Crossover Shipgirls: Building a Dimensional Empire from Scratch

”Mama Tiamat is so cute. Mama Tiamat sings so beautifully.”

The starship slowly descended from the sky.

Tiamat did not attack the group. She simply kept letting out an ethereal sound over and over again.

”Aaaaa—”

Like the sirens of myth.

”Uh, Commander, are the goddesses on their side all this big?”

Belfast leaned out to take a look.

Tiamat was huge as well. It seemed that from now on, Setsuna would have to prepare to drive a big truck.

”This is only her mind-body. You haven’t seen Tiamat’s dragon form yet.”

Setsuna landed on the sea. The black mud did not stain him in the slightest. It was perfectly isolated from him by an energy shield and force field.

The other Heroic Spirits only dared to stay on the ship and watch. If they were contaminated by Tiamat, they would be finished instantly.

”S- Sir Setsuna, maybe you should reconsider?”

Roman watched in fear, afraid that Setsuna might overplay his hand.

”If Tiamat’s authority spreads any further, if the black mud completely pollutes the Persian Gulf and the entire sea, Uruk... no, the human order of this Singularity will be finished!”

”Once she fully awakens, she will inevitably follow her instincts—purge humanity and reshape the planet’s ecosystem. If we can’t stop her...”

Beast II, Tiamat, stood completely above the Heroic Spirits in power. Even if all the Mesopotamian goddesses joined forces, they would not be her match.

The instinct of the Beast of Regression was to remake the planet’s ecology. She wanted to create new ”children” of her own design and transform the world into what she desired.

—To her, humanity was like an alien species, something that would be cleared away in the process of planetary remodeling.

”Relax. I have my own way of dealing with Mama Tiamat.”

Setsuna took out the Holy Grail and waved it in front of her.

”Want it?”

”Aa... Aaaa—”

Sensing the enormous magical energy radiating from the Grail, urgency entered Tiamat’s voice.

BOOM!!—

The surrounding sea instantly surged into a massive tsunami. Waves hundreds of meters high, mixed with black mud, rolled toward Setsuna, a terrifying sight.

Smack—

Setsuna pressed his hand downward. The power of space forcibly suppressed the entire sea.

”No need to get so excited. Here, I’ll feed you.”

Hummm...

He released his hand. The Holy Grail slowly floated toward Tiamat and was absorbed into her body without any resistance.

The next moment—

”Aaaaa!!!—”

Another clear, soaring cry.

The magical energy within her body expanded violently. Spiritual power churned the ocean, forming a vortex over a hundred meters wide beneath her feet.

”Tiamat’s Spirit Origin strength is rising rapidly... This—this exceeds the power of every Holy Grail!!!”

Roman stared at the readings on the holographic interface in shock.

”She’s reconstructing herself! That’s Tiamat’s authority!!! You’re about to face true Human Evil—Beast II!!”

Boom—

Spiraling seawater mixed with black mud wrapped around Tiamat, forming a pillar of water that connected heaven and earth.

Her figure shifted and expanded within it.

The sky changed color. The ocean trembled.

Finally, a ”god” several times larger than Gorgon stood upon the sea.

The ship’s sensors could not even accurately scan her data. A flood of garbled symbols erupted across the display.

”That black mud is also part of Tiamat. She is the ocean itself—and the mother of all Mesopotamia.”

”A being like this... unbelievable.”

Ishtar and Ereshkigal looked at Tiamat with awe.

”Even if Commander wants to drive a big truck, isn’t Mama Tiamat a bit too big?!”

Gudako clicked her tongue in amazement.

”Aaaa...”

Tiamat’s gaze passed over them and turned toward distant Uruk.

RUMBLE!!!—

A tide of black mud roared toward the land like a world-ending flood.

”Tiamat is heading for Uruk!!! The sea level is rising!!!”

Roman calculated frantically.

The ocean surface rose at a speed visible to the naked eye. Within minutes, the coastline of the Persian Gulf was submerged, the land being relentlessly devoured.

”Be careful!!! Even if the Angeloids can hold back the Lahmu, the ocean will drown the continent within three days! At that point...”

”Tiamat must be defeated or dealt with as soon as possible!!!”

”Mm.”

Setsuna nodded.

”If we want to talk to Mama Tiamat, we need to let her vent first. After being trapped in imaginary space for countless years, finally descending upon Earth—it’s understandable she’d be a little excited, right?”

”But... this isn’t a suitable place. If Uruk is destroyed, things will get troublesome for us too.”

”Come on. Let’s change venues.”

He pressed both hands onto the sea.

Hummm...

A wave of energy spread outward, enveloping more than half the planet in an instant.

Flash!!!—

Blinding white light erupted.

Gilgamesh, who had been directing the defenses in Uruk, instinctively narrowed his eyes, shielding himself from the distant glare.

The next moment, he realized that the Lahmu battling the Angeloids along the coast had vanished without a trace.

Millions of Lahmu seemed to evaporate from the Earth in a single instant. The sea level abruptly dropped.

Tiamat and the starship in the distance were gone as well, as if removed from the world entirely.

”What is this...?!”

The golden king was utterly baffled. He could not comprehend this foreigner at all.

Those aboard the starship also found that the scenery before them had changed.

Blue sky. White clouds.

Beach and sea.

Magnificent villa-style hotels, orderly clusters of buildings, and natural scenery so beautiful it seemed unreal...

”?”

Gudako nearly thought she was hallucinating.

Within seconds, she reacted.

”This is the Theater of Domination?!”

”Commander, are you dragging Mama Tiamat here to throw a party?!”

The Theater of Domination created by Setsuna using the authorities of the Herrscher of Reason and Herrscher of Dominance existed independently of this spacetime—like a Reality Marble.

However, the rules and creations within it could be freely customized. Its scale was even greater, approaching that of a star system.

Boom—

Tiamat and all the Lahmu were pulled inside as well.

They floated in the sea, observing their surroundings in confusion, unsure of what had happened.

”You got half of it right.”

Setsuna pinched Gudako’s cheek.

”Mama Tiamat’s movements are too big. On Earth, a slight misstep and Uruk is finished...”

”Doesn’t she want to rampage? Doesn’t she want to return humanity to nothing? She can play here as much as she likes. However many planets she wants, I’ll make that many for her.”

”When she’s tired of playing, we’ll go talk to her.”

As he spoke, he waved at Tiamat in the distance.

”Take your time. Don’t get too excited.”

”Aaaa!!—”

Tiamat responded with a resonant cry.

Her authority as the Mother of Beasts drove countless Lahmu and black tides toward the land.

Flowers, grass, and trees were completely eroded, turning into parts of Tiamat.

The Lahmu originally intended to slaughter humans, but aside from Setsuna’s group, there was no one else inside the Theater of Domination.

Occasionally there were birds and beasts—creations Setsuna had casually molded with his authority.

They wandered aimlessly across the land.

”Ah...”

Ishtar and Kukulkan marveled at Tiamat’s destructive power.

Ordinary Heroic Spirits were a tier below divine spirits. Even a squad of Heroic Spirits working together could not defeat Kukulkan. And goddesses like them were several tiers below a ”Beast.”

Even bound together, they would not be Tiamat’s match.

Setsuna brought the Heroic Spirits onto the ship, had Belfast brew tea and prepare snacks, and leisurely watched Mama Tiamat wreak havoc below.

Whoosh, whoosh—

In just a few hours, the black tide swept across most of the planet, destroying all creations and submerging the land.

The entire world was covered in black mud. All life was extinguished.

Everything returned to being part of Tiamat.

”Aa, Aaaa!!—”

Mama Tiamat sang again to express her emotions.

”Oh? Let me change the venue for you. Keep playing.”

Setsuna snapped his fingers.

Flash—

The authority of the Herrscher of Reason instantly dismantled and reconstructed the entire planet.

A new world, structurally similar to Earth, appeared beneath Tiamat’s feet.

Every natural environment was refreshed, like a newborn world.

”...Aa?”

Mama Tiamat raised her head. There was even a trace of confusion in her tone and eyes.

Acting on instinct, she once again unleashed her authority. Black waves churned in the ocean, tides of mud devouring the entire world.

”Come on, since we’re free anyway, let’s have some hotpot first—consider it welcoming our new companion.”

Setsuna beckoned. Gudako, Belfast, and the Heroic Spirits gathered.

A stove was set up on the bridge. Ingredients were sliced and arranged.

Anna, Ushiwakamaru, and the others were both frightened and fascinated, unable to stop glancing out the viewport.

”Um, Master, is this really okay...?”

”Relax. Inside the Theater of Domination, she can play as long as she wants. Time itself is distorted here.”

Setsuna watched as the chaotic tide swallowed yet another planet and extended his hand.

”Once she’s had enough fun, we’ll go talk to her.”

Snap!—

The planet reset again. Blue sky, white clouds, ocean, and endless tropical rainforest stretching across the continent.

”...”

Mama Tiamat stood in the sea. Just as she was immersed in the satisfaction of regression, she blinked—and found the world changed once more.

She tilted her head.

”Aaa?!”

...

”Aa, Aaa, Aaaaa!—”

Within the world created by the Theater of Domination.

Tiamat released her authority again and again, using overwhelming tides of chaos to drown the entire world.

At first, she even created Lahmu to patrol the land, hunting the ”enemies” from her memories.

But everything inside the Theater of Domination had been molded by Setsuna. Mama Tiamat quickly abandoned this meaningless behavior.

She stood in the ocean, controlling tsunamis formed of black mud, violently eroding the planet.

Hundreds-of-meters-high chaotic tidal waves swept across the globe, coating the once-azure planet in a black-red shell.

Boom—

All life and all creations formed by Setsuna were completely annihilated, returning to Tiamat’s side.

And then, once she completed the ”regression” of the entire world—

The planet would shatter and reorganize before her eyes, reconstructing into a brand-new world.

Each newly created world differed slightly from the last. The planet’s size, continental plates, climate—each underwent subtle changes.

”Aaa?...”

Mama Tiamat looked up more than once, letting out confused, ethereal calls toward the sky.

Receiving no response, she would once again follow her instincts and begin regressing the entire world.

”...”

On the other side, aboard the starship.

”Mm. I call landlord.”

”Three with one.”

”Seventeen-card straight.”

After eating and drinking their fill, Gudako and a group of Heroic Spirits, bored out of their minds, had even gathered to play cards.

”Waaah!—”

Kukulkan and the other pre-Common Era deities were extremely curious about this modern form of entertainment.

”Uh, I feel like this process of dealing with Tiamat is different from what I imagined.”

Ishtar twirled her hair as she spoke.

”So we just eat, drink, and have fun here while Master goes to tame Tiamat?”

This could no longer be called ”strategy” or ”defeat.” It was ”taming.”

Like a parent facing a child throwing a tantrum—let her make a fuss until she’s satisfied, then talk properly.

”That’s about right. If we tried to clash head-on with Mama Tiamat, we’d just be asking for trouble.”

Gudako answered casually, wagging a finger in front of Ishtar.

”Remember this. When the Starsea Empire goes on expedition, there’s one rule you must follow: fight the enemies you’re meant to fight. If it’s not your battle, don’t rush in and cause trouble.”

”In short—listen to the Commander’s orders.”

”Oh...”

The new members nodded, half-understanding.

The previous Three Goddess Alliance had been left to them as training. But at Tiamat’s level, to ensure absolute certainty, Setsuna had to handle it personally.

”Mm... Or is it just my imagination? I feel like Tiamat is getting stronger.”

Ishtar looked at the black tide below.

She noticed that the scale of the chaotic waves was growing larger each time.

”That’s correct.”

Setsuna glanced downward.

”While Tiamat regresses the entire planet into herself, she also absorbs the planet’s energy as her own.”

”This world is constructed from Honkai energy that I shaped, mixed with some magical energy. In a sense, I’m indirectly recharging Tiamat.”

At first, Tiamat could only stand upon the black mud on the ocean’s surface—she could barely support her own weight.

As she absorbed more and more energy, her range of motion expanded.

Her movements were no longer restricted. The chaotic tides she raised became increasingly exaggerated, capable of annihilating most of a continent in the blink of an eye.

Guided by instinct, she repeated the act of regression again and again.

”Aaa?... Aa?...”

”Mama Tiamat seems to be having fun. No rush. Let’s wait a bit longer.”

Setsuna signaled everyone to keep the music going and continue the fun.

”Although... she’s regressing the world using my power. Somehow, it feels like we might end up with some strange connection.”

...

Several days later.

No one could count how many worlds Tiamat had already eroded. Her power swelled continuously through the process of regression, and the speed at which she reduced entire worlds to herself grew faster and faster.

Finally, after once again drowning a world in chaotic tides and returning the entire planet to herself—

Snap—

Setsuna casually reconstructed the planet.

”Aaa...”

This time, Tiamat did not move.

She stood motionless upon the sea, unusually restraining her authority as the Mother of Beasts.

The black mud did not spread. She did not summon her grotesque ”children.”

She simply stood like a statue upon the ocean, observing her surroundings.

”Report, Commander. Tiamat’s behavior has changed.”

The Sophon had just delivered its report when Gudako, Ishtar, and the others crowded against the viewport to look down at Mama Tiamat.

”Aaa... Aaa...”

She stood at sea for quite some time, her gaze crossing the endless ocean and stretching toward distant horizons.

Riding the tides, she toured the planet.

”What is Mother doing?”

Ishtar asked curiously.

”Has she given up on regressing the world? Does she think it’s meaningless now? Was she discouraged by Master?”

It was easy to imagine.

Resetting the world again and again, regressing it into herself—only for it to be restored before her eyes each time.

Even with Tiamat’s current level of rationality, though she could not understand what was happening, she could sense that something was wrong.

”She’s observing this world.”

Kukulkan said.

Tiamat wandered across the empty planet.

Sometimes she would stare at buildings for a long time in silence. Other times, she gazed at landscapes both familiar and unfamiliar.

Was this her former world?

While she had been trapped in imaginary space and then sealed within a dream, she did not know what her ”children” had become.

After countless regressions, her hatred and sorrow had gradually dissipated. Now she wanted to see what the world originally looked like.

”Aa, Aaa...”

Tiamat drifted over the empty planet but found no trace of civilization.

She looked down at the structures Setsuna had created, bent closer, and a trace of curiosity appeared in her violet eyes.

She carefully examined every creation upon the planet.

From mountains and rivers to flowers, birds, and insects—in Tiamat’s heart, they were all her ”children.”

Yet from these creations, she sensed a subtle dissonance.

Things constructed through the Herrscher of Reason’s authority—even if identical in form and structure—still differed subtly from those created by her own authority of genesis.

Tiamat could not comprehend where that difference came from. She gazed at them for a long time.

”Aaa...”

After wandering across the planet once more, Tiamat stood upon the land.

She looked toward the starry sky in the direction of the starship.

Then she raised her head.

And released a clear, penetrating call.

”Uh... are we playing riddles with Mama Tiamat?”

Gudako scratched her head.

”Is she trying to say something?”

”Mama Tiamat seems to be done playing. She’s regained a bit of rationality.”

Setsuna stroked his chin.

”Also, she’s sensed us. She likely realizes this world is being artificially manipulated.”

”Huh?”

Ishtar shuddered.

Fortunately, Tiamat couldn’t fly. That gave her a small sense of security.

Boom—

Setsuna leapt down from the starship.

”The timing’s perfect. I’ll go have a chat with Mama Tiamat.”

...

Whoosh—

After space twisted briefly,

He hovered not far from Tiamat.

Her body was incomparably massive. Before her, he was nothing more than a speck.

Being stared at by eyes larger than a human body and facing the pressure of Beast II—an ordinary person would have fled immediately.

Setsuna felt no discomfort at all. He waved at her.

”Yo. Had enough fun yet?”

”Aaa!...”

Tiamat’s attention was drawn to him.

She slightly raised her head and carefully observed Setsuna.

She did not move, staring at him for a full five minutes.

Just as those aboard the starship began to feel uneasy—

”Aaa... my... chi... child.”

Tiamat called out haltingly.

Her voice was gentle, yet it echoed throughout the entire world, audible even aboard the ship.

”?”

”There are plenty who want to be Commander’s daughter or wife, but wanting to be his mom? Truly worthy of Mama Tiamat.”

Gudako stared blankly.

”Standard Mama Tiamat procedure. Sit down.”

Setsuna was unsurprised.

”Aa, Aaa...”

Tiamat extended her hand and cupped Setsuna in her palm.

”Child... come back...”

—

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