Crossover Shipgirls: Building a Dimensional Empire from Scratch

Setsuna spent a bit more time efficiently wrapping up the remaining matters.

”This period of rest won’t be long. Once Chaldea is ready, we’ll head to the Grand Temple of Time for the final battle.”

”After restoring human history, we’ll start working on conquering the remaining worlds. Mm, I’ll also find a way to get Grey the wetware she wants.”

He realized that his future schedule was packed once again and let out a yawn.

”Ah, I really want to throw a party—”

”Throw a... party?”

Tiamat slowly repeated the unfamiliar word.

”What does that mean? ... Are we celebrating something? ...”

”I don’t understand... I need to learn... mm.”

Tilting her head, Tiamat thought seriously about it.

”Commander, your true nature is showing, hey?! You’ve even corrupted a Beast of Humanity!”

Gudako complained wildly.

”I’ve always had the courage to face myself directly... As if you’re any different?!”

Just as they were bickering back and forth—

”Master, there’s one more thing—please help me!!—”

Bang bang bang—

A blonde girl with red eyes threw herself in front of him.

”Oh? Isn’t this Ereshkigal?”

Setsuna laughed.

”Weren’t you just saying you were fine and planning to ask someone to go shopping?”

”Mm...”

Ereshkigal fidgeted.

”That’s true... This place really is fun...”

”But I suddenly realized something—there’s no Underworld here!”

”My power hasn’t recovered at all. My authority doesn’t work either. In the Starsea Empire, I’m basically useless!!”

As the goddess of the Underworld, she had originally been unable to leave it. Only with the immense magical power provided by Setsuna had she been able to manifest in the real world.

However, Ereshkigal could no longer use her former authority. First, the Seventh Singularity had disappeared after being resolved. Second, a goddess of the Underworld could not enforce her rule across different worlds.

”Isn’t it nice being a carefree good-for-nothing?”

”Eat, sleep, sleep, eat. When you’re bored, you can travel around the capital or elsewhere. Isn’t that the life you’ve always dreamed of?”

Ishtar clicked her tongue.

”Hey!”

Ereshkigal stomped her foot and puffed up her cheeks in protest.

”You and Quetzalcoatl and Gorgon can all use your powers. I’m the only one who’s turned into a useless person. That’s way too unfair!”

As she spoke, she edged closer to Setsuna.

She tugged lightly at his sleeve in a pleading manner.

”Um... Master.”

”I know this is a bit sudden, but didn’t you say before that you could help me build a new Underworld? ...”

Feeling she had been too blunt, she waved her hands awkwardly.

”Ahhh, I’m not trying to make demands of you! Life in the capital is great. It’s just that without an Underworld, I have to rely on your power to manifest, and I can’t use my authority at all. I can’t be of any help to you.”

”I just want to become a little stronger... so I can be useful in the future...”

Pfft—

Setsuna found Ereshkigal far cuter than Ishtar.

He rubbed her head.

”No problem. I remember.”

”I can build you a brand-new Underworld within the Starsea Empire, and we’ll design it according to your preferences.”

”Your original Underworld was too gloomy. It doesn’t really suit this place.”

”Huh?”

Tohru and Elma beside them froze.

”Build... an Underworld? The kind I’m thinking of?”

”Yes.”

”Ereshkigal is the goddess of the Underworld. Since she’s come to the Starsea Empire, she naturally needs an Underworld.”

Setsuna nodded.

”Just like how the two Venus goddesses, Quetzalcoatl and Ishtar, have Venus here, the goddess of the Underworld should also have her own Underworld.”

”...That logic sounds really strange...”

”No, wait—can something like the Underworld even be built by humans?!”

Tohru was stunned.

”In the Type-Moon world, during the Age of Gods, the Underworld was simply located underground. It wasn’t that complicated.”

”The goddess of the Underworld is the Underworld itself. As long as she gathers her authority and releases it in a suitable location, she can transform that place into the Underworld.”

Setsuna took a sip of tea.

Ereshkigal’s situation was somewhat similar to that of Eris and Aqua before. Though they were goddesses, they were still bound by various rules.

”Mm... I did once try to drag all of Uruk into the Underworld. So as long as you give me a suitable place, that’s enough...”

Ereshkigal nodded.

”Uh...”

Tohru clutched her head and thought.

”I have another question. How large of an area can your authority manage?”

”Well...”

Ereshkigal played with her hair.

”In principle, the Underworld governs all life and death in the world. In Uruk, even a demigod like the King of Heroes would fall into the Underworld and under my control after death.”

”In the Starsea Empire, I can’t do that. There are definitely lots of people I can’t govern...”

Over the past few days, she had learned about the situation in the capital.

Shipgirls and Angeloids were mechanical lifeforms governed by engineering. They had nothing to do with the Underworld.

Outer Gods like Nyarlathotep and Kuuko, as well as gods and demigods like Lucoa and Tohru who did not belong to the Type-Moon world, were also beyond her authority.

Her preliminary estimate was that her authority would affect most of Chaldea’s Heroic Spirits and ordinary humans.

”Uh, so if I accidentally die, my soul would fall into your Underworld?”

Gudako pointed at herself. As a native of the Type-Moon world, she figured she probably fell under Ereshkigal’s jurisdiction.

”Mm... I could release you again? Then you’d be revived.”

Ereshkigal scratched her cheek.

”If you died without permission, Master would be troubled, right? ...Mm, I think that counts as helping Master a little?”

”It’s the same for everyone else. If someone accidentally falls into the Underworld, just tell me, Master~”

”Damn, so from now on we’re all insiders with the Grim Reaper? Looks like nobody’s dying anymore.”

Gudako felt an inexplicable sense of absurdity.

Immortality had been achieved in a strangely roundabout way.

”Haha—”

Setsuna laughed.

The Underworld and its goddess were not as gloomy and terrifying as imagined. Ereshkigal was, at heart, a soft and gentle girl. Having her rule the Underworld might even be a good thing.

In the more distant future, as mysticism and the magical side developed further, the Underworld’s scale could expand even more—eventually governing the cycle of life and death across countless worlds.

For now, this was a good beginning.

”Though, you probably won’t have much business in the Starsea Empire for the time being.”

”Aside from shipgirls, dragon girls, and spirits, everyone here is either long-lived or has undergone super transformation surgery.”

Since the founding of the Starsea Empire, local casualties had been zero.

Occasionally, a little girl in the lab or at the Magic Academy would attempt something dangerous, but Mio, Grey, or someone else would always resolve it before anything serious happened.

”That’s even better!”

Ereshkigal puffed her cheeks.

”Back in Uruk, whenever disasters struck, the Underworld would be packed every day. Souls pouring in everywhere—I didn’t even have time to rest...”

”Sometimes a few troublemakers would break in too... Though in Mesopotamia, no one in the Underworld could beat me. Um, except for Tiamat.”

”If everyone lives happily on the surface, I can relax more... Mm, I still want to explore the Starsea Empire properly.”

A goddess of the Underworld hoping to have no work—Setsuna found that rather novel.

”All right. It’s settled. I’ll rebuild an Underworld for you.”

”Mm... Your original one was underground. That doesn’t really suit the Starsea Empire. Earth’s interior here has already been fully developed, and it’s not large enough anyway. Not impressive enough.”

He stroked his chin in thought.

In ancient times, an underground Underworld had indeed seemed mysterious.

In the interstellar era, if the Engineering Department accidentally drilled into the Underworld during a survey, the situation might be slightly awkward.

—The goddess of the Underworld filing a complaint about construction noise disturbance.

”I’ll give you an independent subspace separate from this world. We’ll build a brand-new Underworld there.”

Setsuna snapped his fingers.

”As for constructing it... that’s simple. You can have whatever style you want. Even turning it into a resort is fine.”

”Ohh, thank you, Master~~”

”How long will it take? Is there anything I need to do?”

A faint blush appeared on Ereshkigal’s face.

”Very soon. Come here.”

Setsuna summoned several Colored Bulins.

”Burin Commander, what do you need, burin? Are we building houses, burin?”

The Bulin foreman carried the Ultra Rainbow Hammer and looked at Setsuna with a cute expression.

”Bulin is full of power now! Not just spaceships and starbases—even galactic wonders can be built, burin!!!”

The Colored Bulins surrounded him, hopping up and down excitedly.

From repairing ships for shipgirls in the beginning, to building starbases, Galactic Wonders, and even ancient god EVAs—Bulin dared to face every challenge.

”A small task. Come with me. This time, we’re building an Underworld.”

”?”

”Burin?”

...

Buzz—

Setsuna brought Ereshkigal and several companions down onto the starbase.

It was 150 million kilometers from Earth. As far as the eye could see, silver-gray structures floated in space alongside towering fleets docked in shipyards, forming something like a colossal space fortress.

Below the starbase, the Sun burned fiercely. It was enveloped by something like a metallic shell, framing the entire star.

”That’s a Dyson Swarm. It’s used to harvest energy from the star. A full Dyson Sphere would turn the entire star system into a frozen world—not very cost-effective.”

Setsuna casually explained.

”Oh...”

For someone from the era before the Common Era, the Sun had been an object of worship. In most pantheons, sun gods held supreme status.

Yet before an interstellar civilization, the Sun was merely one ordinary star among countless others—something that could be fully controlled through technology.

”Aren’t we building an Underworld? ... Why did we come here?”

”Um... Don’t tell me we’re building the Underworld inside the starbase? Isn’t this too far from Earth?”

Ereshkigal clutched Setsuna’s sleeve. The starbase was so enormous she was afraid she might get lost.

”When I guide souls in the future, do I have to send a spaceship to pick them up? ... Isn’t that a bit too sci-fi?”

”You want to build a cyber netherworld? Quite the imagination.”

Setsuna rubbed her head.

”I suggest keeping it normal. Mm.”

They proceeded from the docking area toward the Research Division within the starbase. This facility primarily handled astronautics and advanced technologies related to space.

Around them, starships equipped with newly installed weapons were conducting test flights.

Inside the experimental center, Setsuna met some familiar faces.

”Yo, Commander, you came up to see me~—”

Observer floated over while seated on her octopus-like rigging. Several Siren girls from the Arbiters were with her.

They had mastered parallel-universe technology long ago and were responsible for the Starsea Empire’s spatial research.

”Hehe... Did you bring me anything fun? Even a fun new species would do?”

The little girl rubbed against Setsuna while the tentacle-like appendages of her rigging waved around, making Ereshkigal’s eyelids twitch.

After teasing her for a bit, Setsuna got to the point.

”What about the pocket universes we obtained from the Three-Body world?”

”I remember No.1 was only city-scale. Later you drafted plans for larger production. I saw in the reports that you were experimenting with planetary-scale universes?”

”Take me to see them.”

Pocket universe technology—originally a god-tier civilization’s ultimate refuge in the Three-Body world, used to escape dimensional collapse—had been studied and applied by the Starsea Empire for quite some time.

It could create an entirely independent universe separate from this spacetime, free from any external interference.

”Oh, that thing?”

Observer perked up immediately.

She led the group through a long corridor to another complex on one side of the starbase.

”We’ve had the tech for a while. So far we’ve created dozens of pocket universes of varying sizes—from city-scale to planetary-system-scale.”

”As for galaxy scale or full cosmic-structure scale ones, the resource and energy consumption is too massive. We’ll probably have to wait until the Aetherophasic Engine is completed.”

Inside a vast, empty hall, she pulled out something like a remote control and pressed a button.

Buzz—

The ground trembled as three rows of door-like frames rose from below.

They looked like rectangular outlines drawn in faint white lines, about the height of a person, hovering in midair.

The frames were thin and emitted only a soft glow. If one wasn’t paying attention, they could easily be overlooked.

The experimental department had deliberately installed silver metal thresholds beneath each ”door,” engraved with different numbers.

”Pocket Universe No.1 was the first we created—city-scale. We went inside before.”

Observer introduced them one by one.

”No.2, 3, and 4 are slightly larger than No.1—experimental models. No.5 to 10 are continent-scale... roughly the size of Eurasia.”

”From No.11 onward, they’re planetary-system-scale, ranging from moon-sized to Jupiter-sized. No.16 and up are planetary-system-scale, using the Solar System as a standard reference.”

”Man, building these things was exhausting. Good thing we’ve got plenty of resources—and Bulins.”

The Research Division had previously given Setsuna a budget estimate. Each pocket universe required resources equivalent to about five times its own volume.

A planetary-scale universe meant dismantling five or six planets of similar size. A planetary-system-scale one required tearing apart even more planetary systems.

Only the Starsea Empire, which possessed dozens of universes and nearly limitless resources, could afford such a money-burning project.

”Commander, how many pocket universes do you need?”

”Mm.”

Setsuna looked at the rows of pocket universes and pondered briefly before nodding.

”Let’s pick one first. I plan to use it to build an Underworld.”

”I also have another idea—installing pocket-universe tech on the Eternal Snowfall... but let’s solve the Underworld matter first.”

”Hah?!”

Observer jumped in shock.

”Modifying a spaceship I can understand, but building an Underworld? What the heck is that about?!”

”Commander, this is the Spatial Technology Division of the Science Department. Please respect my profession. If you need that kind of service, go to Mio or Tohru at the Magic Academy. Thanks.”

After some commotion, and once she learned that Ereshkigal was truly a goddess of the Underworld, the little girl puffed her cheeks and pointed to one of the doors.

”Pocket Universe No.26. planetary-system-scale. Spatial radius 1.2 light-years—roughly equivalent to the distance from the Sun to the edge of the Oort Cloud.”

”Based on what you said, her Underworld in Mesopotamia was underground and not even planet-sized. This should be more than enough, right?”

”If it’s not enough later, we can always expand it.”

”Eh?”

Ereshkigal had once thought her Underworld was quite large—a subterranean kingdom beneath Uruk.

But on an interstellar scale, faced with calculations in light-years, she was completely stunned.

She began counting on her fingers.

”A light-year... how long is that again? ...”

”Light travels at 300,000 kilometers per second in vacuum—enough to circle Earth’s equator seven and a half times. A diameter of 2.4 light-years means the distance light travels in two years. Do the math yourself.”

Observer explained mercilessly.

”Wha—?!”

”Sixty seconds in a minute, sixty minutes in an hour... so the distance light travels in one day is... then multiply by 365...”

”Ahhh?! It’s that big?!”

Borrowing a calculator and seeing the string of zeros on the display, Ereshkigal nearly fainted.

”Small stuff. Don’t get too excited. In the future, I plan for the Underworld to expand until it governs the cycle of reincarnation across countless universes.”

Setsuna pinched her cheek.

”This is just the beginning.”

”Hiss—”

Ereshkigal looked at him with pure admiration.

”Come on.”

They stepped through the doorway of Pocket Universe No.26.

Strictly speaking, it was merely an entrance. The universe itself existed in another world bubble along the superstring.

Shua—

At first, there was only primordial darkness.

Then time within this universe began to flow. A dim glow appeared across the vault of space.

They floated in boundless emptiness.

As far as the eye could see was the same dark deep space as the universe outside. But since no stars had yet been placed inside, there were no planets either.

”We’ve only constructed the basic framework of this pocket universe. It hasn’t been filled in yet.”

”Mm... Just shape it however you like. All the universal constants here can be modified—time flow, gravity, and so on.”

Observer waved her tentacles as she spoke.

”Whatever kind of Underworld you want to create depends on your preference.”

”Also, if you’re using it as an Underworld, you’ll need to anchor it to the main universe. Otherwise, the two universes won’t be able to communicate or interact.”

”It’s so big!!—”

Ereshkigal looked around frantically, unable to see any boundary, once again overwhelmed.

”Go on. Tell me your dream.”

Floating before her, Setsuna pointed into the distant void.

”What kind of Underworld do you want?”

—

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