New Moon, Old Blood - A Demon Slayer AU
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A life without sickness, without suffering, without death. That is what it could mean to be a demon.

What would you do to give that life to those you love?
Chapter 1 - Asphodel

Lepidoptera

Mother of Monsters, Unverified Impuritas Civitatis
Location
Indeterminate Unknown
New Moon, Old Blood

Chapter 1 - Asphodel

Kanae Kocho was going to die.

She could feel it in every breath she took. Normally, a person was not too aware of their own breathing. It was automatic; just something their body did naturally without them even having to think about it. But for a demon slayer, the act of breathing was more than that. It was a lifeline that brought them strength beyond human limitations. Strength that they could use to protect those who were weaker, who could not defend themselves. For that reason, even a novice demon slayer was thoroughly acquainted with the feeling of each breath and it entered and then left their lungs.

But now, that strength was waning. With each inhale, the air seemed to scrape at the inside of her body. It felt as if she was being cut apart, slowly but surely, from the inside out. Each breath was shorter and more shallow than the last. She could still stand now, but even that was a struggle. Her muscles burned at the strain to simply move. It was a sort of total physical exhaustion Kanae could hardly recall. Even during training, when she and Shinobu had been pushing their bodies to their absolute limits in order to master their breathing techniques. Soon enough, she would fall. That was a certainty. Still, she could remain standing for a little longer, at least. Maybe a day, maybe less. She would at least be able to hold on long enough to say goodbye to Shinobu and Kanao.

It was almost certainly a technique of the demon before her. The red-clad man had taken an evasive approach once it had become clear that she could match him in direct combat. Maybe if things had stayed as they were in that moment, Kanae could have won. If it was simply a direct battle, strength and skill tested against one another. But expecting such a thing would have been the height of foolishness. Life was never fair. That was why it was important to fight for it, to take what happiness you could in the world around you.

The demon had worn her down, slowly but surely. Every time she saw an opportunity to close in, another wave of ice rose up and blocked her path. She couldn't close distance against the demon like this, and from far away she simply had no way to strike him. He was stalling, dragging things out until Kanae could no longer defend herself. At the start she could easily avoid being struck by the swirling gusts of frigid wind the demon scattered about, but bit by bit the cold had sapped the strength and feeling from her body. She was slowing down, and it was costing her. She had suffered several blows already as a consequence, but now the numbness pervading her flesh had taken even that. She could barely feel her sword, broken in half blocking a nearly-lethal attack minutes ago, held steady in her hands. All that was left was a dull ache, a slight burning feeling that filled every inch of her body. Kanae was not as adept in the world of medicine as her sister, but even she knew that once you were cold enough that your body was starting to feel warm, you were near the end.

Kanae wouldn't go down without a fight, though. The demon's smile, empty of all life even from the start, was becoming more and more brittle. Sunrise was near. If she kept it up, perhaps the demon would make a mistake. There was still time to make a difference here.

"You're going to make me feel disappointed." The demon said, that bright yet hollow smile still painted across his face. "You're dead on your feet right now, but I'm still having trouble striking you! If I'm being honest, I'm getting a bit excited here. I can hardly recall the last time I was challenged like this! And yet…"

The demon's smile dimmed just a fraction.

"The way things are going, I'm not even going to be able to see how it ends. You understand how disappointing that is, right?"

Kanae was hardly listening. The world was starting to blur. It was all she could do to stay standing. There was still a chance, however slim. She would just need a moment's distraction, an opportunity, anything that could give her an edge…

"Why not let this end properly? At least that way we can both walk away feeling satisfied, right?" the demon continued. Kanae didn't answer, taking a single, slow breath that rattled in her lungs and stung her skin. She could still do this. She could still last until sunrise, at the very least.

And then it happened.

The sound of wood creaking, or perhaps crashing and breaking, echoed through the streets. The demon turned to look towards the sound, seeming oddly perplexed. It was a distraction, if only a momentary one.

Kanae tried to leap forward, sword reaching for the demon's neck, but to no avail. A violent, wracking cough tore through her, forcing the Hashira to her knees. Ah, so she had already reached that point. The coppery taste of blood filled her mouth, a few drop spilling out from her lips. Despite the pain, the tiny bit of warmth held in the bleeding felt like an incredible blessing. Scant as it was, she would take what she could get.

The demon didn't so much as look back, entirely transfixed by the source of the noise. If she were in better condition, Kanae might have been insulted by the total disregarding of her presence. Now, she was just thankful for the momentary reprieve.

Something had… sprouted in the end of the alley, ash-colored roots crawling across the road and up the walls of the nearby buildings. It almost looked like a tree had suddenly appeared behind them. The pale bark of the abruptly-arriving foliage split open in several places, revealing an emptiness inside that seemed unnaturally deep and dark.

The tree cracked open, and Kanae's senses were suddenly overwhelmed with the scent of spring. Blossoming flowers, a gentle breeze, the warmth of sunlight. It all seemed to flow through the alleyway, easing the pain that had enveloped Kanae's entire body. For just a moment, in spite of the confusion she was feeling at the sight before her, in spite of the circumstances, Kanae felt hope. She couldn't identify why, but something about this sight was reassuring. Everything would be alright, it promised.

And then the demon stepped out of the gap in the tree, and that hope died.

The new demon was dressed in a black haori with dark green branch patterns extending across the surface. Though it may have been Kanae's own inherent bias, they formed a shape reminiscent of wings. Messy, red-brown hair fell down just barely above their shoulders. The demon's face was hidden, a root-like wooden mask covering their eyes and most of their mouth. At the demon's side, they wore a sheath that looked disturbingly similar to those carried by members of the demon slayer corps. They looked a little older than Kanae herself, though she knew that with demons appearances could be deceiving. Even a weak demon entering the fight at this point would have been a disaster. But this demon was clearly not weak.

Though their eyes were hidden and Kanae couldn't see if they were marked as one of the Kizuki, they were without a doubt an equal to the demon she had just been fighting. Not just in the clear way the first demon was looking at them, but also from the way they carried themself. This was without a doubt a trained warrior. Their stance, the way they scanned the surroundings as the entered… but there was something beyond that.

Kanae Kocho was not particularly inclined to believing in the supernatural. Demons existed, of course, as did the afterlife, but having a talented scientist in the family tended to incline one more towards skepticism towards folklore and myths. Even so, there was something inherently otherworldly about the arriving demon. Their outline was strangely distinct in the near-dawn light, and the details of their body seemed sharper and clearer than the dreary alleyway or even the demon that stood staring at them. If the world were a painting, then this demon was given far more time and dedication than any other part of it. It was somewhat surreal just to look at it.

"Douma, you said this would only take a moment. It's getting close to morning, and you're still out here. What's going on?"

Kanae was taken somewhat aback. This new demon actually sounded concerned for the first. To most demon slayers, the very idea of that would be unthinkable. It was the generally-accepted opinion of the corps that all demons were, without exception, monsters without a shred of empathy or care for the lives of humans and other demons alike. Kanae knew better, knew that demons were capable of a full range of emotions just like any other person, but actually seeing it was still highly unusual.

Moreover, it was practically proof that this demon was one of incredible power. There was no other way they would be able to speak to one of the Twelve Kizuki, specifically the one ranked as highly as her opponent.

Said opponent looked delighted at the new arrival, though Kanae could still not detect a hint of genuine emotion behind their smile. "Tanjiro! I'm touched that you came all the way out here. I just ran into a particularly interesting woman and was having a bit of trouble closing things out. I'll be done in just a moment."

"You should've retreated by now." the new demon… chided? "If I hadn't shown up, you might not have made it back to safety before the sun rose. You really need to learn to take better care of yourself, Douma."

"Ah, it's a bit embarrassing getting scolded like this in public, you know? But you're right. We can just end this here, and then-" the first demon said before being cut off.

"No." the second demon interrupted, suddenly sounding stern instead of the gentle, caring tone they had taken before. "Lord Muzan would be unhappy if you risked yourself without a good reason. Head back now. I'll take care of things here."

The first demon stopped, and though his smile remained as wide and brilliant as before, Kanae could tell he was deep in thought. She remained in position, trying to draw up whatever vitality she could for one final fight. If the first demon cared about maintaining their easygoing facade, then she would be ending the night with a different opponent. Against Douma she hasn't been able to get close, but maybe this new demon would have a poorer matchup. It was a slim chance, especially given her condition, but she would take it. Kanae had two, maybe three Breath Forms left in her. She would make them count.

"If you say so!" the first demon answered cheerfully before stepping back towards the tree and vanishing into it. For a moment, Kanae and the second demon stood alone in the alleyway, silent. Then the demon spoke.

"I'm really sorry about Douma. He doesn't really understand people, so he tends to act without thinking about how other people are affected."

"I know." Kanae replied, the words weak and hollow. Even speaking burned, now. That demon was empty, even as he smiled and laughed in their fight. Soft as it might seem, Kanae couldn't bring herself to hate somebody who lived such a pitiable life.

Her words were followed by a fit of coughing. Kanae could feel more blood spill into her mouth. Seeing the moment of weakness, the demon rushed forward. Kanae tried to raise what remained of her sword, but her arms wouldn't respond.

So this was it, then. She had hoped to at least see her sisters one last time before she died. Hopefully they would go on to live long and happy lives, free from all this mess. It was a vain, impossible hope, but Kanae held onto it. That was where people found the strength to do the impossible.

The demon closed in, and Kanae held its gaze. She would not die with her eyes closed. Its claws reached her, ready to tear her apart, and…

Began wiping the blood off of her face?

"Careful!" the demon warned. "Your lungs are severely damaged. You'll only hurt be hurt if you keep exerting yourself. Here, try to match my breathing."

Kanae stared in shock as the demon fretted over her injuries. What was happening? Was she starting to hallucinate? She had lost a lot of blood…

Oh. How ironic.

This demon was truly a kind person. Just like she had always wished to meet. Proof that demons and humans weren't destined to war with each other for as long as they both existed. And when she finally met that person, finally had her hopes proven true…

"It doesn't matter now, does it? I don't have much longer."

The demon shook his head. "Don't say that. You're not dead yet, I can still-"

"I won't become a demon." Kanae said, putting as much finality as she could muster into those five words.

The demon seemed genuinely distressed by her refusal. "Please, I don't want to watch somebody die in front of me when I can save them."

"I'd have to eat people. I won't-" Another coughing fit. More blood stained Kanae's skin. She couldn't feel it this time. "I don't want to live at the cost of other people."

"There are ways to feed without killing people. You won't lose your memories, either, as long as you don't resist the transformation." the demon insisted, voice strained. They sounded strangely far away, even though they were now kneeling right in front of Kanae. "I promise, you won't have to become a monster. Just don't throw your life away like this. Don't you have people who would miss you?"

Shinobu. Kanao. It would hurt them if she died like this. Would they be able to go on without her? Kanao was still too young to tell. It was practically a guarantee that she would become a demon slayer like her sisters, no matter how Kanae wished for her to be safe. And Shinobu…

Shinobu had always been passionate. It was beautiful, the way she attacked a challenge from every angle and found solutions to problems that anyone else might see as impossible. But it was burning her, too, the way she pushed herself. No matter how strong her sister become, Kanae always worried for her. How could she not? She was one of the only two family members Kanae had left. If Kanae died here, without a word to her sisters….

"I know what it's like to want to protect your family." the demon said, with a weight that Kanae could barely begin to understand. So many emotions, all packed into a single sentence. "So please, trust me. If you leave them behind, if you disappear… it'll hurt them. Don't make that mistake."

Kanae tried to answer, but found she could no longer speak. Her body was finally starting to give out. This was it, then. There was no more time to decide.

Maybe this would be a mistake. Maybe she was being tricked, putting her trust in the wrong person because she was desperate to hold onto proof of the dream she had held onto. None of the other Hashira would have ever fallen for such a thing. Maybe that made her naive, believing that all this was just a possibility instead of a certainty. But when she considered the alternative…

Kanae Kocho reached out and took the demon's hand.
 
Oh, wow, that's quite the AU. The obvious one from the first sentence aside, Tanjiro being already a demon is such a massive change you gotta wonder what's actually going on to lead to this. That's some massive timeline change right there.
 
Chapter 2 - Jikininki
New Moon, Old Blood

Chapter 2 - Jikininki

Akaza was starting to feel concerned.

Not worried or nervous yet, but if the situation didn't improve then that might change.

Douma had been out doing whatever it is Douma did when he wasn't bothering somebody (probably still bothering somebody, just not somebody Akaza knew), and it was getting close to morning. Akaza could not possibly have cared less about that, but of course Tanjiro did. And so Tanjiro had left to go check on the Second Rank with one of those creepy tree portals. It had been a good few hours since then, and while Douma had unfortunately reappeared Tanjiro hadn't emerged with him.

"No need to look so stressed! Tanjiro's really strong, so there's no way he would've actually been in danger back there! You ought to have more faith in your comrades, Akaza!" Douma chirped from his side of the safehouse.

"Stop talking if you aren't going to say anything useful." Akaza snapped. The other demon didn't seem to understand the stakes present. Tanjiro was weirdly friendly, a bit like Douma but significantly less creepy and overbearing. It was unlike him to go off and do something without informing one of the other Kizuki of where he was and what he was doing. "So that people always knew where to go to ask for help," he said. The issues with that sentiment aside, it was convenient. Tanjiro's Blood Demon Art was one of the most versatile to ever exist, and even Akaza had to admit how useful it was to keep some of its products on hand. For him to go off somewhere without telling anybody? Not only was it uncharacteristic, it would be very, very bad if something were to happen to one of Lord Muzan's favorites while Akaza and Douma were in the area. They wouldn't be killed just for that, but there would absolutely be punishment.

"Who knows? Maybe he's just savoring his meal. That woman was really impressive, after all. I'm a bit jealous that I lost out on the chance to eat her, but at least she didn't end up going to waste." Douma continued in a teasing, casual voice. Akaza resisted the urge to tear out the other demon's throat to make him shut up. There were more important things to deal with right now.

"I'm going to check his house. If he's not there, I'll find the other Kizuki and ask if they've seen him." Akaza decided. Douma shrugged and took a seat.

"If you're really so worried, I could poke around. If it's for a good friend, then I'd be happy to-"

Akaza ignored the other demon's rambling and stepped through the Wayfaring Tree. Somebody had to do real work around here.

—————————

Akaza stepped lightly as he walked through the inside of the Wayfaring Tree, careful to remain on the outlined path. He had never tested exactly what would happen if he left the leaf-covered trail bordered by stones and wandered out into the dense forest surrounding it, though Tanjiro had warned strongly enough against it that any curiosity Akaza might have had was thoroughly crushed. It didn't help that there was something distinctly wrong with the whole place.

Akaza was a demon. Not only that, but one of the strongest demons to ever exist. There was nothing in this world that he would consider himself scared of. Fear was for the weak, after all. But something about this place was different. He could swear that there were people further in, watching him. There was no fighting spirit from the depths of the forests, and yet Akaza could never shake the feeling of being watched by something. And then there were the voices.

"We don't blame you. You didn't fail us."

"Won't you come and look after me again?"

"You can rest. Just stop fighting and stay here."

The voices echoed from beyond the trees, beckoning Akaza to leave the path and wander out into the woods. He ignored them. They didn't mean anything. Even if there was this nagging insistence that they should be familiar.

Akaza opened the door that he knew led to Tanjiro's house and stepped through, feeling the pricking weight in the air from the passage removed from his skin. He preferred using Nakime's transportation, but she didn't take requests from the Kizuki and only acted at Lord Muzan's command. So if a demon wanted to get from place to place, they usually had to use the Wayfaring Trees. At least it was done with for now.

Tanjiro's house was large, as befitted a demon of his strength. Not in that vain, grandiose way like Douma's complex, but in a more practical sense. The foyer was the largest room in the building, a simple space that connected the two floors and branched off to each of the different wings of the manor. Strangely, it was quiet today. Whenever Akaza had visited before, he could always hear the sound of movement filling the manor. Between the patrolling staff and the house's various temporary residents, there were normally noises everywhere. Now, Akaza could only hear two sounds: the work in the medical room, and somebody walking around in one of the guest rooms upstairs. Everything else was dead silent, save the light pattering of rain on the roof outside.

Akaza proceeded upstairs, since he really didn't want to head down to the medical room. The third-ranked Kizuki stalked through the bear-silent halls, listening as the sound of movement got closer. Right as he reached the door, the noise stopped.

Tanjiro opened the door before Akaza could reach for it. The younger demon was wearing his green haori, but had shed the rootlike armor he sometimes wore. His mask was slung around his hip by a cord made from something between a vine and a tendon thrown over one shoulder, revealing the flame-shaped marking that covered half of Tanjiro's face. With the mask gone, the demon's dark maroon eyes and the letters inscribed into them were fully visible. That on its own was unusual. While Tanjiro wasn't afraid to show his face, he tended to keep his mask on more often than not, even in private. Akaza didn't know why and didn't really care.

"Akaza! I wasn't expecting to see you today, is something the matter?" Tanjiro said cheerfully. At least that was normal. The swordsman's smile seemed to hold all the sunlight he had been denied in his demonhood, endlessly gentle and warm with every word no matter who he was talking to. On anybody else, Akaza would've called that kind of attitude soft. Weak. He had done exactly that when they had first met.

And only that once.

"There's nothing specific. You just disappeared after you went to get Douma and nobody knew where you went." Akaza explained. Tanjiro pulled back, embarrassed.

"Oh! My apologies. I didn't mean to make anybody worry, I just- Well, I had a lot to deal with, and- It'd be easier if I just showed you." the younger demon said frantically, backing out of the doorframe and allowing Akaza to see inside. Specifically, allowing him to see the patient currently sleeping in a bed.

She was the demon slayer who Douma had been fighting, that was certain. She had long, dark hair with a butterfly pin on either side of her head. Her skin was unnaturally pale, nearly matching the white patient gown she was currently wearing. Though her uniform has apparently been bloodied during her fight with Douma, a butterfly-patterned haori was hung on one of the nearby medical devices. The Flower Hashira, if Akaza remembered correctly from Douma's rambling. But more important than that was the smaller, easily-missed details. The pallor of her skin going slightly beyond an ordinary human's. The unnaturally slow heartbeat. The low tremor of her body even as she slept. Slightly inflamed veins, especially around the neck.

This woman was in the early stages of transforming into a demon.

Akaza couldn't help the smile breaking out on his face. A Hashira finally chose to become a demon! To take the path of growth and strength instead of withering and dying! And not just any Hashira, but one strong enough to stall Douma for hours even with all the weaknesses of a human! Akaza could only imagine what sort of demon they would become. They would no doubt rise into the ranks of the Demon Moons. It seemed possible that they'd knock Douma from his seat as Second, even. What fortune!

"How did you get her to accept it?" Akaza asked, breaking the silence that had fallen over the manor's wing. "It seems like every demon slayer that gets that strong would rather die than become a demon."

"She was stubborn about it at first, too. I'm just thankful I was able to convince her to live before it was too late." Tanjiro said, letting out a sigh that contained more relief and exhaustion than Akaza could have ever expected. Was he really that desperate to convert this woman? From the sound of things, it seemed like this was personal. But why would Tanjiro have cared about a demon slayer he only just met?

Well, none of that was Akaza's business. He didn't really care when there were more important things to be talking about.

"So how long do you think it'll be before she becomes one of the Kizuki?" Akaza asked casually. Tanjiro smiled.

"It'll take some time for Kanae to acclimate to Lord Muzan's blood. Not only that, but to understand what it means for her to be a demon."

Akaza almost rolled his eyes. "Demons grow stronger and serve Lord Muzan. What else is there to understand?"

Tanjiro's breathing slowed, and for a second Akaza thought he had said something wrong. But the swordsman merely exhaled and shook his head. "Not everyone can see life in such a straightforward way, Akaza. Especially when other people get involved."

———————————————

Taisho Secret
Tanjiro's manor was purchased with help from Gyokko and Douma in exchange for a few favors. The nearby area is practically always raining or snowing, so it's safe for demons even during the day.​
 
A new chapter! Nice, I was worried that this would be a one-shot.
 
I am barely familiar with Demon Slayer, only watched the first season and a movie, but I am very interested in this.
 
For all those going into this who aren't familiar with Demon Slayer, be aware that there are going to be lots of spoilers. Mostly regarding the Kizuki and their stories, but also some important canon events. Demon Slayer is one of the better-executed stories I've seen, so it's worth a watch if you haven't yet.
 
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