"Splendid. Jin, was it? Your mental waves possess more than enough frequency and intensity to manipulate ether."
No. 700672 narrowed his eyes, sounding genuinely pleased.
"This means that every Automata you create possesses that same capability."
Facing Jin's puzzled expression, No. 700672 spoke with the patient, guiding tone of a mentor.
"No matter the organism, mental wave intensity alone is insufficient to manipulate ether across a distance. That is why we use a 'mental catalyst,' however..."
No. 700672 trailed off, hesitating for a moment. Jin voiced the thought that had just surfaced in his mind.
"...Could it be that a 'mental catalyst' also exists naturally within the brains of certain creatures?"
No. 700672 could not hide his astonishment.
"To 'see the whole from a single fragment'—you are exactly as described. Yes. A mental catalyst is obtained by collecting and processing the brains of specific organisms. Anyone capable of wielding magic possesses a trace of it, but..."
"But the amount gathered from a single creature must be minuscule," Jin finished for him.
No. 700672's surprise deepened. "Precisely. If one were to attempt to gather it from the lifeforms on this planet, they would likely require brains by the millions."
Jin had no intention of committing such an atrocity, nor would he ever permit it.
"...The lifeforms on this planet?" Jin caught the hidden implication in the phrasing.
"Perceptive. Indeed, my own artificial brain contains a vast quantity of that catalyst."
Then, his gaze turned searching as he watched Jin. "And so does the Failure. What will you do? If you were to extract my brain, you would have an easy source of mental catalyst. You might even be able to stop the Ether Stampede."
Neige silently moved to stand in front of Jin and Reiko. To extract No. 700672's brain was synonymous with killing him.
Jin reached out and lightly patted Neige's head, a reassuring smile on his face. "I'm not that kind of pathetic human. As a Magi Craft Meister, I'll find another way."
No. 700672 stared at him for a moment before letting out a silent chuckle.
"Heh, you truly are a fascinating man. Neige, there is no need for worry. It seems he has no intention of harming me."
Neige's tension finally dissipated.
"As you have likely surmised, the Failure may be using mental catalysts harvested from the other eleven units."
Jin's expression grew grim. They didn't know how many times the Ether Stampede had been triggered during the Great Magic War, which meant they had no way of knowing how many charges were left. In the worst-case scenario, it could still be used nine more times.
"Rather than worrying about how many uses are left, I'd rather focus on making sure it never happens again."
That answer clearly pleased No. 700672.
"A fine sentiment. To a man such as you, perhaps I can share the truth."
"?"
"Our kind—the homunculi—are vulnerable to low temperatures. If the environment drops below freezing, we cease to function."
"..."
It was unexpected information, but Jin merely tilted his head in thought.
"Is something wrong, Jin?"
"I was just... questioning the veracity of that information."
"Oh?"
"If you've based yourselves in these high latitudes, there must be a reason for it. The abundance of ether is certainly one, but if you were truly that vulnerable to the cold, I imagine you would have moved further south. You certainly have the means to collect ether wherever you go."
No. 700672 nodded deeply and repeatedly, looking satisfied with the explanation.
"Exactly. Forgive me for testing you. I have seen now that you do not harbor even a shred of malice toward me."
"You were testing me?"
"I apologize. But it has confirmed that you are worthy of my trust."
No. 700672 gave a command to Neige, who brought over a small metal box roughly the size of a business card case.
"This is for you."
"What is it?" Jin asked as he accepted the box from her.
"A mental catalyst. The last one I possess."
"What?! I couldn't take something so precious..."
No. 700672 looked Jin straight in the eye. "No. Dealing with the Failure was a duty I should have inherited from my masters. Since I am entrusting that task to you, this is only right."
Jin gazed at the box for a long moment before offering his thanks in a quiet, steady voice. "I will make good use of it."
No. 700672 nodded once and explained the procedure.
"If you intend to use it in a Magi Machine rather than a living organism, you must add it to a light-attribute magi-crystal. Can you manage that?"
"I can use Fusion or Add."
"I am unfamiliar with those terms, but if you say you can do it, then I believe you. Use a ratio of one part catalyst to one hundred parts magi-crystal."
Doing so would allow the generation of mana waves perfectly suited for ether interference.
"You likely already understand this, but while all-attribute magi-crystals are indeed versatile, it is far more efficient to use a magi-crystal of the specific attribute matching the magic you wish to cast."
"I understand."
Jin's mind immediately went to audio speakers. Full-range speakers had their advantages, but they had limits to their output—or in magical terms, their intensity. In Reiko's case, her "limit" was already far beyond anything a human could achieve, but the principle remained. In high-end audio, one used a multi-way system: tweeters for high frequencies and woofers for lows.
Magic was even simpler. Fire-attribute magic was best handled by fire-attribute magi-crystals, and water-attribute magic by water-attribute magi-crystals.
"It seems you've grasped the concept. Now for the final point; whether you can comprehend this is entirely up to you."
With that preface, No. 700672 moved on to a more advanced theoretical explanation.
"It concerns the medium through which mental waves and mana waves travel."
Jin listened in silence.
"Even my masters did not fully solve the mystery, but they believed there are particles even smaller than ether, and that waves travel through this medium."
"Something like an 'under ether,' then."
"Oh! Precisely. You really are quick to understand."
If under ether was compared to air molecules, then ether was like a balloon. The flow of air—the wind—could move the balloon. The way mental waves manipulated ether followed a similar relationship.
Mana also traveled through this under ether. Theoretically, if one could manipulate the under ether itself, they could counter any form of magic.
"Well? Do you understand?"
"Yes. At the very least, I can see a path forward."
"Oh?"
"I've been considering several ways to counter the Failure. I was worried about it using teleportation to escape, but I believe I have a solution for that now."
No. 700672 nodded. "Jamming the teleportation? Naturally, you know of teleportation markers? You simply need to radiate mana identical to the marker's signature."
Based on the theory he'd just learned, Jin was confident he could create the necessary mana waves.
"Yes, that's what I'm thinking. Essentially, I'll overwhelm the area so it can't distinguish the signal from the teleportation marker."
"Exactly. You truly are magnificent. At this rate, you might actually be able to handle the Failure..."
"I've intended to do so from the start."
No. 700672's face contorted slightly in what looked like a smile.
"Looking at you reminds me of my masters. Back when they were still full of vigor and ambition."
His eyes narrowed as he looked toward something far beyond the walls of the room.
"Thank you for everything."
Jin expressed his gratitude and announced his departure. He still had many questions, but time was short. He had a mountain of work waiting for him back at the laboratory.
As Jin turned to leave, No. 700672 called out to him again.
"...Forgive me, but could you come a little closer?"
His voice was stiff, different from the tone he'd used until now.
"What is it?" Jin walked back to the bedside.
No. 700672 stared intently at him in silence for a moment, then reached out his right hand and took Jin's.
"U-um?"
But he released Jin's hand almost immediately and lay back.
"...Forgive me for stopping you. Jin-dono... regarding the Failure, I apologize for forcing the cleanup on you, but I must ask you to handle it."
"That's the plan."
"...Will you return?"
"Yes, once this is all settled."
"I look forward to it. I enjoyed our conversation."
"Until then."
Jin stepped out of the room. Left behind, No. 700672 lay still, staring at the ceiling—or perhaps at the sky he knew lay beyond it.
"...Father?" Neige, the white girl, called out to him with concern.
"...Neige, for a moment, I felt as if I had returned to the past."
"What do you mean, Father?"
"...That young man, Jin... he reminds me so much of my former masters. Even though his face and build are entirely different."
No. 700672 narrowed his eyes with nostalgia.
"I was granted knowledge by the master who created me, and I passed that knowledge to his children, and then to his grandchildren. He made me remember those days."
No. 700672 closed his eyes. Neige watched over her creator with a sorrowful gaze.