Demons (鬼 Oni?) are immortal and malevolent beings, which kill and eat humans. Demons are not born and were once humans, until they had transformed. They possess a murderous instinct and almost lost all memories of their human life time. Demons still retain their personality, except that it turns into a twisted, dark version. There are a few Demons, who retained their humans emotions and memories such as Nezuko Kamado, Tamayo and Yushiro.
AppearanceDemons 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 still retain their normal appearance, but others gain monstrous features like multiple heads or limbs. Stronger Demons have Demon Marks, dark markings that come in various patterns across their body. There seems to be a subclass of Demons called Demon Summons. These are Demons who have 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.
The first demon that is said to exist 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, due the fact that he was diagnosed with an disease, which would kill him before he would turn twenty. Muzan however killed the doctor out of frustration in the believe that the doctors 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 worked and that he had transformed into a Demon. However the treatment of the doctor was not finished, and Muzan was restricted by the sun, due the fact that the sunlight would kill him. Finding an opportunity to overcome that weakness, he has created more Demons to assist him in finding a way.
All Demons are not born and were once humans. If a human comes into contact with the blood of Muzan Kibutsuji through inflicted wound or through drinking, then they will transform. Demons like Tamayo are also stated to be able to transform humans into Demons.
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 Demons depend on how many humans they ate and they can grow stronger by receiving additional blood of Muzan. Some weak Demons however have a limit in how much they can grow stronger and they might be not compatible with a higher amount with Muzan's blood.
Sunlight - The sunlight is deadly to all Demons, making it a 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 not affected by sunlight is Nezuko.
Nichirin Blades - Nichirin Blade 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 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.
List of known Demons
- Muzan Kibutsuji (Progenitor)
- Akaza (UM3)
- Daki (UM6)
- Doma (UM2)
- Enmu (LM1)
- Gyokko (UM5)
- Gyutaro (UM6)
- Hand Demon
- Hantengu (UM4)
- Head Demon
- Kaigaku (UM6)
- Nezuko Kamado
- Kamanue (LM6)
- Kokushibo (UM1)
- Kyogai (LM6)
- Nakime (UM4)
- Mukago (LM4)
- Rokuro (LM2)
- Spider Family
- Swamp Demon
- Tamayo, the only other Demon aside from Progenitor that was able to turn a human into a Demon
- Wakuraba (LM3)