The Demon Slayer Corps is founded around a thousand years ago during the Heian period after a distant relative of the current Ubuyashiki family named Muzan Kibutsuji inadvertently became the very first demon. On advice of a priest, the Ubuyashiki family established the organization and assumed the responsibility of leading the battle against the demon progenitor and his demons, as eradicating the world from demons would break the curse that bound them.

The Twelve Kizuki is an organization of the twelve most powerful demons in existence who are under the direct command of the Demon King, Muzan Kibutsuji.

Kokushibo is a very tall man of muscular stature and pale skin complexion. He possesses long, spiky black hair with red tips that he keeps in a ponytail. His most notable feature is the three sets of eyes on his face with yellow irises and red flesh sclera that have black straight lines diverging from each iris. His middle set of eyes has the daiji of "Upper Rank (上じょう弦げん Jōgen?)" and "One (壱いち Ichi?)" engraved in them while his other sets of eyes feature black pupils and a cracked pattern in the irises. Kokushibo also has red markings resembling flames on the top left side of his forehead and the bottom right of his chin. Kokushibo wears a purple-and-black hexagonal patterned nagagi kimono and black umanori-styled hakama pants tied with a white uwa-obi. He carries a fleshy katana at his waist that has eyes in the space between the tsuka ito wrapping of the handle of his sword. The tsuba and blade are shown to have eyes and veins. The scabbard is also fleshy in appearance. Kokushibo is the first demon that utilized Breathing Styles. His Breathing Style, in particular, is one of the most dangerous and powerful ones displayed thus far. Enhanced with his Blood Demon Art, he can create many chaotic crescent moon blades when slashing that vary in length and size in crescent shaped sword attacks. Kokushibo had continued to develop this Breathing Style and had created over a dozen techniques over the centuries he had lived.

Demons are a carnivorous, vampire-like species whose primary diet is humans, consuming their flesh and blood. Sunlight is deadly to them and will burn them to ash, so they strictly operate at night. Demons have an abnormally powerful physical constitution and potent regenerative abilities, making them immune to normal weaponry and granting them the ability to heal from any wound with ease, even re-growing their head. Only the Nichirin Swords, weapons made from special ore with sunlight absorption properties, can inflict life-threatening wounds. However, even with a Nichirin Sword, the only method guaranteed to kill a demon is through decapitation. Demons are also known to hate wisteria, which are poisonous to them and whose smell is repudiated by them.

Breathing Styles are swordsmanship styles that makes use of an esoteric breathing technique known as Total Concentration Breathing practiced and taught by members of the Demon Slayer Corps and are used in combat by Demon Slayers (later passed on as traditional dances post-time skip).

All known Breathing Styles currently taught within the Demon Slayer Corps are derived from Sun Breathing, the first one ever created. The majority of known Breathing Styles mimic a certain element of nature (i.e. flame, water and wind) and replicates it with the user's movements, techniques and abilities. The weapon attacks of Demon Slayers who use Breathing Styles are also longer ranged, possibly due to shockwaves or air pressure generated from their weapon itself when swinging and striking.

The Blue Spider Lily is a mythical and enigmatic flower that is described to be a spider lily that is blue in color instead of red. The flower only blooms at daytime for two to three days a year, after which it closes back up, resembling a large horsetail weed.[2] In some cases, depending on the climate, the Blue Spider Lily doesn't even bloom at all. This explains why Muzan Kibutsuji never found it despite searching all over Japan for centuries: he was hindered by his inability to go out during the day.

This flower has medicinal properties, being used to cure Muzan of an unknown illness that would've caused him to die before turning 20,[3] albeit the medicine was in its experimental stage and thus extremely difficult to replicate.[4] The Blue Spider Lily medicine was also the cause behind Muzan's demonification, granting him a strong body and changing his blood to have transformative properties, enabling Muzan to create more of his kind as well as hold dominion over them. However, this medicine also had the side effect of causing Muzan to disintegrate into ash if he was exposed to sunlight as well as a craving for human flesh.[5] It is also implied that the Blue Spider Lily can nullify a demon's weakness to the sun if it’s consumed, which is the reason Muzan was spending centuries searching for the flower.

Muzan is a tall and muscular man, with an unnaturally pale skin complexion that can be easily confused as being unhealthy and sickly. He has curly black hair, and sharp, almond-shaped eyes. His irises are cat-like in appearance and colored plum red. He also possesses sharp, blue nails on his hands. He appears to be in his twenties.

This flower has medicinal properties, being used to cure Muzan of an unknown illness that would've caused him to die before turning 20,[3] albeit the medicine was in its experimental stage and thus extremely difficult to replicate.[4] The Blue Spider Lily medicine was also the cause behind Muzan's demonification, granting him a strong body and changing his blood to have transformative properties, enabling Muzan to create more of his kind as well as hold dominion over them. However, this medicine also had the side effect of causing Muzan to disintegrate into ash if he was exposed to sunlight as well as a craving for human flesh.[5] It is also implied that the Blue Spider Lily can nullify a demon's weakness to the sun if it’s consumed, which is the reason Muzan was spending centuries searching for the flower.