Hello, I'm OMG04 and this is my first fanfiction. This is a crossover between Demon Slayer: Kimetsu no Yaiba and RWBY. Basically, both worlds and stories will be combined together. I plan on starting out with both stories playing side by side like in cannon, but eventually, our beloved heroes will meet and fight against a common enemy. Also, I'm all caught up with RWBYbut I've only watched the anime for Demon Slayer, so I may make up some stuff not canon to the manga. In other words, expect a lot of plot holes.

Again, this is my first time writing a full fanfiction. So if you want to avoid boring or cringe-writing, I'd keep scrolling if I was you. That said, if you do stick around, I will be open to all criticisms. However, if there is too much negativity, I will likely lose all motivation to continue and just drop the story. If that happens, I'll probably put it up for adoption for a more talented writer.

I decided to write this because I was honestly a little disappointed with the small amount of compelling Demon Slayer stories. I also really enjoyed most of the RWBYcrossovers I've read. I'm a huge fan of both series, in case you didn't notice. This is just an idea I had in the back of my head and I don't really expect it to gain a lot of attention. I do not own Demon Slayeror RWBY, both stories go to their respective owners. With that said, I really do hope you enjoy it. Now let's get this show on the road!


Prologue

Remnant, a world filled with life and warmth but also death and malice. Man was born into an unkind world, one filled with creatures of darkness known as Grimm.

The Grimm had only one goal, to destroy all humans and their creations. However, humanity fought back against the Grimm.

The warriors of peace who faced these creatures were known as Huntsmen or Huntresses. These warriors were known for their exploits of Dust, a naturally occurring energy propellant that could be triggered by the aura of humans and Faunus.

This Dust was used to power the weapons of the Huntsmen. They also used Aura, a manifestation of the soul present in every living creature.

The Huntsmen were able to amplify and control their Aura, using it as a force field to shield themselves from potentially fatal blows.

However, there was a limit to how much damage one's Aura could take before their reserves were empty.

Fortunately, Huntsmen could use Aura for offense as well as defense through Semblances.

Semblances are the users' Aura in a more tangible form, giving the humans special abilities that made them more than human.

Through their use of Dust, Aura, and Semblances, humanity pushed back the darkness of the Grimm to a point where chaos was almost nonexistent. This left strength, civilization, and peace to take its place.

With this new age of civilization came four kingdoms spread out all across Remnant. These kingdoms are Vacuo, Atlus, Mistral, and Vale. At least, that is how things are in one world.

However, we are talking about a Remnant with not four, but five kingdoms.

This fifth kingdom was a slightly smaller, less advanced, yet more peaceful chunk of land separated from the rest of the world.

This kingdom was known as Japan. Japan was located far from the other lands and the people who live there do not experience much negativity, making Grimm attacks quite rare over there and easier for the Huntsmen located there to defend against.

Japan was usually neutral when it came to the wars of the other four kingdoms and even served as a safe haven for refugees of the wars.

However, their neutrality during wars earned them the label of "coward" in the other kingdoms.

Although they were behind on technological advancements compared to their rivals, Japan was still prosperous and above all, peaceful.

Thanks to the efforts of the five kingdoms, their Huntsmen, and academies used to train them, Remnant experienced a long period a peace.

Sadly, all good things must come to an end. That includes the peace in Remnant which came to a swift end with the arrival of a new, potentially more dangerous foe. Demons.

The demons are humanoid monsters with superhuman strength and durability. What sets a demon apart from a human was their sharp teeth, slit pupils in their eyes, differing skin complexions, and thirst for human blood.

The demons bear a few similarities with the Grimm. Those similarities being that no one knew where they came from, and they only lived to kill and destroy.

Those are where the similarities end, however. Grimm can be killed through normal means if a Huntsman is strong enough, the same could not be said for demons, unfortunately.

Demons could not seem to be killed at all, they had regenerative capabilities that were out of this world. Weapons, Dust, Semblances, and even the legendary Silver Eyes that were the undoing of many Grimm did laughingly little against these creatures.

However, it wasn't long before it was discovered that sunlight is fatal to demons. As soon as it even touches them, it completely disintegrates them until they are nothing but dust, and not the good kind.

It was also confirmed that these creatures did not have Aura, for reasons nobody knows. However, this meant demons only came out at night and retreated to the darkness come dawn.

Any poor soul unlucky enough to encounter one would either have to outrun and escape them, or hold them off until sunrise. Easier said than done, especially if the demon was much stronger than their opponent.

The demons also got stronger with each human they devoured, whereas Grimm got stronger and wiser with age.

It wasn't long before the demons infested and terrorized all of Remnant, especially in Japan. They had killed over a million people in just the first five years of their arrival.

The Huntsmen were not prepared for such an enemy and many of their lives were claimed in their desperate attempt to combat them.

The Huntsmen tried their best, using traps to keep the demons in place until sunrise, but they were nearly helpless against the seemingly endless hordes of demons and Grimm.

It seemed that humanity had reached its end, until one man from Japan found a way to stop the demons.

He was not a Huntsman, but a Japanese swordsman with Hanafuda earrings, long, ruffled black hair with burgundy tips tied into a ponytail, and a strange mark on his forehead which resembled flames.

The sword he wielded was no ordinary sword, but a Nichirin Sword. This sword was a blade forged from a special ore which absorbed the demons' one weakness, sunlight.

When the man decapitated a demon with this sword, the sunlight imbued inside prevented the demon from regenerating, causing it to fade away.

Even though the man was able to kill demons much more efficiently, he was only human. It was said he had Aura, but no Semblance, making him about as strong as a lower-class Huntsman.

He would stand no chance against the stronger demons, or so people thought. While he didn't have a Semblance, he had invented a special breathing technique that supposedly allowed him to harness the power of the sun.

This breathing technique gave the user superhuman strength, speed, and durability that surpassed even the strongest Huntsmen.

To top it all off, the effects of the technique could be doubled or even tripled if the user uses their Aura to enhance it.

The breathing technique could even give the user a unique form of a semblance which best compliments the technique despite what their original semblance might have been.

This is because of the effects the breathing technique has on the users body and Aura, this special Semblance was the body's way of adapting to the technique so it could be easier to use.

The swordsman's semblance which he gained through the breathing technique was called "Sunlight Absorption."

This semblance allowed the man to store more of the sun's energy within himself. This not only made the man stronger in every way, but he could also release his stored energy in the form of fire, making his slashes more dangerous to be near.

He then combined the breathing technique with his sword techniques, inventing twelve forms of the breathing technique.

The man who was dubbed the "Slayer of Demons" called this technique, "Sun Breathing". With this breathing technique, the "Slayer of Demons" was a force to be reckoned with.

Soon, other swordsmen and women from Japan would obtain their own Nichirin Swords and fight alongside the sun breathing user.

However, only he could use the Sun Breathing technique because the after effects of the technique left the user weak and helpless if he or she wasn't strong enough.

It also consumed Aura a lot faster than a regular Semblance, making it easier for one to run out of Aura in the middle of a fight.

Thus, the Slayer of Demons taught these swordsmen and adapted his breathing technique to fit his student's' capabilities.

This resulted in five new breathing techniques branching from Sun Breathing. Water Breathing, Thunder Breathing, Flame Breathing, Stone Breathing, and Wind Breathing.

Each technique has a certain number of forms. These new breathing techniques were much weaker than Sun Breathing,but were easier to use consistently and still very effective against the demons and even Grimm.

Like the Slayer of Demons, they all gained special semblances due to the effects on their bodies from the techniques. Users of Water Breathing gained water summoning abilities, users of Flame Breathing could set their bodies and weapons ablaze, and users of Thunder Breathing were said to have been able to move as fast as lightning.

These breathing techniques, including Sun Breathing, were labeled Total Concentration Breathing techniques.

Total Concentration Breathing is how the sun breathing user gained increased physical stats. This could be used without the use of Aura, which was a great substitute for using Aura.

That said, Aura could still be used to further increase the power of their attacks.

When this was discovered, the swordsmen decided to only use Aura against more powerful foes and only as a last resort in order to save Aura for protection.

The technique of combining Aura with Total Concentration would come to be known as "Aura Enhanced Total Concentration Breathing" (or AETCB for short).

The users of the breathing techniques and Nichirin Swords fought side by side along with the Huntsmen who used weapons coated in Wisteria Poison.

It turned out that demons actually have another weakness, the Wisteria Flower.

With the combined effort of the kingdom Huntsmen and Japanese swordsmen, the demons were pushed back to Japan and forced to go into hiding.

The demon threat was finally over after seven long, bloody, and tragic years. Humanity had triumphed over darkness once again.

Soon after the long battle ended, the five kingdom governments met in Japan where it was unanimously decided that the demons would be kept under a watchful eye in Japan.

They would plant Wisteria trees all across the border to prevent the demons from escaping the kingdom. Huntsmen would be placed at the border to keep an eye out for Grimm trying to reach Japan while the swordsmen of Japan who fought the demons would eliminate all demons they could find.

These swordsmen and women would come to be known as "Demon Slayers." Once the meaning was adjourned, the Demon Slayers came together to form an unofficial organization of swordsmen and women who would protect humanity from the demons.

And thus, the Demon Slayer Corps was born.

The people of Japan were not happy when they heard they would be trapped with the demons, but still refused to leave the homes that they and their ancestors had built.

Fortunately, demon activity was at an all time low after the event which would be known as the "Demonic War."

Things weren't perfect, the demons and Grimm were far from down for good. People still died at the hands of those monsters from time to time, but they were disposed of just as easily by the Huntsmen and Demon Slayers.

Peace was once again restored to Remnant and would stay that way for many centuries until the present or the "Taisho era" as the people of Japan call it.

The demons weren't publicly seen for centuries and most civilians nowadays don't even think demons existed in the first place even though the Demon Slayer Corps was still active, operating away from the public eye.

This also applied to the civilians in the outside world, and even most Huntsmen. It seemed like the Demonic War was far behind them now.

For some in the present, the long lasting peace would endure for another two years. But for others, the end of peace had already arrived at their doorstep.

This is a story about two people, one from each side.

On one side, there is a girl with black hair with red tips, a red hood, a mechanical scythe/sniper rifle,…and silver eyes.

On the other, a boy with ruffled black hair with burgundy tips, a green and black checkered haori, a burn scar on his forehead,…and Hanafuda earrings.

The girl lives on a small island called Patch near the city of Vale with her father and older sister in a small house in the woods, her late mother being a skilled and powerful silver eyed huntress.

The boy lives in Tokyo, Japan with his mother and five younger siblings in a small cabin on Mt. Kumotori, his late father being a simple coal merchant. At least, that's what he believes.

The girl wants to become a Huntress and protect others like the heroes she heard about from the stories her father read her as a child.

The boy simply wants to live a good, long, and simple life with his family. Even if they didn't have much.

Both are happy and innocent...and naive. They are convinced nothing could ever go wrong.

The thirteen-year old girl named Ruby Rose gets to spend another two years in her little Fairytale

While the thirteen-year old boy named Tanjiro Kamado is thrown into the dark, cold abyss of suffering known as Reality...


And that's the prologue, it was pretty much just a crappy attempt at world building. I hope you like how I set up this world, make sure to review so I can learn what I'm good at and what I need to improve on as a writer. That said, this is my first time so if you have to criticize, break it to me gently please.

Now as I said before, this story is basically gonna be the same two stories you heard before playing side by side. I'll try to throw some extra dialogue and events into the mix to make things more interesting before we get to the juicy bits, but I understand if you might get bored just reliving both stories all over again.

If you decide to stick around, thank you very much! I plan on releasing Chapter 1 very soon, I'll probably already be done by the time I post this part. If this gets enough attention for me to continue, my posting schedule will most likely be random. Don't worry though, I don't plan on leaving you hanging for like three weeks with each chapter.

Also, I hope I didn't make the Huntsmen sound too weak against the demons. I'm sure the more powerful Huntsmen are more than capable of taking on at least a mid-level demon until sunrise. But I don't see them doing much to the Upper Rank Demons or Muzan. By the way, Salem vs Muzan, who would win? I think Salem but you guys let me know your opinions.

Finally, what did you guys think about my idea of Aura Enhanced Total Concentration? I thought it was a neat idea. Also, the whole "breathing techniques unlocking unique Semblances" thing was my way of making the so-called "special-effects" of Demon Slayer a reality. I really don't like the idea of "breathing make normal human op" because that kind of implies that anyone could do it. Of course, the demon slayers can still used Total Concentration even when their Aura reserves are depleted, the attacks will just be a lot weaker. I hope you guys like the idea, I've gotten really invested in it.

Well, see next time on Chapter 1: Fairytale vs Reality