Demon

Demons (鬼) are immortal and malevolent beings. Most Demons possess a murderous instinct to kill and devour humans and have lost most of their memories from their human life. Demons do still retain their personality, except it often turns into a twisted, dark version of it, although there are a few Demons, who have retained their human emotions and memories, these being Nezuko Kamado, Tamayo and Yushiro.

Appearance
Demons come in a variety of forms. Most look human at first, only with slight changes like paler skin, catlike eyes with unusually-colored sclera, dark markings that come in various patterns across their bodies, or a change in hair color. The changes differ from Demon to Demon, as some Demons can still retain their normal appearance while others gain monstrous features like multiple heads, mouthes or limbs.

It's been noted that the more humans a Demon has eaten, the more physical changes they may undertake.

Origin
The first Demon that is said to have existed was Muzan Kibutsuji. The one who turned him into a Demon was a generous doctor from the Heian Period, who wanted to save Muzan from death since, at the time, he was diagnosed with an disease which would kill him before he turned twenty. However, out of frustration and believing the treatment would be useless and only cause him more suffering, Muzan killed the doctor before the latter could finish the treatment by letting Muzan consume the final part of his "Blue Spider Lily" medicine.

Shortly after, Muzan came to realize that the treatment had in fact worked, prolonging his life, although at the cost of his humanity. He soon regretted killing the doctor, as he found out that becoming a Demon made him unable to expose himself to sunlight anymore, as it burned him, and the only way to overcome the sunlight was by consuming the "Blue Spider Lily" medicine of the doctor, whom he had killed. The Blue Spider Lily however could not be found. Obsessed with walking under the sun once more and getting rid of his weakness, Muzan has made his the life goal to find the "Blue Spider Lily" by creating Demons with his blood, so they may help him finding it. This makes Muzan the progenitor of all Demons.

Creation
Demons are not born naturally, instead reproducing by being converted from a human after 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. People resistant to Demon blood exist, however they are very rare.

Tamayo was also able to convert a human into a Demon, although through science rather than the sharing of blood, however this method is nowhere near as efficient as it took her two hundred years to develop, and was put into practice only once.

Abilities

 * Immortality: 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.
 * Enhanced Regeneration: Demons have a 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.
 * Inhuman Physical Capabilities: Demons possess inhuman strength, speed and endurance.
 * Blood Demon Arts :  These are spells that Demons can cast due to their special blood. The look and abilities of these spells can vary, for example, Rui can manipulate fine, sharp threads and Daki can turn into and manipulate special sashes.
 * Growing Power: 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.

Weaknesses

 * Sunlight : The sunlight is deadly to all Demons, making it their most major weakness. Sunlight ignites the Demon cells and hampers the regeneration to the point where the entire Demon body burns to ashes, resulting in its death. The only Demon so far to have developed resistance to sunlight is Nezuko.
 * Nichirin Blades: Nichirin Blades are blades made out of Scarlet Crimson Iron Sand and Scarlet Crimson Ore, which absorbed sunlight to such an extent that a decapitation strike with such a weapon can instantly kill a Demon. There are exceptions, however, as Muzan, Akaza and Kokushibo have been able to overcome this weakness and become immune to being killed through decaptication by a nichirin blade.
 * Wisteria: The Wisteria Flower is poisonous to Demons and also gives off a smell which they don't like, making it useful as a tool to ward them away. Wisteria Flowers can be used as a base ingredient to create deadly and highly potent poison mixtures, which can easily kill lower ranked Demons and even ended up playing a large part in the death of Doma, Upper Moon Two and one of the strongest Demons in existence.
 * Curse of Muzan: Every Demon created through Muzan's blood is also inflicted with a deadly curse which activates if they call out Muzan's name or divulge any information about him. Demons like Tamayo and Nezuko, however, have freed themselves from the curse through a special method.

List of known Demons
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Demon