Humans and demons have been living for long in centuries, many of the humans were ignorant towards the existence of demons. Demons possess eternal youth and can live for centuries, sometimes even longer if they're incredibly strong. They can't die through conventional means, but can be killed by sunlight or with one of the Demon Slayer Corps special Nichirin Blades. The Wisteria flower is also poisonous towards demons.

Demons possess inhuman strength, speed and endurance. They also have strong regenerative ability more than any living being or human, able to heal any sort of injury caused by a conventional mean in mere seconds. That includes regrowing limbs and organs. Demons also have Blood Demon Arts. These are spells that Demons can cast due to their special blood. The look and abilities of these spells can vary.

Demons could also grow powerful. The strength of a specific Demon depends almost entirely on how many humans they have eaten, and they can also grow stronger by receiving additional blood from Muzan, which is how the Twelve Demon Moons gained their overwhelming power. Some Demons, however, have a limit to how much they can use to grow stronger, and they might be not compatible with a higher amount of Muzan's blood.

Demons are not born naturally, excepting Muzan, the very first demon who was born as a result of the Blue Spider Lily medicine. Most members of the race reproduce by converting a human through exposure to the blood of a very strong demon; only Muzan and his top two Twelve Demon Moons, Doma and Kokushibo, have been shown to possess this ability. The other Upper Rank demons also have the authority to transform humans they view as worthy into demons by giving them blood, and upon receiving permission from Muzan, their blood will transform into Muzan's. However, it is unknown whether or not the other Upper Rank demons have turn anyone into a demon or whom the identity of the one who has been turn into a demon.

The Twelve Kizuki (Jūnikizuki) also known as the Twelve Demon Moons is an organization of the 12 strongest beings amongst demon-kind, second in strength and authority only to the Demon King himself, Muzan Kibutsuji. Each member of the Twelve Kizuki has received a large portion of Muzan's blood, greater than any other demon, thus exponentially increasing their power. As a testament to this, every member of the Twelve Kizuki can use Blood Demon Arts, supernatural powers and techniques manifested through blood that only powerful demons have access to.

The Twelve Kizuki are divided into two groups, the six Upper Ranks (Jōgen), numbered one through six, and the six Lower Ranks (Kagen), also numbered one through six. The number signifies their ranking among the Twelve Kizuki, determined by each member's level of strength, Upper Rank One being the strongest and Lower Rank Six being the weakest. The Lower Ranks have their numbers and ranks engraved only into one eye, whereas the Upper Ranks have their numbers in one eye and the kanji for "Upper Rank"(Jōgen) in the other.

The Upper Ranks each possess a level of power that far surpasses that of the Lower Ranks; as proof of their strength, the strongest of the Demon Slayer Corps, the Hashira, have killed numerous Lower Ranks over the years, while the Upper Ranks have killed and eaten many Hashira.

The Twelve Kizuki have been given three tasks to complete by Muzan's command:

- To find the Blue Spider Lily (the key ingredient of the experimental medicine that made Muzan a demon).

- The destruction of the Demon Slayer Corps.

- After over a hundred years of failing at these two tasks, Muzan has decided that demons should serve him with suicidal devotion to ensure success.

Since the demons existed, there were also the existence of The Demon Slayer Corps (Kisatsutai). They are an organization that has existed since ancient times, dedicating its existence to protecting humanity from demons. There are hundreds of Demon Slayers within the organization, however they are not officially recognized by the government.

In order to become a Demon Slayer, an individual must pass the Final Selection, a test used as an entrance exam to join the Demon Slayer Corps. During this selection, examinees must survive for seven days on a mountain top infested with demons without any aid from the outside world.

Candidates who complete the Final Selection are issued the standard uniform, as well as being given the opportunity to choose the ore for their special sword, which is designed to eliminate Demons efficiently. Each new Demon Slayer is also assigned one of a rare species of talking bird for communication purposes.

Members of the Demon Slayer Corps are separated into ten different ranks. From lowest rank to highest, these ranks are:

Mizunoto - Mizunoe - Kanoto - Kanoe - Tsuchinoto - Tsuchinoe - Hinoto - Hinoe - Kinoto - Kinoe

Once a Demon Slayer has fully settled into the organization, they are classified as a member of the lowest rank: Mizunoto. They must complete a variety of missions to ultimately climb the ranks and reach the highest Demon Slayer position: Kinoe. Following the Final Selection, each swordsman has their rank inscribed on their hand through a process called "Wisteria Flower Engraving." As they continue to progress as swordsman, they can check their rank by saying a password and flexing their arm to reveal their status.

Within the Demon Slayer Corps, there is a group of elites known as the Hashira (Pillars). This group consists of the most powerful combatants in the organization. The Hashira, in accordance with their great strength, are the most highly ranked members of the Corps, second only to the leader of the organization. In order to become eligible to join the Hashira, a Demon Slayer must either successfully kill 50 demons or a member of Twelve Kizuki as a Kinoe.

Mizunoto are roughly paid around 20,000 yen whereas Hashira are paid as much as they want.

There was also others outside of the Demon Slayer Corps who worked on exterminating Demons and form an alliance with the Demon Slayer Corps. They were called the Uchiha. The Uchiha was reputed to be the humanity's strongest because of their Sharingan and natural battle prowess.

Uchiha are subject to powerful emotions: these emotions typically start as love for a friend or family member that then become overpowering hatred when the object of their love is lost. This loss, whether actually experienced or merely threatened, awakens an Uchiha's Sharingan. It is due to the Sharingan that the Uchiha have received much of their fame and notoriety.

The Uchiha have been fighting the Demons for centuries. Muzan did not take any action yet towards the Uchiha because the Uchiha usually kills the demon who were near the Uchiha Estate and also the one who are near the Demon Slayer Corps Estate since their alliance prompted them to watch each other's backs. Usually the Demon Slayer Corps were the one who sent their people to work outside far way while the Uchiha mainly are for patrolling since they have natural visual prowess.

Since Muzan never ever once take any action yet towards the Uchiha. The Uchiha through out the centuries have become too comfortable and overconfident thinking they were invisible towards the demons.

But what if all changed? What if one night Muzan set up a plan by himself to dealt with one of his sworn enemies using his own great intelligence and ability to think tactically? Does this will affect a certain Uchiha? How does the outcome will be?