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. There are a few Demons, who have retained their humans emotions and memories such as Nezuko Kamado, Tamayo and Yushiro.

Appearance
Demons come in a variety of forms, most look human at first, only with slight changes like gaining pale skin, catlike eyes, or a change in hair color. The more humans they eat, the more changes they undertake. Some demons still retain their normal appearance, while others gain monstrous features like multiple heads or limbs. Some demons have dark markings that come in various patterns across their body. There seems to be a subclass of demons. These are Demons who have completely lost their minds and have gain incredibly monstrous features. They turn into combinations of animals and demons, with disgusting features like multiple heads and mouths.

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. Muzan at the time was diagnosed with an disease, which would kill him before he would turn twenty. Muzan however killed the doctor out of frustration believing that the doctor's treatment would be useless and only make him suffer, till he would meet his untimely death. After killing the doctor he would soon realize that the treatment of the doctor had in fact worked and he had transformed into a demon. After his transformation, Muzan found out that he was unable to walk in the sunlight. Finding out how to get rid of this weaknes, became his life goal. To help him find a solution, he started creating other demons by exposing them to his blood.

Creation
Demons are not born naturally instead they were once humans, until they either got exposed to demon blood or in Muzan Kibutsuji's case transformed using a drug called "Blue Spider Lily". Normally it is led to believe that only Muzan can transform humans into demons, hovewer few of the Upper Moons have shown this ability as well. These being the Upper Moon One, Kokushibo and the Upper Moon Two, Doma. Tamayo was also able to transform a human into a demon, however in order to transform one it took her 200 years.

Abilities

 * Immortality: Demons possess eternal youth and can live for centuries, maybe even longer. They can't die through conventional means.
 * Regeneration: Demons have a strong regenerative ability, which can heal any sort of injury caused by a conventional mean in seconds.
 * Inhuman Physical Capabilities: Demons possess inhuman strength, speed and endurance.
 * Blood Demon Art : Blood Demon Arts are spells demons can cast due to their special blood. The look and abilities of these spells can vary, for example Rui can manipulate sharp threads and Daki can turn into and manipulate special scarfs.
 * Growing Power: The strength of the demons depend on how many humans they have eaten. They can also grow stronger by receiving additional blood from Muzan. Some demons however have a limit in how much they can 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 major weakness. Sunlight ignites the Demon cells and hampers the regeneration to the point where the entire Demon body burns to ashes, resulting in death. The only demon so far to have developed resistanse 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 kill a demon.
 * Wisteria: The Wisteria Flower is poisonous to demons and also gives off a smell, which they don't like. Wisteria Flowers can be used as a base ingredient to create deadly and highly potent poison mixtures, which can easily kill lower rank demons.
 * Curse of Muzan: Every Demon created through Muzan's blood is also inflicted with an deadly curse. The curse activates, if they call out Muzan's name or divulge information about him. Demons like Tamayo and Nezuko have freed themselves from the curse through a special method.