Erika stood right in front of Akira in a small clearing behind the house. Shinobu, Tamayo, and Yushiro were watching from the side. It was nighttime, but the moonlight illuminated the area enough to make everything visible.
Akira looked at her and asked, "Are you ready?"
"Yes," Erika replied confidently.
"Alright then," Akira said, stepping back slightly. "Do it."
Erika took a deep breath, muttering softly, "Blood Demon Art: Blindfold."
In the next moment, her body began to fade. Slowly, she vanished from sight right before everyone's eyes, everyone except Akira, whose Mystic Eyes could still see her perfectly.
Shinobu and Tamayo's eyes widened in surprise. "She actually did it," Shinobu murmured, impressed. Even Yushiro, though trying to look uninterested, glanced at the spot where she had just disappeared.
A few seconds later, Erika reappeared, breathing lightly but smiling. "Did I do it?"
Akira smiled back. "Yep, you actually did it."
It had been a few days since Erika's "birthday," and both she and Akira had been working hard, Akira on his research, and Erika on mastering her blood demon arts. With Akira's help and by sharing his Mystic Eyes through Yushiro's blood demon art, she had trained nonstop. It took her some time to adapt to his vision, but now she could handle it for a few minutes.
And today, she had finally succeeded in learning her first Blood Demon Art, Yushiro's Blindfold. Originally, she had been trying to replicate Tamayo's blood demon art, but after using Yushiro's blood demon art to share Akira's vision for a while during her training, she managed to learn his blood demon art instead.
Tamayo said with astonishment. "It's actually impressive. Even Yushiro needs to use his talisman to perform it, but you managed to do it without one. I wonder how you did it."
Yushiro, as usual, looked irritated but said nothing.
Erika smiled modestly. "I just used my own blood inside my body as the medium. But I can only turn myself invisible with it. If I wanted to use it on others, I'd probably need those talismans too."
Akira nodded. "Makes sense. What about the vision-sharing ability? Can you try that?"
Erika thought for a moment. "Let me try something." She closed her eyes for a moment, focusing deeply. When she opened them again, Akira suddenly felt a strange connection form between them. His vision flickered, and before he realized it, he could see himself standing in front of him.
But the world looked different. The usual radiant colors that filled his vision through his Mystic Eyes were gone. Instead, everything looked normal, the way it did in his previous life. Yet at the same time, his sight pierced the darkness easily, everything appearing brighter and sharper.
It didn't take long for him to realize he was seeing through Erika's eyes. And not only that, he could still see through his own vision at the same time.
"You actually did it," Akira said with a grin in amazement.
Erika blinked and then canceled the connection, smiling proudly.
Tamayo asked curiously, "Did you just share your vision with Akira-san?"
"Yes," Erika replied.
"But didn't you say you'd need a talisman to use your art on others?" Tamayo asked, a bit puzzled.
Erika tilted her head. "I don't know how, but I can use it on him without one. Maybe… because I've drunk too much of his blood?"
Akira chuckled. "Yeah, that actually makes sense."
Before either of them could continue, Selena's voice echoed telepathically in both their minds. "Actually… hmm, no, forget it. I'm not sure yet."
Both Akira and Erika exchanged confused looks, wondering what she was about to say, but in the end, they decided to leave it for later.
Akira grinned, a mischievous glint in his eyes. "We can do a lot of things with this, hehe…" he said with a low, evil laugh.
Erika blinked. "Like what exactly?"
Akira's grin widened. "Think about it, we can use this for aerial reconnaissance. If we attach the talismans to our crows and use my Mystic Eyes to read the energy signatures of demons, we can basically scan entire areas from the sky. It'd be like infrared aerial photography, we'd be able to find hidden demons anywhere."
Shinobu, who had been quietly observing, raised a finger and said, "That's actually a really good idea. It would make locating demons during the day much easier; we could eliminate them before they even come out during the night and attack anyone."
Erika looked a bit unsure. "That sounds good, but… I don't think I can handle that much yet. For now, I can only maintain the connection for about fifteen minutes."
Akira nodded. "Yeah, that makes sense. Unlike demons who have infinite stamina, you're still part human; your body has limits." He crossed his arms thoughtfully. "How about this: after we identify an area where we already know demons exist and we can use this method to pinpoint their exact locations. That way, you won't need to hold it for too long."
"That sounds reasonable," Erika smiled faintly. "Yeah, I can do that much."
Shinobu nodded approvingly. "That's a solid plan. We should report this idea to Oyakata-sama, along with the demon vaccine we developed."
Erika blinked. "Wait, you finished the vaccine already?"
Akira tilted his head. "Didn't I tell you about it last night?"
Erika scratched her cheek, looking embarrassed. "You did? Sorry, I was too exhausted from training to remember."
"Yeah, that tracks." Akira chuckled, but deep inside, he did think she was probably too busy drinking his blood, but he didn't say it out loud because he had a feeling it would endanger his life.
"So," Erika asked curiously, "can it turn a demon back into a human?"
Akira shook his head. "No, it's not a cure, it's a vaccine. It can prevent someone from turning into a demon, but it can't reverse the process once it's complete. We'll still need more blood from an Upper Moon demon to work on the cure."
"So… no new demons can be born?" Erika asked hopefully.
"Kinda," Akira said slowly, his tone turning serious. "But there are some complications."
Erika tilted her head. "What kind of complications?"
"Well, how should I put it…" Akira hesitated, glancing at Shinobu before answering. "This vaccine works a bit differently from a normal one."
Shinobu folded her arms and said flatly, "It kills the person turning into a demon."
Erika froze. "What…?"
Akira sighed. "Yeah. The thing is, Muzan's cells evolve too fast. A normal vaccine wouldn't work; they'd just adapt and bypass it. So I had to make something new. This vaccine doesn't destroy the demon cells themselves. Instead, when the infection starts, when the muzan's cells begin rewriting the human cells into the demon cells, the vaccine forces those cells to self-destruct."
Erika asked grimly. "What do you mean by self-destruct?"
Akira paused, then, in a grim tone, he continued. "In short, it kills the host before the transformation can complete. They die as a human, not as a demon."
"That's…" Erika struggled to find words. "That's not really a solution."
"It prevents new demons from being born," Akira said simply.
"But it kills innocent people who are about to turn!" she protested.
Akira exhaled heavily. "Yeah. It's not perfect. But it's the only thing we could make with what we have right now. If we can get Upper Moon blood, we might be able to create a real cure."
Erika lowered her gaze, conflicted. "So… are you really going to use it?"
Akira shook his head. "That's not my call. We'll let the Corps decide."
Shinobu nodded. "We'll present it to Oyakata-sama and discuss it at the next Hashira meeting."
Akira then smirked mischievously and said, "If it's up to me, I'd just spread it across Japan by mixing it in the water supply or something. Problem solved!"
Everyone turned toward him in horror.
Akira quickly raised his hands defensively. "I was joking! Joking!"
Everyone sighed in relief, except for Erika and Selena, who were both unconvinced. They knew him well enough to realize… he might actually do it if left unsupervised.
The two of them silently agreed to keep a closer eye on him from now on.
Tamayo asked gently. "So, most of your objectives here are achieved, and the rest of the experiment can only be continued with the blood of the Upper Moon demons. What are you going to do next?"
Shinobu answered first, with her usual smile. "We need to bring the vaccine back to headquarters and decide its fate there."
Akira added with a sigh, "And we still need more upper moon blood. Our research is at a standstill without it, so I guess we'll have to head out again for a while."
Tamayo nodded. "I see. When will you leave?"
"Tomorrow," Akira said.
"I will continue work on the vaccine and the cure," Tamayo replied. "Try to bring any Upper Moon blood you can." To most people, that would sound like a suicidal request, but Tamayo knew Akira fought the Upper Moon Three, so she asked without hesitation.
That night, as Akira rested in his bed, Selena's voice echoed in his mind. "Brother, why did you really make that kind of vaccine?"
Akira blinked, then replied, "What do you mean?"
"You know exactly what I mean," she said. "If you wanted to, you could create a vaccine that actually stops Muzan's cells without killing the host. In fact, I know you could even make a cure, one that turns demons back into humans. So why did you create a self-destruct vaccine instead?"
Akira smiled faintly. "Well… It's all part of my master plan."
"Master plan?" Selena repeated, curious. "What are you planning now?"
"Remember Kaigaku, the one who'll become Upper Moon Six in the future?" Akira said. "I made this specifically to kill him. When he tries to turn into a demon… he'll die instantly."
For a few moments, Selena didn't respond. Then her voice came, incredulous. "Why go that far? He's not even that strong. Zenitsu kills him later anyway. I don't understand why you'd waste time designing something for him."
Akira smiled faintly. "It's not for him. It's for Zenitsu's master, the old Thunder Hashira. Remember? When Kaigaku turned into a demon, that old man committed seppuku right. It's kinda sad in the anime, and I don't want that to happen. If Kaigaku dies before turning, that old man won't need to take his own life. That's why I made this vaccine."
Selena sighed softly in his mind. "Why care about him? You haven't even met the man."
Akira said in a happy voice. "Because I want to give this world a happy ending. Keeping people, I believe, deserves to live alive; that's part of it. Only if everyone survives and finds happiness can it be considered a true happy ending. So yeah… I'm going to save that old man, too."
There was silence for a few seconds before Selena asked, "Okay… but why not just create the cure right now?"
"Because it's not time yet," Akira replied in a serious tone, "Nezuko still needs to conquer the sun during the Swordsmith Village Arc. Only then can we lure Muzan out for the final battle. Otherwise, where are we going to find him?"
Selena chuckled lightly. "Makes sense. So… what's next?"
Akira smiled, turning onto his side. "Next up, the Entertainment District Arc."
The next day, they bid farewell to Tamayo and Yushiro before setting off for the Demon Slayer Headquarters.
Yes, unfortunately for Akira, that also meant more walking.
It was, in his words, an arduous journey.
In reality, only he seemed to think so. Erika and Shinobu walked ahead, perfectly fine, while Akira trudged behind, dragging his feet like a sulking child.
"I don't get why you can't walk properly," Erika said, glancing back at him. "You have enough stamina to run for an entire day if you wanted to."
"It's not about stamina," Akira muttered, slouching. "Walking for a while is fine, but walking an entire journey on foot is just boring. I want my bike back…"
"Oh yeah, about that bike," Erika said thoughtfully. "Do you think it's fixed by now?"
Shinobu smiled faintly, answering without turning her head. "It should be. Our swordsmiths are very skilled; they should have repaired it by now."
Erika looked at Akira again. "Didn't you also ask them to build you a special sword or something?"
Akira straightened up a bit, suddenly more cheerful. "Yeah. It should at least let me fight without breaking in a single slash this time."
"What's so special about it?" Erika asked curiously.
Akira smirked, eyes glinting with mystery. "Well… It's a custom design."
"Really? How does it look?" She asked.
"Hehe," Akira chuckled, tapping the side of his nose. "That's a secret. You'll see soon enough."
Erika rolled her eyes while Shinobu hid a small, amused smile behind her hand.
And so, after a long (not really) and arduous (only for him) journey, the group finally reached their destination, the Demon Slayer Headquarters.
However, as soon as they arrived, they were met with unexpected news.
The Sound Hashira had already taken the Tanjiro squad to investigate the Entertainment District.
Akira looked up at the sky and sighed deeply. "Of course… they just had to go on ahead."