Kimetsu no Yaiba does not belong to me but to Koyoharu Gotōge ... I only do this for fun and hobby

Warning: This is an AU. I think there is a slight Out of Character, don't blame me! I'm new to fandom.

There are also changes with the manga canon.

It had been 3 years since Tanjiro and Nezuko's arrival at the infinite fortress, growing and strengthening to be of help with their father's ambitions. Much of the Upper and Lower moons (Needless to say which one I mean huh?) Became attached to this pair that they now consider family. In that time, many anecdotes are necessary to tell about those experiences of this pair of demon brothers but, it is better to count some necessary.

The first was in relation to food. Because they don't consume the same food as the rest of their species, they had to survive in 2 ways: His father's blood and sleep, the latter being something strange and unusual for others. The moons, above all a certain platinum blonde with rainbow eyes, tried by all means that they could eat the humans, but their failures were more and their success was null, so they chose to surrender. One day they decided to accompany their father on a mission to finish off a demon slayer as a study excursion type, however, unexpectedly, the father saved one of his lower-class "subjects" but, he did not have the luck to his side since he was quickly a victim of the brothers.

"..." Muzan was shocked, watching his sons tear the limbs of that demon, tasting like a saucer the flesh and blood of that creature that begged for help to be silenced by the girl since she cut his neck.

"You ..." Kibutsuji giddy whispers. He had to control his nervousness because of what he was observing: The cannibalism of his own species by his adopted children.

"It's delicious!" The brothers exclaim with joy, completely ignoring their father since they only wanted to satisfy hunger.

From there, Muzan noticed that there was a new way to satiate the appetite of his offspring ... although deep down it causes him to repel (All the moons weighed the same). He decided to accept that his children satisfy his hunger with his weaker followers that he believes was not worth it.

The second thing that happened when the brothers were barely 2 years old since they lived with demons and has a name: The incident of the bloody moon. It happened that their father gave them the first mission to carry out, being to finish off a squad of Demon slayers who were in the vicinity of the Red District, where is Daki's territory. That day there was a lunar eclipse, Tanjiro and Nezuko had been fighting with slayers of Kinoe and Kinoto rank, however things took an unexpected turn when a wind breath user cut off the pink eyed girl's left arm and the red-eyed boy's leg right. Seeing how someone you love is hurt by a stranger was the breaking point, causing them both to unconsciously gain power by transforming into larger versions of themselves. Nobody lived to tell the terror that the two demons caused, ruthlessly ending and tearing each slayer, bathing the place and themselves with blood but, the worst thing was that they lost their reason ... they were attacking each other despite the fact that they annihilated to a squad of 10 people.

Kibutsuji had felt that something strange was happening with his children so he went to see that it was his whereabouts ... the words died in his throat since those boys he adopted attacked each other like wild beasts. Why were they attacking each other?. He used his power to exert his control over them with his blood but it was useless, why didn't they obey him? They have their blood circulating in their veins, their own cells move in their bodies, he didn't understand. He opted for another measure and have Nakime call his 3 main moons and Daki who is nearby.

Kokushibo, Douma, Akaza and Daki were shocked to see the stage with their protagonists. Muzan did not explain as he also did not know the factors that caused that problem, he gave the order to stop them and leave them out of action regardless of the method they applied. The higher moons present are amazed by the unexpected increase in power of both Berserks modes and found it difficult to achieve their boss's request without having to kill them, Douma was able to deliver the final blow by cutting their heads with their golden fans and causing the Gyoutaro's sister lock them in her girdle.

"What the hell happened to them? That overwhelming power surpasses any of us" Akaza screams gasping as his wounds regenerate.

"I can't deny it, despite being rank 1, they surpassed my power" Kokushibo sighs as he watches the heads of his apprentices fade into the ashes.

"I'll find out what happened. But knowing that they have opened a new door of power is gratifying, but not with these circumstances" Muzan comments as he asks the sixth moon to take the brothers to a room that he usually uses in the fortress.

1 week later, Tanjiro and Nezuko opened their eyes. They felt exhausted and confused. What happened? they were supposed to be on a mission but were wounded by the enemy ... What happened after that?

"I see that you awake" Muzan watches him without emotion.

"Father / Dad" The brothers say at the same time. Sighing, the man relates he saw it with his own eyes and the way his moons used to stop them. When he finishes narrating, he opens his eyes and is surprised to see them. Cry?

Between sobs, the two recounted what happened before they lost control of their new powers, making it possible for the dark-haired man in elegant clothes to tie the dots and better understand the panorama. In silence and not very characteristic of him, he embraces them, whispering that they did not need to feel guilty for what happened. He listens to the lament of his children, they were ashamed that their father was a spectator of such unfortunate vulgar behavior caused by them, and they even apologized to each other for the aggressions caused between them.

Months after that incident, moons 1 and 3 worked to get their students' Berserk mode controlled, during that time, a third event occurred that left their father speechless and would lay the groundwork for a high-risk mission to which they would perform. It all started at dawn in a common forest in that country, Tanjiro and Nezuko were fighting against Akaza and Kokushibo as part of the training that they ignored that the sun started to give its first signs.

"Let's suspend up to here, Nakime opens the door to go to the fortress" The six-eyed demon says seriously when he notices that the dawn begins to appear.

Everyone makes a race against the clock so as not to be incinerated by the sun, however, the pink-eyed female stumbles on a root protruding from a tree which alerts her older brother and goes to help her. The adult demons arrived on time to a door that appeared out of nowhere, but before closing, they notice that the young people were not by their side. The pink-haired man with tattoos realizes the situation of the brothers, although he wants to help, he realizes that the solar rays begin to cover the landscape, which prevents them from being rescued, something that the black-haired man with six eyes knows very well for his misfortune, if one or both go to the rescue of their wards, they would not make it in time even if they use their bloody arts. Amid the sadness and frustration that they could not save them, they close their eyes to avoid seeing the cruel imbalance ... if there was one.

Seconds before, Tanjiro and Nezuko note that time is against them, so they decide to surrender and accept their cruel destiny by embracing each other since at least they would die together. Sunlight bathes and screams at them from the pain of burns, but that suffering slowly fades to the point of feeling nothing. They open their eyes and notice that the scars from sunburn gradually disappeared because of their regenerative cells, erasing from their skins the most desired test by all of their species including their own father: Defeat the sun

The surprise of both upper moons that could observe that phenomenon was enormous. Demons that can walk in the sun without being cremated? It was the first time that they were spectators of something unusual. Out of nowhere, they were teleported to the fortress by the power of Nakime (She didn't care if Upper Moons 1 and 3 were at the blissful gate she put them), where a Muzan watches them with a smile.

"So ... after a long time, someone could finally beat the sun" Kokushibo and Akaza prostrate themselves before their boss, Tanjiro and Nezuko do the same.

"My lord, we ..."

"It isn't necessary to say something. I rejoice that my children could meet one of my many goals."

"Dad ... it is disturbing that we beat the sun while you and the rest can not achieve that desire."

"My sister is right, why us? We are not special."

"Tanjiro, you're wrong. In your genes, you carry that monstrous and disgusting vestige of that being that almost annihilates me, and that you could perform that power is a test."

"Father ... I ... I think you are wrong" At those words, his father's gaze hardens, causing fear in those present

"Wrong ?, I'm all ears to know those reasons" The red-eyed boy shudders to hear the voice and rage, so he tries to control himself so as not to anger him more than he already is.

"You see ... having these skills may be a matter of luck, as well as being able to beat the sun, but that doesn't mean that we are special."

"And the point is? ..."

"We haven't shown our worth to be at the level of which we are worthy. The Upper Moons and one of the Lower Moon ones have shown their power but in our case ..."

"Brother, perhaps ..." Tanjiro listens to Nezuko's whisper and with a glance he answers in the affirmative.

"Let us carry out a mission that will be of great benefit to all of us."

"Mission? What would? If you can know."

"Infiltrate the Demon Slayer Corporation to finish them from within" 10 words were able to open the eyes of the demon progenitor. And it was not the only one, since the moons present were impacted.

"That idea was supposed to be just a joke. But if with that we can demonstrate the capabilities of our power ..." The pink-eyed girl determinedly observed her brother as a support.

"If you allow me, my lord ... that mission is very dangerous. I don't doubt the capacities of Tanjiro-sama and Nezuko-sama, however, it is impossible to hide their demonic essence and more if it is for so long, the enemy will annihilate them if they discover the truth."

"I agree with Kokushibo, if they pose as demon slayers, they would have to face those of our class and above all, if they come to confront us, they must finish us to show their worth and confidence to the enemy."

Muzan raises his hand as a signal to silence those present. He took minutes to analyze the pros and cons of that idea, on the one hand he thinks that it was ideal for his children to carry out this type of mission and more if it was to annihilate their enemies, but it was also dangerous if a slayer discovered the identities of they. Half an hour after thinking and amid the abrupt and disturbing silence, he decides to take the floor.

"I must say that this idea is interesting and striking, it is true that the proposal has some holes that allow the decline to be made."

"Father / Dad ..."

"However, for some rare reason ... just imagining you betraying your trusted "comrades", causes a strange and gratifying emotion in my person" The smile that the dark-haired plum-eyed person causes a chill in those present .

MONTHS LATER-IN A SLOPE IN A DESOLATED MOUNTAIN.

Winter progresses slowly despite the fact that snow and ice decorate the landscapes of that mountain, however, the whiteness of the snow contrasts with the carmine that sprouts from some bodies, victims of a slaughter caused by a vile creature.

"The stage is ready" Muzan Kibutsuji turns to watch his children in disguise. Nezuko has her hair messy and loose, a yukata mangled from cuts, and a wound from her chest to her neck as well as her head. Tanjiro for his part, wears his green and black plaid haori, a peasant outfit and a pastel blue scarf, his shoulder length hair is tied by a high ponytail. The funny thing was that they both show their human disguise, hiding their demonic characteristics.

"From here we will separate, I hope a lot from you" See how his children bow, the last one they would see for a while.

"We will not fail" They say at the same time, when they open their eyes, they notice that their father disappeared.

"Brother, do you know what to do?"

"Yes, I can feel someone coming ..."

"This will be fun, ne?"

"So is. Let the show begin."

.

.

.

When he wakes up, he watches his unconscious sister, with a muzzle-shaped bamboo adorning her mouth. Listen to someone's deep, cold voice, call him.

"What ... what happened?" Tanjiro asks pretending to hurt his head. A man with black hair, dark blue eyes of which he wears a haori half red and half patterns in green and orange.

"Wear a sword and that uniform ... is a demon slayer" He still remembers the pathetic fight he had to give to deceive that young man with a cold look and ... sadness? and prevent him from killing his sister.

"What would you do if your sister came to eat humans?" Asks the young hunter in a cold voice.

"What an idiot, eating humans is disgusting to us ... That will not happen, I have faith that she will not eat anyone."

"How are you so sure?" Tanjiro gives him a steady look, fighting with his instinct to finish off that guy.

"I know there is some humanity in my sister. If I must sacrifice years of my life to find a cure, I will" After saying that, a silence overwhelms both. Watch the pillar sigh as he turns his back on him.

"Go to Sagiri Mountain, you will find a man named Sakonji Urokodaki. Tell him Giyu Tomioka is sending it" That said, he disappears fleetingly. Tanjiro uses his nose to verify that Tomioka was out of his range.

"Apparently this is a mere introduction of what will be in store for us ... Nezuko, are you awake?"

"Something ... Did that guy leave?" She asks in a whisper as she releases that uncomfortable muzzle.

"I think. We will have to go to the Sagiri mountain, we must tolerate this path if we want the mission to be successful. Can you bear without speaking of what I become a slayer and infiltrate their ranks?" The hug from his sister was the answer he wanted.

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2 YEARS LATER-OUTSKIRTS OF SAGIRI MOUNTAIN.

Tanjiro observes or more admires his new clothes that he would be wearing from now on. a dark-brown gakuran jacket, a white belt, baggy pants that extend into tabi socks, a pair of zōri with red straps, and bands of white cloth wrapped tightly around his calves, all beneath his signature checkered haori. He also uses a large wooden box on his back, which was a gift from Urokodaki to transport his sister throughout the day.

As he walks his first mission, his thoughts approach excitement, as the infiltration was completed when he managed to pass the Final Selection and obtain the title of Demon slayer. He is saddened that Nezuko could not actively participate since part of the deception in her is pretending that the sun is dangerous to her as well as the primitive and childish behavior.

"My family was attacked by a demon during the night, despite the injuries, I was able to survive but my sister Nezuko ... I don't know why they turned her into a demon. It is true that she tried to eat me but I could make her see reason, I saw her tears when I motivated her to fight that new instinct. I don't know why she doesn't speak, or why that creature did that to us, but I'll do whatever it takes to find a cure and get Nezuko back to the way she was before. "

It was the lie the boy uttered to both Tomioka and his teacher with the Tengu mask, and it was relieved that they at least believed him.

During his walk, he called the crow to ask him something ... or well, that animal believed that. Tanjiro inserted his claws into the black-feathered bird, and enjoyed listening to the crow's cry of pain and agony.

"I wonder if it is possible to turn animals into demons. Hopefully it works, because I want you to be useful to me" He whispers maliciously, being the last thing the bird listens to before he becomes unconscious.

At night, the brothers finished eating a low-class demon and it goes without saying that the creature was a victim of their appetite, since in the words of the pink-eyed girl "That happens for intruding on our way" . While resting, the red-eyed boy's Kasugai raven woke up, Nezuko called his brother to look at the result of that bird's demonization.

"Who'd say? It worked" Tanjiro proudly observes his creation. 4 pairs of carmine eyes, black feathers with red tips and a pattern fire mark from the chest.

"From now on, this creature will serve us and our father Muzan-sama. You will be their informant and you will also be a spy in the Demon Slayer organization. You will be a double agent and you will inform our father of everything you discover about that organization."

With his squawk and a nod of his way, was the answer, accepting his new life, where his mind has only one purpose: to serve Tanjiro, Nezuko and Muzan faithfully.

With a smile of pure evil, Tanjiro and Nezuko begin preparations for their mission to be successful. Longing to enjoy the despair of their enemies when they deliver the final blow and achieve one of their father's goals ... The destruction of the Demon Slayer Corps.

TO BE CONTINUE.

Part 2 is now available and it's time to ...
Taisho Secrets!
* The Berserk mode of our brothers. Because they unblocked that mode unconsciously, it took them 2 years to control that power so they only use it in drastic measures. With Nezuk it is the same that we saw in the manga but for those who have not read it yet, leave the description.

-Nezuko: She grows to an adult stature, a short white horn grows on the right side of her forehead, and a venous pattern appears around her left eye, she gains a red and green vine-shaped pattern covering her skin , going around his arms, legs and chest.
-Tanjiro: He grows to an adult stature, gets a red horn on the left side, a venous pattern covers around the right eye and right cheek. The reddish black flame pattern covers her arms, legs and chest. His hair is tousled and unruly

* Why Muzan did not eat one or both brothers when he learned that they defeated the sun? In the next chapter will be the answer.

* And most importantly: Being on a mission to infiltrate the Demon Salyers Corporation, what is seen in the manga / anime is valid in this fanfic ONLY that in the vs with Rui, Tanjiro and Nezuko... they never faced that moon but rather it was Tomioka. That was because they were both secretly eliminating squads that came with the pillars.

That's it. Chapter 3 will take time to publish because I have problems with the development of this.
See you!