Ereshkigal.

”Ah... ah...”

She stared blankly at the vast expanse of emptiness before her.

This was an alternate spacetime independent of the main universe. It truly deserved the name ”Underworld.”

A diameter of two light-years might not seem large to shipgirls accustomed to interstellar travel, but for a goddess from thousands of years ago who had spent her entire existence underground, it was an unimaginable scale.

”Uh...”

”Like before? ... No, no, that old Underworld was way too gloomy.”

Ereshkigal shook her twin tails.

”Master, do you have any suggestions?

”Me?”

Setsuna thought for a moment.

”Tell me what kind of Underworld you want, and I can use the Herrscher of Reason’s authority to create a rough prototype for you.”

”As for the finer details and future expansion work, the Bulins will help you handle it.”

Behind him stood a Bulin construction team, carrying Ultra Rainbow Hammers, drills, and wrenches.

They had originally thought Setsuna was going to make them dig an Underworld out of thin air. That was far beyond Bulin specifications. There were no blueprints for something like that, nor could there be.

In the worldview of shipgirls, there simply was no concept of a netherworld.

But if there was a clearly defined world to build within, following instructions was much easier.

”Mm...”

Ereshkigal hesitated, then poked Setsuna lightly.

”Master, you’re so knowledgeable. Give me some ideas?”

”Well...”

Setsuna pondered for a moment, then waved his hand.

Shua—

The Herrscher of Reason’s authority rapidly constructed something within the void.

First, a massive palace in traditional Eastern style took shape. It appeared solemn and imposing, forged entirely from dark materials that exuded an overwhelming sense of gloom and dread.

Below the grand hall stretched an intimidatingly long staircase, ghostly blue flames burning along both sides.

Ereshkigal shivered involuntarily just from glancing at it from afar.

The palace road extended endlessly forward. Greenish-black mist filled the entire space, blurring visibility.

It was as if the cosmos itself had transformed into a sinister realm of spirits.

Looking up, one could see a towering gate reaching toward the heavens. On its archway were three large worlds written horizontally in traditional script:

[Gate of Ghosts]

”Eeeek!!!—”

”What is that supposed to be?!?!”

Startled, Ereshkigal clung to Setsuna’s arm, trembling.

”The Underworld.”

Setsuna answered concisely.

”I created it based on Dragon Empery mythology. From now on, the Gate of Ghosts can serve as the entrance to your netherworld—the first stop for souls arriving here.”

”As for the internal architecture, I shaped it according to my impressions. It’s only a replica, but it counts as a type of Underworld.”

”Oh, and if you want, we could even have some Angeloids come cosplay Black and White Impermanence and ghost officers to help you work and enhance the atmosphere.”

”Of course, they have nothing to do with actual gods or spirits.”

”!!!”

The goddess of the Underworld was thoroughly frightened.

This place felt even more ”underworld” than her original Underworld.

Yat Sen, Harbin, and the other Dragon Empery shipgirls who had followed Setsuna were also struggling to keep straight faces.

Angeloids cosplaying White Impermanence? What kind of setup was that?

After dying, you arrive in a cyber Underworld and run into high-tech ghost officers?

Were the old spirit soldiers who once captured souls with magic now switching to arc emitters and corrosive missiles for ”physical salvation”?!

”N-No, no, no. Living here would be terrifying. I’d have nightmares.”

Ereshkigal waved her hands frantically.

”Stuff from another mythological system doesn’t suit me. Let’s do something more normal.”

”Mm...”

Setsuna thought again.

”An Underworld needs to maintain a certain level of dignity, so the style should be solemn.”

”But... if you prefer a more relaxed environment, we can separate your palace from the Underworld itself.”

Shua—

This time, it was constructed entirely according to Mesopotamian mythology.

A dark wasteland stretched endlessly into the distance, its scale thousands or even tens of thousands of times larger than her previous Underworld.

Several colossal stone pillars that seemed to connect heaven and earth were embedded with cages meant to imprison souls.

Encircling the Underworld was a river of the dead that stretched beyond sight.

At the entrance stood a massive gate that nearly filled the entire space, its surface etched with complex ancient script.

Setsuna clenched his hand, gathering Honkai energy before him.

Before long, a gigantic three-headed beast was created.

”The hellhound? Cerberus?!”

”Isn’t that from Greek mythology?! Master, you’re mixing universes here!!!”

Ereshkigal was stunned.

”It’s fine. That thing makes a pretty good guard dog.”

Setsuna lifted his chin.

”This will be your work area, a place to display the majesty of the goddess of the Underworld.”

”I’ll create a living area for you as well.”

Weng—

In a space isolated from the Underworld, he opened up another small independent realm, accessible only through special means.

A resplendent palace was created, more than a hundred times larger than the most luxurious royal palace on Earth.

The main hall alone was comparable to a small city. Endless seas of flowers spread out from beneath their feet, surrounding the entire estate.

Farther away lay a vast artificial lake and lush forests, where birds and beasts roamed freely.

Beyond the horizon shimmered a blue ocean and sandy beaches.

At the same time, a warm sun rose into a cerulean sky, casting golden light across the land.

Birdsong, fragrant blossoms, open skies, and boundless seas.

Two light-years of space was so vast that Setsuna could easily mold a planet-sized resort within it.

”This will be your new palace, independent from the outer Underworld. You can think of it as your private garden. What do you think?”

Setsuna said...

”I’ve created a basic layout. If you want more decorations or additional buildings, go find the Bulins. They’ll remodel it for you.”

”If you want to raise small animals... the Life Sciences Division can help with that too. There’s more than enough space here to sustain an entire ecosystem.”

”Wow...”

Ereshkigal was completely dumbfounded.

Compared to this, the stone house she used to live in really did look like a beggar’s shack.

So even a goddess needs someone powerful to rely on... mm.

”This one!! Thank you, Master!! I love this Underworld!!!”

She closed her eyes and clasped her hands together.

Shua—

The goddess of the Underworld’s authority spread outward.

In some unseen manner, Ereshkigal established a connection with this world.

”Uh...”

She quickly realized that this space was far too vast—countless times larger than her previous Underworld. With her own power alone, she could barely control the entire realm.

Just as she grit her teeth and forced herself to endure, Setsuna gently placed a hand on her shoulder, pouring more magical power into her.

Weng—

A tide of magic surged throughout the entire miniature universe.

The goddess of the Underworld completed her dominion over the realm.

A faint pressure enveloped the entire area, and even the surrounding temperature seemed to drop by several degrees.

”Tch, that feeling again.”

Ishtar hugged herself as goosebumps rose across her skin.

It was the same familiar aura as when she had fallen into the Underworld in Uruk.

Any living being here would be naturally suppressed by the goddess of the Underworld.

With Setsuna’s magical support, Ereshkigal’s strength had grown further, and her control over the laws of the Underworld had become even stronger.

”Ah...”

Ereshkigal let out a long breath.

Returning to her own domain made her feel much more at ease, and her power was rapidly recovering.

”Thank you.”

She expressed her gratitude to Setsuna once more.

”From now on, I can fulfill my duties as the goddess of the Underworld again.”

”Mm... Master, when you have time, come visit me down here—”

As she spoke, Ereshkigal rose onto her tiptoes and gently kissed Setsuna on the cheek.

”Huh?”

Ishtar stared in shock.

”Th-That was just thanks! Just thanks!!”

Blushing furiously, Ereshkigal stomped her foot.

”If you accidentally die someday, I’ll welcome you in the Underworld. We can have a meal together, enjoy some afternoon tea, and then I’ll send you back.”

”As for outsiders... they can either go play with Cerberus or get locked in cages and hung up, just like before.”

She decided to grant a few willful little privileges within her authority.

”Good grief.”

Gudako froze for a moment.

”So I’m a VIP member of the Underworld? Priority resurrection rights? ...”

”This is advanced. Way too advanced.”

...

”Mm...”

Ereshkigal strolled through the Underworld that the Starsea Empire had rebuilt for her.

She was extremely satisfied with this new realm. At its entrance stood a hellhound molded from Honkai energy.

When it saw Ereshkigal, it lowered its heads and whimpered softly.

On the surface, its strength was comparable to an Emperor-level Honkai Beast. But once within the Underworld—where most living beings were suppressed by its laws—there were hardly any who could defeat it.

Vast and imposing, the two-light-year expanse was unimaginably larger than her former Underworld.

Ereshkigal had to rely on teleportation magic just to move efficiently within it.

Within this miniature universe, the entire space constituted the Underworld itself. After walking to the end, one would return to the origin from another direction, like a Möbius strip.

In the dim Underworld, at regular intervals, colossal stone pillars that seemed to connect heaven and earth stood tall. Embedded within them were black metal cages.

They had originally been used by the goddess of the Underworld to imprison souls. Every spirit that fell into the netherworld would be confined within them.

Unfortunately, in the Starsea Empire, she had no business for the time being.

Shua—

Ereshkigal opened a portal and stepped through, arriving in the lower layer of the Underworld—her ”palace.”

Blue skies and drifting white clouds greeted her. A gentle breeze brushed across her face.

The resplendent palace and flower-filled estate exceeded all her expectations. Setsuna had fulfilled every one of her wishes.

Simply being here lifted her spirits.

Managing the Underworld no longer seemed so dull and tedious.

”Mm...”

Ereshkigal brushed back her hair and sat on the sofa.

”Master said the Underworld is mine to manage from now on... In the future, it will oversee the reincarnation of countless worlds within the Starsea Empire. I need to build it properly.”

The master may lead you to the door, but cultivation depends on the individual.

Setsuna had used a miniature universe and the Herrscher of Reason’s authority to construct the Underworld’s basic framework.

As for what it would ultimately become, that depended entirely on Ereshkigal herself.

”Burin!”

”Whatever you want to build, just tell Bulin! Commander said Bulin will help you, burin!”

Several Colored Bulins stood by within Ereshkigal’s estate.

They looked at her cheerfully, scampering about the palace.

”Eh...”

Ereshkigal stared at the adorable group, slightly dazed.

”Anything at all?”

”Of course, burin!”

The leading Colored Bulin patted her chest proudly.

”As long as it’s reasonable and Bulin can understand it, we can build it, burin!”

”Uh...”

During her earlier tour of the Starsea Empire, Ereshkigal had witnessed the Bulins’ capabilities. They could hammer out almost anything.

Now, however, she found herself at a loss.

”My palace is fine... but the Underworld itself does feel a bit empty. Should I create some assistants like Master suggested? ...”

Back when her Underworld had only been the size of a city, managing it alone had posed no problem.

But now, faced with a light years-scale Underworld, handling everything by herself felt nearly impossible.

”Burin?”

”Do you need Angeloids, burin?”

A Bulin tilted her head.

”Mm... I want assistants that fit the characteristics of the Underworld.”

Ereshkigal considered carefully.

In her vision, she would remain in her palace, while the upper Underworld would generally be managed by her subordinates.

After thousands upon thousands of years guiding souls, she felt it was time for modernization.

As the dignified goddess of the Underworld, handling every task personally no longer seemed appropriate. Only Master or other special figures would warrant her personal reception.

”So you need robots?”

The Colored Bulins scratched their heads.

”Engineering type, combat type, or domestic type? Bulin can make them all, burin!”

”...”

Ereshkigal fell into deep thought.

She had borrowed a mountain of mythological texts from the Starsea Empire’s library.

From the Classic of Mountains and Seas to Greek mythology, she had read through nearly every description of the ”Underworld” across major religions.

”Mm... Come to think of it, maybe I should just adapt to local customs?”

After sifting through the vast ocean of information, she finally decided to preserve the Underworld’s core characteristics while incorporating some local elements.

”In the Underworld Master created earlier, there were ghost soldiers and spirit generals. Can you make those?”

Ereshkigal asked.

”They’re a bit creepy to keep nearby, so just let them handle things in the upper Underworld for me.”

”Burin?”

A cluster of Bulins crowded around, staring at the eerie holographic images of ghost soldiers for a long while.

”Those are mythological entities, not Bulin’s responsibility, burin!”

”But Bulin can build robots that look exactly the same!! And we can add some magic too, burin!”

...

Elsewhere.

The subsequent construction of the Underworld was left to Ereshkigal, and Setsuna refrained from interfering for the time being.

”The Underworld will function as a subsidiary realm attached to the main universe.”

Observer adjusted data across a holographic interface, linking the miniature universe with the primary cosmos.

Under normal circumstances, one could only enter the Underworld through death. However, since the palace existed within a miniature universe, it had a physical ”entrance.”

At Ereshkigal’s request, the entrance was placed within the Starsea Empire’s administrative center, allowing her to drop by occasionally.

”This is the Gate of the Underworld. The living must pass through it to enter the Underworld.”

They added some simple decorations to the miniature universe’s entrance.

”If that’s the case, couldn’t we use miniature universes to build a Heaven or something as well?”

”If the miniature universe grows large enough, wouldn’t our main universe effectively become a composite cosmos?”

The Siren girls began brainstorming all sorts of ideas.

There were literal goddesses of Heaven residing in the capital, though they seemed to prefer staying in the city and playing with everyone.

”Uh... Are you planning to handcraft the Three Realms or something...”

The shipgirls felt they were embarking on a truly grand undertaking.

A few days later, news of the Starsea Empire’s Underworld spread rapidly throughout the capital.

Curiosity and confusion abounded.

The Research Division had studied this world for years. Suddenly announcing the existence of an Underworld left many astonished.

”Mm... I really want to conduct field research in the Underworld.”

”Studying soul reincarnation and the mechanism of the Underworld goddess’ authority could easily produce hundreds of papers.”

A group of young girls carrying survey equipment eagerly prepared to investigate.

At Chaldea’s base.

Someone had already begun putting theory into practice.

”Hit me. As hard as you can.”

In the training ground, Gudako pointed at herself.

”Hm?”

Across from her, Scáthach held her spear, staring at her as if she had lost her mind.

It was the first time she had ever heard such a request.

”I want to experience what it feels like to have my soul return to the Underworld and then be revived. Visiting normally would be boring.”

Gudako explained.

”It’s a good way to test whether the Underworld functions properly in the Starsea Empire. Otherwise, wouldn’t my VIP Underworld membership be wasted?”

”If there are any bugs, we can fix them early.”

”...”

Scáthach’s brows furrowed tightly.

Because she was a VIP client of the Underworld, she had decided to die once for the experience. The logic was questionable at best.

After some thought, she realized this decision was not hers to make and contacted Setsuna.

”Master, Ritsuka Fujimaru is attempting suicide.”

”Huh?”

Setsuna rushed over immediately to observe the scene.

”You’re serious about this?!”

”Mm... It’s not like I can actually die for real.”

Gudako scratched her head.

”There are so many ways to revive people here. You’re not going to just let me stay dead, right?”

”I just want to experience what it’s like for a soul to fall into the Underworld. The Starsea Empire hasn’t had a single casualty in years. Everyone’s long-lived. Ereshkigal might not receive a single soul for ages.”

”I’ll go test it out first. Scout ahead?”

Setsuna was astonished by her curiosity.

He sighed.

”Fine. Go take a look around down there, then have Ereshkigal send you back.”

The resurrection process had now been simplified to the point where the goddess of the Underworld could simply open a backdoor and release someone—far easier than conventional magic.

”Ha!”

Gudako stood tall, chest out, indicating she would not use any defensive measures—like a hero marching to execution.

The next moment—

”Then, excuse me.”

Whoosh—

Scáthach hurled her spear.

Boom!!—

A violent impact struck her body. Gudako’s vision went dark.

”Commander... I... I’m seeing my life flash before my eyes...”

”I think I see my uncle again... he’s talking to me...”

”Get lost!!! Hurry up and go see the goddess of the Underworld!!!”

—

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