Multiverse: The Daily Life of the Inventor in another world

Underworld:

"I see... so this genius teenager managed to summon Serafall to be his contractor, and she's bound by their agreement to keep his work a secret," Sirzechs said thoughtfully, reviewing the report.

He was considering how to keep the arrangement beneficial while also finding a way to profit from it. If his queen's information was accurate, the devil kind could gain immensely from this.

"And Serafall wanted to make him her queen, but he turned her down?" Sirzechs asked, double-checking the most important detail.

"Yes, he's rather prejudiced against the entire Evil Pieces system," the silver-haired devil confirmed with a nod.

"I see... well, that doesn't matter much. He might change his mind when he's older."

'Then he'll find a way to get eternal life... I've already warned you about how dangerously intelligent he is!' Grayfia seethed silently. Sirzechs didn't seem to grasp how truly lethal Marcus was.

It was eerily similar to what happened with the original Lucifer! He had underestimated the threat of Solomon simply because he was human—and look where that led them? Solomon nearly conquered them single-handedly! A lone human had walked into their world, taken a few devil ladies, and left as though he owned the place.

"As you wish. I've completed the mission you requested," Grayfia said, bowing slightly.

"Yes, now I'll need to contact Serafall and request one of those... 'signers' for myself."

"I suggest we reach out to him through her. It's better if he doesn't think Serafall has failed on her end. Instead, we can frame it as you noticing her increased productivity and wanting the same results," Sirzechs' queen advised, repeating what Marcus had just conveyed to her via their mental connection.

"Exactly what I was thinking."

Sirzechs smiled at the thought of reducing his paperwork while letting Serafall maintain her contract with this brilliant human inventor.

He had no idea how the teenager had managed to summon a Satan, but if he could do that, his potential was definitely worth noting.

The Crimson Lucifer made a mental note to keep an eye on this human for future recruitment, while Grayfia kept a neutral expression. She attempted to resist her bindings—but the moment the thought crossed her mind, she immediately forgot about it.

The power of Ars Goetia—or whatever subspecies it was—was simply too overwhelming, particularly against her very essence. As Serafall had mentioned, the maids' bindings were even stronger due to her being from a house directly serving Lucifer, which gave her a much deeper connection to Ars Goetia.

~~~~~~Back in the 'Garage'~~~~~~

"Did you just say Grayfia's been added to the fold?!" Venelana's legs gave way, and she collapsed onto the couch.

A member of her family had been pulled into this mess! She quickly realized that the pillars were virtually identical to Lucifer's, meaning the Gremory pillar was still vacant. Marcus could summon either her son, the leader of their race, effectively making him the de facto ruler of the Underworld—or he could summon her daughter. Both scenarios were deeply concerning.

"Yes. Turns out she's been tracking my movements for months. The Diamond Core Heart has been masking me from her, which is impressive," the brown-haired devil lady explained.

Venelana nodded slowly. She thought of the massive diamond downstairs—the size of a basketball—and wondered whether something like this could be installed in Kuoh to protect her daughter. But how should she approach it?

A deal? Money? She snorted at the idea of offering cash to someone who could make jewelry from clay...

Perhaps an exchange of magical knowledge? But Marcus could just ask her anything, and she'd tell him regardless.

Then she considered it from a different angle: What if he visited Japan, specifically Kuoh? Since it was devil territory, stray devils appeared there more frequently.

In the end, she decided that, as an inventor and researcher, he would likely be most interested in gathering "data."

"Lady Leviathan…"

"Call me Serafall. We're pretty much connected for life," Serafall said, sighing as she flipped through a book on cars.

"Then Serafall, now that Sona and Rias will be attending school in the human world, have you thought about asking our Master to install something like a Diamond Core Heart in Kuoh?" Venelana asked.

"I did the moment I saw its effects. We knew the moment Grayfia was in the city. Where she thought she was tracking us, we were actually tracking her… It's the most advanced territory surveillance magic I've ever seen."

"So, yes. I've been thinking about it for weeks now," Serafall sighed.

"We could easily get him to Japan as part of a field expedition. But installing another Diamond Core Heart would be impossible. There's nothing of value for him there," Serafall said, another sigh escaping her.

"We could just tell him he can leave one behind to collect data," Venelana suggested thoughtfully.

"...That...might work... I'll ask him after one of our sessions," Serafall said, giving a lewd smile.

"Good. The more interest he has in Kuoh, the better for our family there."

"Indeed. Have you thought about asking for his help with Rias' marriage contract?" Serafall raised an eyebrow.

"No... I haven't. I hope Rias will think of something now that she's moving to the human world," Venelana sighed.

Serafall said nothing but wore the look of someone who didn't believe Rias would do anything at all. The girl was simply running away, unlike her younger sister, who was just going for field experience.

But, of course, it wasn't Serafall's business. She'd try to help Sona if possible, but her priority was ensuring Marcus didn't view the devils as enemies.

"Time will tell."

~~~~~~Marcus~~~~~~

Now that Grayfia was in place, I could breathe a sigh of relief. With her around, I would know exactly what Lucifer was up to.

This incident had been a close call, reminding me that I needed to have some means of defense and be prepared to go on the offensive if necessary.

So, the moment she left, I dove back into my work on the guns.

The designs were almost complete. I quickly discarded the Glock design since the circles weren't going to fit. That's why I opted for hand cannons.

A larger, bulkier version of the Desert Eagle for the right hand, and a more slender, sleek one for the left.

After all, they were Castor and Pollux.

And since Pollux was female, the left-hand design was slightly more slender, with a shorter barrel. Since my left hand was less dominant, I planned to use the scatter mode more often with it than my primary hand.

The guns themselves featured star inscriptions. Along the barrels were carvings of stars and wind, symbolizing that the stars would guide the way… to their targets' graves.

While the design was finished, I wouldn't be using them just yet. Instead, I opted for something simpler to begin with.

It was time to test them. I had a feeling that I wouldn't be able to use them as they were—the recoil would be significant.

A few minutes later, the pair of guns were finished. One of the perks of having an assembly line was efficiency, though the downside was the need to constantly update and upgrade. But in the end, it was worth it.

"Is this it?"

"Yeah, Castor and Pollux."

"You named them after the Greek twins? I wonder what they would think if they knew a Pseudo Sacred Gear was named after them," Serafall chuckled.

"I'm not sure. I named the first gun after the word 'cast,' as in casting magic bullets, so 'Castor.' But then, I realized I was making a one-handed weapon, so I decided to make another. Remembering the Gemini twins, Castor and Pollux, I named the second gun 'Pollux.'" I explained the reasoning behind the names.

"I see... I never would have guessed it came from such a simple idea," the black-haired Satan said, intrigued.

"Well, it came up naturally, so I'm sticking with it. As for how the Greek Pantheon will react… I don't care." I shrugged before picking up Castor.

I frowned. Even with an experimental metal alloy, the gun was still quite heavy—just as heavy as the original Mark II.

The alloy had made it lighter, but the difference in size didn't reduce the weight.

The real issue, though, was the recoil. That's why I needed to test it before figuring out how to fix that.

"Well, I doubt they'll react too harshly... probably... I'm not sure," Serafall replied, though even she seemed uncertain.

"Aren't you responsible for the diplomatic side of things?" I asked, inspecting Castor and gauging its weight.

"I am, but the Greek Pantheon is ancient. They don't care much for our faction, calling us 'newcomers.' I wouldn't know how they'd react to a Sacred Gear named after one of their lesser gods when the more prominent ones don't have one. It could go either way. Zeus would probably get jealous. Poseidon might join in, just for fun."

"As I said, it would be a coin toss, depending on their current mood," she explained, speaking quickly.

"Wow, I had the same thoughts. And I haven't even met them yet!" I said, shaking my head. The myths weren't too far from reality.

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