I do not own Kimetsu no Yaiba or MHA

This stort contains spoilers for Demon Slayer: Kimetsu no Yaiba past the anime and near the end of the manga. If you aren't on Dimensional Infinite Fortress yet, be warned. There's a lot of spoilers here.

Prologue

The battlefield at its current state is a mess. A whirlwind of fists and slashes could be seen in a deserted room where two people were duking it out in a battle to the death. Upper Moon number 3, Akaza was cockily exchanging blows with a tall figure in a green and yellow haori, who was desperately slashing his sword as hard as he could, using form after form of the Water Breathing technique, in a desperate attempt to cut off the head of the demon.

"You've used up all of your Water Breathing forms now, Giyuu! But honestly, I'm amazed that you've lasted this long against me by yourself. I'll do you a favor, and end this quick. Farewell!"

Giyuu Tomioka, the Water Hasira, knew that he couldn't go on anymore. His body was at its limit, and his sword was snapped in two, a short edge in his hands without much use. He had no way of defending himself against the demon's superior moves. He looked behind the demon, and saw the unconscious figure of Tanjirou Kamado, the boy that he had found on a snowy hill, whose whole family had been murdered by Muzan Kibutsuji. The one that he had been protecting for the past 5 minutes with his life.

He's still out. I can't hold out any longer. If I die here, he'll be finished off right after!

Giyuu saw the demon's hand stretched out, claws reaching for the middle of his chest, which were reaching to pierce his heart. He knew he was too slow in blocking the fatal strike.

I'm done for.

He lost hope when suddenly, a splash of blood appeared between the two of them, where the arm which would have sealed his fate had suddenly disappeared. He looked up, and saw that same arm soaring through the air, and a man who's hair was sun-red crouched down with his Nichirin Sword in his hand.

Tanjirou cut his arm off in the middle of his attack. The sound is different from his regular breaths, even his hair and eyes are different. I was saved by Tanjirou.

Akaza was confused, and also terrified about what just happened. How did he block that attack when he was so near-death? He didn't even move! However, he had no idea that there was someone behind him.

Every single cell in my body down to my cheek hair is telling me to kill this guy right now. Something about him has changed. This is bad! I have to kill him, here and now!

"Technique Deployment, End Style: Chaotic Blue Metal Afterglow!"

Tanjirou opened his eyes, and what he saw was a scene that he had never seen before. Everything around him was in infrared. He could see the muscles, brains, and everything inside the two figures doing battle.

So this is the place that Dad was talking about… the See-Through World.

Azaka was standing upright, after throwing a hail of assaults at Giyuu.

"Come on Giyuu. Don't die like Kyoujiro and Tanjirou. Become a demon." He once again threw the offer out to the Water Hashira, only to be ignored. However, Giyuu was looking behind the demon.

He hasn't noticed Tanjirou yet! There's your chance, while he can't see you! Go for his head!

Instead of taking advantage of his momentary element of surprise, the now sun-haired Demon Slayer chose to yell at the top of his lungs. "AZAKA! I WILL CUT OFF YOUR HEAD! RIGHT NOW!"

Giyuu was beyond confused about what he just did. Idiot! Why did you call out his name like that!

Azaka finally noticed the imminent threat and turned around.

How is he still alive after that attack! It doesn't matter. My compass will be able to detect it. But something's strange. It's like he has no battle spirit!

He turned around, and was greeted with an upside down Tanjirou, with his sword cutting a deadly arc to his exposed neck.

Dance of the Fire God: The Setting Sun

Tanjirou struck at the right side of his neck, and dragged his sword through in one motion. Giyuu and Azaka both had bewildered looks on their faces.

Impossible…

The demon's head then fell off, and his body was starting to disintegrate, all the way down until he was just powder in the wind. Tanjirou flipped back around from his position in the air upside down, and tried to land on his feet, but lacked the energy to move any of his muscles. The last thing he saw was the ceiling, and Giyuu struggling to get up from his position from the ground.

Crap, I'm losing consciousness! Muzan is so close by, I just need to get to him, and cut off his head!

Tanjirou struggled to keep his eyes open, but the jarring headache forced him to close his eyes, as he faded into unconsciousness.

The sun blazed brighter than Tanjirou was normally used to when he regained consciousness. He was laying up on what felt like long grass. He tried opening his eyes, but the sun was directly on his eye, and he was forced to shut them again.

What just happened? Where am I? I'm supposed to be at the fortress with Giyuu and all the Hashiras...

He sat up, covered his eyes with one hand, and sat up with the other. He stood up and got a look at his surroundings. His eyes were greeted with a familiar sight; the sight of a large wooden cabin in the woods.

Is this my house? No, it looks like my house, but this isn't the same, or it doesn't feel the same. It never felt this bright before, it was always snowing back home. How am I here though? It's so quiet, and there's no sign of destruction or a battle anywhere...

Behind him, leaves rustled, and the sound of dirt and leaves crunching was apparent, followed by the voice of someone he had never heard before.

"You're awake."

Tanjirou was startled for a moment, and he turned around him to find the source of the voice. He looked towards the trees, where there stood a man who was wearing a plain red haori over an orange kimono with a sword strapped to his waist. He had red hair, with the same shade as his own, with a mark in the shape of what appeared to be a fire on the right side of his face. What stood out the most to Tanjirou was that he had the same hanafuda earrings that he had. He recognized that from the description from somewhere.

Is that…. the first breath user? Yoriichi, right?

He spoke again. "Your name is Tanjirou."

"Ummm…. yes. I'm Tanjirou Kamado."

"Kamado. Makes sense, I almost mistook you for Sumiyoshi."

Sumiyoshi? Is that my ancestor's name? Probably, from what his words were implying.

"You're the same as him…. except you've been hardened to become a Demon Slayer. But your eyes radiate compassion for others, even for demons."

After the century-old slayer had finished saying his assessment of Tanjirou, An awkward silence passed between the two, one waiting for the other to speak. Yoriichi decided to break that silence. "If the slayers from my time were like how you are now, things would have been different. I wouldn't have been banished from the Corps otherwise."

He was banished? For what? I thought he was the one who had the most powerful breath, and the one who taught all of the others how to improve their fighting! Why would they do that?

"You must be wondering what my story is. I'd recommend sitting down for it. After all, we have all the time we need."

Unsure of what to say to that, he just agreed to his request. Tanjirou and Yoriichi both go to the porch of the house, and take a seat, both looking at the horizon.

"Over here, it's just been me for the longest time. I wanted someone to listen to me. What came up were the faces of Sumiyoshi and Sukayo, his wife. You and Sumiyoshi are nearly identical in every aspect." Tanjiro didn't know how to respond, so he just let Yoriichi continue.

"From time to time, when I take glimpses into the real world, and I see the smiles of all of the others there, it makes me happy. I'm happy to see all of them happy."

I guess I don't count as "them" anymore. If I'm here, that means I'm probably dead.

"This world….. is beautiful in every single way. From the people in it, to the blue skies above. I believe being born in this world is a blessing in itself." Yoriichi had a very sad look on his face, as he started to recall past memories from so long ago.

"My mother was a very religious woman. She frequently prayed for strife to disappear from this world, every single day. She prayed every day to the sun god to shine on my deaf ears. She was the one who gave me these earrings as charms. At the time, I never spoke, so I made her worry often, and I felt very bad about that. My brother was a very nice boy. He was constantly worried about me, even when my father struck him and told him to forget me. That day, he gave me a flute and told me, 'Blow this when you need help, nii-san will save you.' He smiled, face bruised, and told me, 'you don't have to worry about a thing.'"

That's such a sad story. I'm not sure what I would do when I was in his situation.

"As a child, I was unwanted. After my mother had died from her illness, I had left the house. I had been told to enter the priesthood, but I never went to the temple. Instead, I opted to run as much as I could, as far as I could, and see how far the sky reached."

Yoriichi paused in his monologue, staring at the now darkening sky, the slightest glimpse of stars beginning to make its way into the sky. He then sprung a question on Tanjirou.

"Have you ever sat down on a hill at night, and taken a look at the stars? Have you taken some time, and appreciate the beauty of it?"

Tanjirou pauses and thought about his answer before giving it. "Yeah, I have. After a long day of training, when you get tired and your limbs are sore, it really helps you get your energy back. Being outside at night helps me keep training to get stronger."

"That's how I felt when I ran away from home. I ran the entire night, but my feet never felt tired, or wanted to stop. I kept running and running to the mountains, where I ended up in a place that was filled with compact fields and paddies. And in the middle of one, I found someone standing there all alone. She was a girl that was about my age. She was holding a bucket, and wasn't moving for the longest time. I was confused, so I asked her what she was doing. That was when she told me her story. Her entire family had died from a disease. She was lonely, because she had lost everyone that she had known, so she brought tadpoles from the paddies back to her home."

It was obvious to Tanjirou that the girl the first sun used was speaking about was very important to him. Listening intently to Yoriichi's memories, he tried to find out how he came to be the way he was now.

"But when the sun started to set, she released all of the tadpoles that she had collected in her bucket, and put them back into the paddy. I asked her, 'You're not going to bring them home?' She told me that she felt bad about separating the tadpoles from their parents and siblings. She was lonely, and had no family left. She was just like me, just searching for what made them happy. To her, it seemed like having someone there was that happiness. So then I asked her, 'How about I go home with you?'"

Once again, Yoriichi paused before continuing. "When I said that, she turned around, and started to stare at me. Her eyes were glistening with tears, and they were like obsidian. They were beautiful under the reflections of the sunset. The girl's name was Uta. From that day on, we started to live together. She was someone who loved to talk from day to night. But, thanks to that, I learned that people saw things differently than I did. She told me she never heard of being able to see through living creature's bodies."

Wait, is he talking about the see-through world, the one that I accessed when I was fighting Azaka? He was able to use it even when he was a child?

"That was the first time I learned about the reason behind my small feeling of alienation. 10 years later, we became husband and wife, with a baby on the way. During her last month of pregnancy, I went to fetch a midwife to prepare for the birth. However, on the way, I ran into an old man trying to cross the mountains, trying to rush over because his son was on the verge of death after suffering a wound in battle. I accompanied the old man and his son to their place. I decided that I would wait until tomorrow to get the midwife. I got home as fast as I could, but it has already turned to nighttime. Uta has been killed, along with the child in her belly."

He came home to his family murdered, just like I had.

"That day, I had learned a lesson. Even if something is more precious to you than your own life, others can and will easily trample over it."

Remembering those terrible moments had taken a toll on the old slayer, as his voice started to waver towards the end of the sentence. Tanjirou could hardly blame him. He knew how it felt to experience that, to come home one night and find your family decimated. He wouldn't wish that on anyone else.

"My only dream was to live a quiet and happy life with my family. A small home with our futons standing side by side, at a distance where I could see their faces and easily reach out and touch their hands whenever I wanted to. But that would never come true, only because demons exist in this beautiful world."

After the monologue was finished, a long silence ensued. That was when Tanjirou finally spoke. "Was this why you became a Demon Slayer? To get revenge for your wife and child?"

"I became one to make the world into a better place, so that what happened to me doesn't have to happen to anyone else. That was my purpose of becoming a Demon Slayer. Just as your purpose of becoming a Demon Slayer to save your sister from being a demon, and turn her back into a human. Everything happens because of a cause."

Hearing Yoriichi mention Nezuko, Tanjirou had suddenly remembered something.

If I'm here now, that means I'm dead, and I can't do anything to turn her back into a human! Tamayo is dead too, so that means there isn't a chance of them figuring out a cure, either!

With a worried tone etched in his voice, he asked the million-yen question. "Yoriichi-san! If I'm here, does that mean I'm dead? Does that mean there's no way I can turn my sister back into a human?"

Yoriichi didn't respond immediately, but when we did, he calmly explained to Tanjirou the situation. "Before Tamayo left to intercept Muzan at the home of Kagaya Ubayashiki, she had completed the prototype medicine for the transformation from a demon to a human. It has already been administered, and Nezuko, along with Sakonji Urokodaki, Shinjuro Rengoku and Kiriya Ubayashiki are with her, waiting for the effects to take place. Even though I do not know the result, there is a chance that she is already able to turn human. If that happens, then Muzan's plan to intercept Nezuko and gain the ability to conquer the sun will be impossible. You have already done as much as you can do, and you went down in battle to your last breath."

Tanjirou listened with wide eyes as he heard what was happening with the people that he cared about back down there.

Tamayo was able to do it. She made the medicine, and she turned her back into a human. Thank you. Thank you so much.

"So does this mean I'm dead now? Do I watch from up here while the rest of the Hashiras fight Muzan?"

However, he did not expect in his wildest dreams the answer to that question.

"That depends on your next decision."

Tanjirou's head perked up at that sentence. "Is there a way I can go back? Is there a way I can go back and help them?"

"Unfortunately, you can't do that. Once someone is dead, there's no way they can go back to how things once were."

Tanjirou's risen hopes came crashing to the ground as he learned the circumstances. He couldn't go back. He was stuck here. What would he do now? Yoriichi mentioned something about a decision earlier, right?

"Wait, didn't you say something about making a decision? Is there something else I can do?"

Yoriichi stood up from the porch that he had been sitting on, and started to walk towards the trees again. He motioned for Tanjirou to follow him.

"Follow me. It would be easier for me to show you."

Tanjirou jumped up, and he sprinted after Yoriichi's speeding form through the woods. They were running for a good 15 minutes until the light started to show between the leaves of the trees. The woods ended, the walls of trees faded away, and what greeted them was an ever expansive beach. Miles of sand, which were seemingly endless stretches out as far as the eye could see in both directions, to Tanjirou's left and right. In front of him was a vast ocean, water was everywhere, no end in sight.

Once Tanjirou had appeared, and stood next to Yoriichi, overlooking the sea, Yoriichi put his hand out as if he were trying to reach the sun, and made a pulling motion. The sun suddenly shifted downwards, disappearing into the ocean, the sky went from bright to dark in a matter of seconds. The moon moved at rapid speeds to replace what had been the sun moments before.

"Before I start explaining, I would like you to keep a very open mind. Things that I explain here are things that you may not believe for a while when you first hear them. When I heard about this, at first I didn't think it was possible. However, this is something you must understand before you make your decision."

Tanjirou nodded, mentally preparing himself for the impossible. He paid full attention to Yoriichi, and what he was about to say.

"To put this as simply as I can, there are other worlds beside this one. Worlds that are similar, yet different. What we mainly want to focus on for now are the differences. The differences can vary, from events in history occurring differently, civilization being more advanced than others, and some things not existing in the first place. Many worlds exist that don't have demons, but have other types of evils. Some worlds exist where people cannot live in a peaceful environment, which leads to constant warfare. However, right now, we are mainly focusing on one world."

"This world, many hundreds of years ago in a country called China, a baby had been born that was radiating light. It was a complete mystery on how these powers came to be, but as the generations kept passing by, more and more of these people had special powers that they can use. Today, every 4 out of 5 people that exist have a special power, and these are known as quirks."

"These powers helped the world grow faster in many ways. For many people, having quirks is a good thing. They make people more powerful, and more able to protect themselves. However, even though these powers exist, there are still people that opt to be evil, and to do things that screw other people over for the benefit of themselves. More and more people are willing to do bad things just because they have quirks that allow them to get away with doing them. Society called them 'Villains'."

"In response to these villains, people who had strong quirks well suited for combat, and the drive to help others in need, opted to solve this problem. They were the better and more effective version of police, and they translate from our world's Demon Slayers. They were known as 'heroes'. More and more villains appeared, which caused more and more people to step up to the plate to take care of them. Heroes. They gained fame, riches, and glory. Soon a new system emerged. The word 'hero' became a new profession, one that provides hope to others."

"They protected people from the villains and other evils that existed, and they got money and fame for doing so. It became something that everyone wanted to be, but it wasn't at all an easy task. It requires determination, willpower, and a good heart in order to become a good hero. In the present day, there is one hero that stands above all others. He is extremely powerful, and is admired by everyone. The people feel safe, knowing that he is there to protect them. He is known as the Symbol of Peace, All Might. He is thought to be invincible, and unbeatable. That he will always be there to protect them."

"What people aren't aware of, is that All Might will not be there to protect them forever, and when he finally is unable to fight, there are going to be many many villains who have been suppressed by All Might's influence, and heroes will be needed in order to stop chaos from erupting. Many people will die because of this, and there is only one way to prevent this from happening. A new symbol must rise. Someone, like All Might, who can protect others fearlessly, and make the people of Japan feel safe."

Tanjirou listened intently to the story of this world, and at the end of it, he was left speechless. As a kid, he has always wondered if there were other places like the one that he had lived in, with other people that did not know the other existed. For that to be real, he didn't know what to think of that. His thoughts instead wandered to this new world. Heroes. Villains. Quirks. How people lived. He has imagined what life would be like if demons didn't exist, but even if they weren't there, there would be some other evil that kept people from living in happiness.

"Having the mark means a lot more than just being gifted incredible strength and the ability to access the see-through world. Having the mark shows our determination to save. To help others that need it. The ability to use that power for the better of others. And now, you have the mark. It's only fitting, a whole world waiting for you out there. A new purpose that can be set out for you. Tanjirou, if you became a Demon Slayer to make the world a better place, to help as many people as you could, and there was a whole world of people in need, what would your decision be?"

"It's just like you said, Yoriichi-san. Seeing the smiles of everyone being happy, it makes me happy. Being able to live in the world is a blessing in and of itself. If I had the chance to help as many people be happy as I could, then why wouldn't I take it?"

Yoriichi then pointed out in the horizon, where the moonlight shone on the water's surface. "The gates are open to you here. All you have to do is go towards it. Keep on pushing, no matter what comes your way, in order to complete your purpose." After ending his speech, he then disappeared in a gust of wind.

Tanjirou looked behind him once again, taking in the view of the trees, and the sound of distant crickets echoing throughout the forest. He turned back to the water, started to run, and jumped in. He started swimming, as fast as he could, towards the moonlight. Towards a brand new world.

A/N This was a short introduction of a crossover that I've had in my mind, and have been inspired by other stories that I have read. I'm planning on having each chapter be about 10K-15K words and I will try making chapters as soon as I can. I'm aiming for the next chapter to be about 15K words in the next 3 weeks to 1 month.

Just a note, the place with the old home is not the same scene as Manga chapter 186. He isn't in Sumiyoshi Kamado's body, he is in his own. Yoriichi is shown to be a pretty depressed man in the manga, but here it's a bit different.

For the people who like to point out edits and try to help give suggestions to make stories better, I'm looking for a beta reader, because I'm not the best of writers out there. I'm not asking for the reader to make specific edits or anything like that, I just need someone to tell me their opinions on the chapter, and how they would rate it, etc. Use the PM feature on FanFic to PM me if you are interested.

Thanks for reading.

(Giyuu and Tanjirou vs Azaka: KnY Manga chapter 185)

(Yoriichi Backstory: KnY Manga chapter 186)