Just what kind of power was this? Roy couldn't quite put it into words.
If he had to describe it—this was the ability to freely manipulate one's own body.
How far that manipulation could go, Roy wasn't sure. After all, this power had been passed down from Tiamat to him, and he didn't even fully understand how to use it yet, let alone control it properly.
Fortunately, the master of life creation herself was right in front of him, so Roy sought Tiamat's guidance on how to wield this ability.
An A-rank Body Modification skill, in simple terms, meant full control over one's own physical form. Not just superficial layers like skin, muscles, bones, and internal organs—but even down to the cellular and genetic level. All of it could be modified.
Of course, such modification had its limits.
Theoretically, A-rank was the pinnacle of what a Servant could achieve in body alteration. However, compared to EX-rank, which approached divine levels, it still fell far short.
With Roy's current physical and soul capacity, obtaining A-rank body modification was already pushing the upper bound.
To ascend to divine-level body modification would require sharing Tiamat's Authority over life—not just physical strength, but a soul elevated to godhood. That was a different league entirely.
But for Roy right now, even just A-rank was more than enough.
At the very least, it meant he could fix this fundamentally defective homunculus body of his, giving it proper organs, a natural lifespan, and normal human genetics.
More than that, he could enhance his muscles and bone density, boosting his physical capabilities and finally shedding the label of being a "frail artificial being."
Tiamat was endlessly patient and gentle.
She answered every question Roy had—if she knew the answer, she would never withhold it.
"All in all, this power is something you'll need to train with over time to master. There's no shortcut. Mom really wants to help you master it instantly, but… Mom can't do that…"
Tiamat said, her head drooping in disappointment as she spoke.
"If it's hard to control, that just proves how powerful Mom's ability is—and how useful it'll be to me, right?"
Roy earnestly reassured her.
"Ah! I see!"
Tiamat smacked a small fist into her palm, suddenly enlightened.
Roy nodded subtly to himself.
He felt like he was starting to understand how to communicate with Tiamat a bit better.
"Yaaaawn…"
Right then, Tiamat let out a small yawn, rubbing her eyes like she was getting sleepy.
"Roy, Mom's been a bit busy lately, so I won't be able to stay with you all the time. You have to take care of yourself, okay?"
She nodded with that dazed, sleepy look—clearly fighting the urge to doze off.
"Then go ahead and rest, Mom."
"Mmm-hmm. Roy is such a sweet child… But if anything happens, you can always call for Mom, okay? No matter how busy Mom is, she'll come to help you, yawn…"
Tiamat let out a huge yawn and then crawled over to Roy, clinging to his chest.
Soon after, her body began to turn transparent and gradually melted into Roy's body, disappearing from the bedroom entirely.
'So that's how she entered my body in the first place?'
Roy looked himself over in surprise.
With Tiamat residing within him, he now had an unimaginably powerful trump card up his sleeve.
Still, he figured Tiamat probably wouldn't adapt well to other worlds' rules. Best not to expose her recklessly. If something went wrong and he lost her… he'd never forgive himself.
But if Tiamat could adapt to other worlds…
Hiss—should I feed the Holy Grail to Tiamat?!
Roy jumped at the thought, cold sweat running down his back.
I just called her "Mom" and I'm already trying to turn her into a cheat code? I'm scum!
He quickly shook his head to banish the disturbing idea.
Still… Tiamat had said she was "busy."
What exactly could she be busy with?
Roy decided to investigate. He checked every part of his body, but found no trace of Tiamat.
After a long moment of thinking, he smacked his forehead and turned inward—probing his own soul.
As expected—
Inside his soul, he discovered a newly-formed imaginary number space.
Floating at its center was a massive silhouette.
Unlike the youthful form she had appeared in before, this version of Tiamat was a gigantic dragon, cradling a crimson sphere in her arms—carved in the likeness of a woman.
"The Black Holy Grail…?"
Roy immediately recognized it and was taken aback.
When he awoke on the Outside Side of the World, the corrupted Holy Grail had vanished from his hand. He'd assumed he lost it during a moment of unconsciousness. But now he realized—Tiamat had taken it.
Why would she be holding the corrupted Grail?
Roy studied her for a while, but Tiamat didn't react. Her eyes were shut, and she remained still—deep in a dreamlike slumber.
Then Roy noticed something odd.
The Holy Grail in her arms was pulsing with red light—rhythmic and intense.
At first, Roy assumed it was the Grail fulfilling Kotomine Kirei's wish. But the longer he watched, the more he realized something was off.
The red light wasn't activity—it was resistance.
Tiamat was interfering with the Grail somehow.
Is she trying to destroy it? No… with her power, she could smash it in one blow. There'd be no need for such slow work…
Then it hit him:
She's purifying it?
The more Roy thought about it, the more plausible it sounded.
Tiamat was likely purifying the Holy Grail, and the curses within it were instinctively fighting back.
Not resisting out of will—Angra Mainyu's soul had already been obliterated—but reacting like parasites to a foreign body.
The curse of "All the World's Evil" was conceptually potent. Even someone like Tiamat would need effort to neutralize it—especially since she wasn't a purification-specialized deity… probably.
Roy retracted his awareness from the imaginary number space and opened his eyes in the bedroom.
If she's really purifying the Grail… then she's doing it for me, isn't she?
A warm sensation bloomed in Roy's chest.
Despite only just meeting him, Tiamat was already helping him in every way she could. That unconditional love from "Mom" truly struck him.
It also made his earlier "feed the Grail to Tiamat" idea feel even more shameful.
"Alright then… time to test out the power Mom gave me!"
Roy slapped his cheeks to stay alert, then sat up, placing his fingers on his skin.
"Wait a second, why the hell would I test it on myself first?! What if something goes wrong?!"
Moments later, he climbed out of bed and headed to the courtyard.
He spotted an ant crawling on the ground, picked it up, and placed it in his palm. Then he channeled magical energy into it.
Instantly, the entire biological makeup of the ant unfolded before his "eyes"—organs, tissues, everything perfectly visible.
Even biology class dissections never looked this clear.
Roy began using the power gifted by Tiamat, cautiously starting his first trial at biological modification…
***
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