Upon hearing that the process would require at least nine months, Kagaya could not help but let out a sharp, involuntary breath. The sheer scale of the undertaking was daunting.
However, Horitake was quick to pour cold water on any lingering optimism.
"Nine months is the estimate for the most ideal, extreme scenario," Horitake clarified, his expression grim. "That assumes, for instance, that each Hashira only requires a single vial to replenish their life force. If Sister Shinobu and I were to devote ourselves entirely to the research, we could potentially produce one vial a month on average."
He paused, shaking his head. "But such a grueling pace is simply not sustainable in reality. No two Hashira are the same. The timing of when they manifest their Marks, the intensity of those Marks, and the sheer physical strain they exert while using them—all of it varies. Consequently, the toll on their lifespan varies as well."
"Some might be lucky," he continued. "One vial might be enough to bridge the deficit. But others might be less fortunate—or perhaps just more reckless in battle. For them, a single dose might not be enough to claw back what they've lost. Therefore, calculating for exactly one vial per person is inaccurate at best."
Horitake shifted his weight, his voice reflecting the weight of his responsibilities. "Then there is the matter of our time. It is nearly impossible for Sister Shinobu and me to churn out a vial every month. Her work isn't limited to research; she is the heart of the Butterfly Mansion, healing the wounded, and she is frequently called away on specialized missions. I am no different. As the Heaven Hashira, I have duties I cannot abandon."
He sighed. "We cannot live in the laboratory. Our progress will be intermittent—starts and stops, pauses for duty. Realistically, if we manage one vial every month and a half, we'll be doing well."
"And there is one more thing we must consider," Horitake added, his eyes narrowing. "We cannot focus solely on the Hashira. It is entirely possible that a common swordsman, pushed to the brink and possessing enough latent talent, could also manifest the Mark."
His words struck a chord with both Shinobu and Kagaya.
Kagaya nodded slowly, a trace of melancholy in his voice. "You are right. While the Hashira are the strongest face of the Demon Slayer Corps, there are many among the ranks who possess extraordinary potential. The possibility of them awakening the Mark cannot be ignored."
Horitake's tone grew more troubled as he pressed the point. "That's exactly the issue. we can't just prepare for the Hashira. We need a reserve—a safety net for those ordinary soldiers who sacrifice their longevity for a moment of power."
As he spoke, Horitake's mind drifted to Tanjiro. Among the rank-and-file, he was certain the young boy would manifest the Mark. After all, wasn't he the protagonist of this story?
Resting his chin on his hand as he sat cross-legged on the tatami mat, Horitake looked truly distressed. "Taking everything into account, I estimate the Corps needs nearly twenty doses of the life-replenishing elixir. Even if we strip that down to the absolute bare minimum, we need fifteen."
"Fifteen vials..." he murmured. "That represents over a year of work for Sister Shinobu and me."
A heavy silence fell over the room. Over a year. For two active Hashira, that was an eternity. For the Demon Slayer Corps, which lived on a knife's edge, it was a gamble they couldn't afford to take lightly.
Breaking the silence, Shinobu spoke up, her voice calm and accented with a small, reassuring smile. "Actually, it might not take quite that long. The initial stages of brewing are complex and difficult, yes. But as we become more proficient with the process, the time required for each subsequent batch will naturally decrease."
"I've already factored that in," Horitake countered gently. "It's true, our speed will improve. But Sister Shinobu, we can't spend every waking hour making medicine. We have lives, we have duties, and we must continue our own training. If we bury ourselves in the lab, who will slay the demons? Who will save the patients in the Butterfly Mansion? And what of our own strength? If we don't sharpen our skills, will we be ready when the time comes?"
He looked down at his hands. "Speaking for myself, I don't believe I am strong enough yet. We cannot afford to be hyper-focused on the elixir at the expense of everything else. And that, inevitably, stretches the timeline back out."
Shinobu hesitated, a look of uncertainty crossing her delicate features. "Could we... perhaps add more people to the team? A few more professionals to assist with the production?"
This was the very question Kagaya had been preparing to ask. Both of them turned their gaze toward Horitake, waiting for his judgment.
Horitake looked pained. "Sister Shinobu, Oyakata-sama... an elixir that can grant life and extend one's years is a dangerous thing. If word of its existence were to leak, the consequences would be beyond my imagination. The fewer people who know, the better. The fewer hands involved, the safer we are."
He looked between them, his gaze intense. "The three of us knowing is already a risk. Bringing in outsiders is out of the question. However... if we truly find ourselves short-handed..."
He took a breath, conceding slightly. "Sister, you could bring Aoi into the fold. Let her act as our assistant. I trust her. But as for anyone else? Absolutely not. They either lack the skill, or quite frankly, I don't trust them enough with a secret of this magnitude."
Shinobu let out a small sigh of relief. "With Aoi helping me, the progress will definitely accelerate. It could cut our production time by at least a third."
Kagaya, having listened intently, reached a conclusion. "This elixir is of paramount importance. It must not be leaked, even if it means we produce fewer doses than we hope for. Horitake's caution is justified. Allowing Aoi of the Butterfly Mansion to assist is a reasonable compromise. Beyond that, however, there is one question that remains."
Horitake and Shinobu asked in unison, "What is it?"
Kagaya looked at them both with profound seriousness. "The question is: how much time do we have left before the final battle?"
Shinobu remained silent, unable to offer an answer. Both she and Kagaya turned their eyes toward Horitake. They were well aware of his uncanny ability to seemingly peer into the threads of the future.
Horitake went quiet, mentally calculating the trajectory of the events he knew were to come. "We have roughly two to three years before the final confrontation," he answered, his voice tinged with uncertainty. "At the very most, it won't exceed five."
Kagaya nodded, his expression resolute. "Then let us work with the shorter estimate. Let us assume we have two and a half years. Within that window—accounting for your lives, your training, and your missions—is it enough time to produce fifteen vials?"
A sense of clarity washed over Horitake and Shinobu. They began to calculate rapidly in their minds.
If they looked at it through that lens, the time was indeed sufficient. In fact, the timing was almost perfect.
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