Shinji Matou at Your Service

In a room on the top floor of the Millennium Castle Brunestud, closest to the corridor.

Arcueid was sitting while Shinji was standing, the latter talking.

"Do you realize your mistake?"

"I do."

"What was it?"

"I don't know."

"You... forget it."

Shinji sighed helplessly.

"You don't know? I'll tell you. You are an executioner of True Ancestor, specifically meant to kill fallen True Ancestor, not humans. If you can't even distinguish between a vampire and a human, how can you be a proper executioner?"

"..."

"Hey, hey, are you listening?"

"Yes, yes, I'm an executioner of vampires, right."

Arcueid stuck out her tongue and grinned, "Hehe, I like the title Executioner."

"Don't laugh, and keep your hands still. Sit properly, with your hands on your lap!"

Shinji was fuming. When he watched Tsukihime, he thought Shiki was too harsh on Arcueid, completely lacking the gentleness he showed to others. Now, he realized it wasn't that Shiki didn't want to be gentle; he simply couldn't afford to be. A moment of inattention and this girl would get carried away.

When ordinary people get carried away, it might not lead to major problems. But if Arcueid did, it could spell disaster, given she was a God of the Present World who could alter reality at will, possibly even on a level higher than a god.

Regardless of whether she would leave in the future, Shinji had decided to teach her properly while she was awake, to prevent her from causing trouble.

Of course, Arcueid didn't appreciate Shinji's good intentions.

Her eyebrows raised, and her expression fell, pouting in dissatisfaction.

"I almost killed you, so what? You're such a petty person. Didn't you almost kill me too?"

"Please, my princess, I'm human, not a True Ancestor. Humans are very fragile, unlike you True Ancestors who can revive even if you die — even if my Harpe cut you in half, it wouldn't matter much to you."

No matter how effective the Refraction of Longevity effect of the Harpe was, it couldn't compare to the conceptual power of the Mystic Eyes of Death Perception. Arcueid could survive even after being divided into seventeen parts by the Mystic Eyes of Death Perception, so the Harpe was nothing.

"Hmph, reviving comes at a cost, and that's not what I meant."

"Then what?"

Shinji was genuinely curious.

"It's that blue light. Because of it, my head is now filled with all sorts of noises, all jumbled up. If it gets worse, I might not even be able to maintain my consciousness."

"Is it that serious?"

Shinji didn't believe it. It wasn't even the complete Third Magic, how could it be so powerful?

"Yes, it is."

Arcueid nodded repeatedly.

"What exactly is it? Rune and Kabbalah secret arts are ineffective against me due to my immunity. Since I haven't developed an immunity — meaning I haven't experienced this type of magic before — it must be something from the East or South America. But could the secret arts from there hurt me this much?"

Arcueid's unique constitution made her naturally develop immunity to magic she had experienced, rendering such magic ineffective. Only extremely high-level mysteries could harm her effectively, such as Mystic Eyes of Death Perception, Distortion Mystic Eyes, or ancient Age of Gods' Mysteries.

"Hey, hey, tell me."

Arcueid grabbed Shinji's arm and shook it back and forth.

"Alright, alright, I'll tell you. Just stop moving, I'm getting dizzy."

Mainly, he was hungry. Since waking up, he had been exploring and battling without eating anything. Not being hungry would be strange. Unlike the True Ancestor in front of him, supported by Gaia, who didn't need to eat and was always energetic and full of vitality.

"Here, it's this."

Shinji extended his hand in front of Arcueid, and with a flash of blue light, the magic activated.

However, just as Arcueid let out an "Oh" of amazement, Shinji's face suddenly turned pale, and he swayed, collapsing forward.

Naturally, he was caught by Arcueid.

"Uh? What's going on?"

Shinji looked at his hand in confusion, pressing his other hand on Arcueid's arm to steady himself.

He didn't feel anything wrong, so why did activating the magic give him a headache? It was like the feeling at the final moment of his confrontation with Arcueid when the blue of his soul and the pure white world burst open.

"Am I... injured?"

"It seems so."

Arcueid brought her face close, staring intently into Shinji's eyes.

"It should be similar to the injury I received. When you activated that just now, I felt my Marble Phantasm... hmm... ouch!!!"

Towards the end, Arcueid's expression also showed pain.

"What about you?"

"I just tried to activate Marble Phantasm, but the noise in my head got louder. It seems your magic and my Marble Phantasm interfere with each other, causing both of us to be affected."

"I see—then why does my head stop hurting now, but yours still does?"

"Maybe it's the difference in the scale of abilities. Your ability is smaller in scale, so the impact is less. My ability is larger in scale, so the impact is greater."

"That makes sense."

Shinji carefully recalled the final clash. Wasn't it just two abilities interfering with and affecting each other? He probably saw Arcueid's memories because of this.

"So Marble Phantasm and Third Magic can affect each other?"

"Wait, what did you say?"

"I said Marble Phantasm and Third Magic can—"

"Third Magic? You used Third Magic?"

Arcueid showed clear surprise.

"Yes, but it's only a small part, still far from being a true magician. It's mainly about thoughts and emotions."

"Third Magic... thoughts... emotions... all related to the spiritual realm. They can interfere with each other. Magic is indeed fascinating."

Arcueid furrowed her brows, then quickly relaxed as if she'd understood.

"That must be it. I can faintly hear pronunciations like 'brother' and 'Shinji'. Is that your name?"

"Yes, sorry, I haven't introduced myself yet. I'm Shinji Matou, a magus with a small grasp of the Third Magic."

Shinji smiled and extended his hand to her.

This time, Arcueid sensibly shook Shinji's hand.

"I'm Arcueid, Arcueid Brunestud. Just call me Arcueid. As you said, I'm the executioner of vampires, a True Ancestor. Nice to meet you, Shinji."

"Nice to meet you, Arcueid."

PS: Correction, there's no concept of mystery in terms of effectiveness. It's just that mystery becomes ineffective in the face of a greater mystery, leading to derived concepts.