Chapter Text
It was almost dawn, yet the intense battle between the Third Upper Moon and the Flame Hashira beside the derailed Infinity Train didn’t look like it was letting up. Even with the Hashira bleeding and had his eye heavily damaged, it was obvious that he was not done yet, not in the slightest. His heart burned brighter and hotter than the pain this demon had inflicted him, and if the only way to defeat the demon was to light his whole being ablaze and take the demon with him to ashes, then so be it.
The two young demon slayers watching meters away felt more than saw what Rengoku Kyojuro was about to do, and they could only scream his name as the Upper Moon demon and the Flame Hashira surged forward. It was obvious to anyone that this was the last strike for both opposing parties, and whoever loses would have to fall on to the ground and die as the first rays of the sun flood the sky.
In a more familiar timeline, the loser died with the young demon slayers listening and crying over his kneeling dead body, yet died without feeling shame. He died with the sweetest, happiest smile on his face as he joined his mother on the other side.
But in this timeline, fate has other plans.
Just as the Upper Moon and the Flame Hashira were mere meters away from one another, a figure so quick that it could only be perceived as a blur even in trained and enhanced eyesight, went them in a flash and pushed both back with a blast of ice cold air. Rengoku flipped in the air and landed on his feet somewhere near the two young demon slayers, his stance immediately on the ninth form of his Flame Breath, while Akaza, the Third Upper Moon Demon, landed further back, a snarl in his throat at the interruption.
Both fighters didn’t know what happened exactly, but they did know that someone powerful had entered the battlefield. And to think that this person had come between such powerful attacks… both of them readied their stances, waiting for the dust to settle and reveal who it was. Even the Upper Moon had difficulty seeing the fighting spirit of this intruder. Both of them weren’t dumb to try and attack this person right away.
But surprisingly, it was Tanjiro who figured out who- no, what the person was, and it made every hair in his body stand up. Oddly enough, the scent reminded him of a certain demon who helped him and his sister back in Asakusa.
“Demon!” He gasped, shaking Inosuke from his trance, even though Tanjiro didn’t look particularly alarmed. The scent was undeniably from a demon, a powerful one at that, yet it didn’t smell foul at all. It was as if he was breathing in a cold breeze while under the heat of the sun, and for a moment he wanted to smell a bit of it more. Then he gasped as the smell suddenly intensified, and he felt like his lungs had frozen at how cold it became around them.
Rengoku was aware of this sudden temperature change as he himself could feel the cold stinging his exposed wounds. The flame in his heart and stance made it more than bearable for him, yet it was so cold that he could see puffs of condensation whenever he breathed out. He heard what the young slayer said, and his eyes narrowed at the dusty area where he and the demon Akaza should be fighting by now.
‘Another Upper Moon? Has to be, for a demon to stop oncoming attacks from an Upper Moon and a Hashira should at least be higher than the title of Upper Moon Three. Or maybe I’ve just underestimated the power of the Upper Moons…’ Rengoku thought, growling at himself for his wrong judgment.
The silence was deafening, and it felt like the world came to a standstill. Rengoku couldn’t stand it as he knew he was losing precious time when he should be finishing Akaza by now, yet he knew he can’t survive the night if two of the top three Upper Moons came to kill him. He needed to get the young slayers out of here lest they lose their lives, and he can’t accept that. Not when he knew these three and the demon girl was essential to the Corps.
All of a sudden, both Tanjiro and Inosuke jerked up, their senses flaring yet couldn’t react with their bodies fast enough. Rengoku could barely see a blur of movement before a hooded figure was suddenly standing behind the two. He felt his body move out of instinct to protect, and before he knew it his sword clashed with the person’s weapon. Rengoku was sure that this was the demon who entered as coldness seemed to seep out of their very being and into the air, the way they moved was seemingly inhumane if they weren’t trained to move so fast, and the pure strength that pushed back on his blade.
But then he noticed that this demon was… weird, in a way. They were hooded so he couldn’t see their face, but the outfit they wore were very odd for a demon to wear- his mind was spinning at the fact that they wore a hood, adorned a haori that he couldn’t see the design of because of the darkness, the strikingly familiar black uniform they wore and the very real nichirin blade that was clashing against his own sword.
How?
Their head tilted upward as if they heard his question, giving Rengoku a view of half of the demon’s face, which showed a small smile stretching on pale white skin. It shook the hashira how genuine the smile was, while also noticing that the demon had to look up to him a bit as they were an inch shorter than him.
“I am not your enemy.” The demon spoke, and the hashira pushed harder against the other's sword when their soft voice almost lowered his guard. He growled as his effort was answered with the demon pushing back as well, yet their smile didn't change. It was concerning.
“You are a demon. Most likely an acquaintance of that Upper Moon.” He answered curtly with a whisper to match the softness of the other’s voice, eyeing where Akaza landed and he could feel the sneer on the Upper Moon's face due to the interruption.
“Why would an Upper Moon allow their acquaintance to have a nichirin sword close by?” The other asked with their smile curling a bit more, as if amused. Rengoku didn’t find anything amusing about their situation, but he did think it was impossible for a demon to wield a nichirin sword. Are demons that powerful now that they can wield the only weapon that can destroy them?
Despite this, it still didn’t make sense for him that this odd yet powerful demon to be an ally to the Corps.
Unsurprisingly, it was Tanjiro who proved the demon’s words to be right.
“Rengoku-san! The demon is right; she is not our enemy! At least that’s what I smell from them…” He grunted as he sat up with the help of a grumbling Inosuke. Rengoku’s eye twitched at his words, and he pushed his blade against the other even harder.
“This demon is tricking you-“
“No!” Tanjiro affirmed, which surprised the hashira. He knew from stories that Kamado Tanjiro had an excellent sense of smell, to the point he can smell someone’s intentions and emotions. Tomioka Giyuu also mentioned in the Hashira meeting after the Kamado sibling’s trial that Tanjiro’s sense of smell was one of his attributes that helped him survive his fight with Lower Moon Six. But was it good enough to smell beyond the stench of a demon and find their true intentions?
“They smell like the demon- no, they smell like how Nezuko smells! I know I only have her and another demon to compare to, but this demon doesn’t smell like they’ve killed a human. They don’t have the stench of death and suffering… they smell sincere and alarmed. I-I do believe this demon is telling the truth!” Tanjiro exclaimed, hitting it home as he groaned in pain after talking too much with his injury still vulnerable, and Inosuke grumbled loudly as he scolded him while examining the wound.
Rengoku’s eyes widened more while the smile on the demon’s face stayed the same after the young slayer said his words. But should he believe it? Why would this demon help him, anyway? Especially now? And a demon this strong without killing a human? It sounded like it was a thought from the realm of impossibility.
But he was mostly hesitant because he was scared of letting these young slayers die if they were tricked into lowering their guard.
The demon then looked in the direction of Akaza so fast that Rengoku thought they broke their neck, just as Inosuke called out-
“The other demon is coming-!“
In a flash the demon he was interlocked with moved, and he saw within that great speed how they let him slash their leg just so they could move away from their stalemate and run towards the incoming Akaza, who headed straight to Tanjiro. The young slayers had their swords ready as Rengoku followed his instinct on shielding them both while he readied the fourth form of his Flame Breath to meet the oncoming attack.
Yet it never came as the hooded demon suddenly came from the left side and struck Akaza with their leg so hard that he went flying back with speeds even Rengoku’s trained eye only saw as a blur. A small crater formed around the hooded demon due to the force of their attack, while the demon slayers felt the remnants of the shaking earth coming from the impact of the kick joined by another small shaking as the Upper Moon hit and slid on the ground away from them.
Rengoku was awed at the raw power the hooded demon oozed as it stood up straight as if they did nothing but just stand upright then.
“Hashira,” The hooded demon called, and Rengoku readied his stance from the fourth form to the fifth, worried that he had to fight this powerful demon as well. But they protected them, showing their power right in front of them and even showing their back to Rengoku, who could’ve cut off their neck while they were preoccupied with kicking Akaza.
As much as he wants to deny it, Rengoku wanted the hooded demon to prove that they were their ally.
“Protect your fellow swordsmen at all cost. With the state of your injuries, you must not enter our fight or we may lose to this Upper Moon.” They stated, their voice soft yet serious as the temperature around them went down again. “There are only mere minutes before the sun shines upon us. I will hold him out here, and he shall burn. I will burn with him if I must if that is what we need to keep him in the light.” They continued, and Rengoku looked back at Tanjiro to affirm the demon’s words. Hope filled his burning heart when the younger sniffed, then nodded with an awed smile.
“Can you hold him out to burn without him moving?” The hashira asked, and the hooded demon turned their head to look at him with a pleased smile before nodding.
“Then I will cut his head as soon as he is immobilized. I won’t waste any time.” Rengoku affirmed, and the demon’s smile turned up to form a small grin.
“That is good to hear, Flame Hashira.” They said before facing upward, following the movement of the Upper Moon who released his Blood Demon Arts as he barreled toward them. In a second the hooded demon had their nichirin sword out, and Rengoku noticed a detail on it that he didn’t see earlier when they were interlocked- the body of the blade glowed a faint blue, and he was sure that the glow made a unique pattern on the blade, like the flame pattern on his own sword, but he couldn't see it clearly in their distance.
He also couldn't help but notice their haori now that the demon had their back facing him- it was loose and fell all the way to the ground, the excess cloth flowing out like a waterfall around their feet. The design looked like the haori was made from broken glass, the broken ‘pieces’ getting bigger as it reached the ends. The pattern looked hypnotizing, but the most eye-catching aspect of it were the colors that adorned the design- it looked like the whole cloth was supposed to be gradient, with the upper part colored in the lightest blue, which then got darker as it went down the haori. But by the ends the gradients turned into different shades of dark blue, making the haori look like it was carved from the clearest block of floating sea ice.
A demon wielding a glowing nichirin sword and adorned a beautiful haori... their existence was nothing short of terrifying and ominous. Rengoku couldn't help the excitement blooming in his chest.
“Protect them at all cost. If I fail, lay your life if you must to get them to the sun. Do not let him take the girl in the box.” That statement shocked him for a second, and he heard a sharp gasp from Tanjiro behind him. How did the demon know? Is that why they were protecting them, for Tanjiro’s demon sister? But if the older brother himself said they were to be trusted, then it was great that they knew about her, right?
None of his questions ever made out of his mouth though, as the hooded demon jumped up to meet the Upper Moon with their sword on the ready. And thus an odd battle commenced- a battle between the Third Upper Moon and a mysterious hooded demon who wielded a nichirin sword- another demon demon slayer.