Demon

Demons (鬼) are a group of malevolent creatures that kill and eat humans. The first Demon is said to be Muzan Kibutsuji, and it is from his blood that new demons are made, although this is not absolute, as Tamayo has also been stated to be capable of this.

Description
They are immortal beings that can live centuries. Their diet consists of humans. Demons are able to change their body structure at will, heal their wounds rapidly, and there are also Demons that possess special abilities. They can only be killed by sunlight, and if their heads are cut off from their body using a special blade. They are also repelled by wisteria flower, which is how they keep Demon on the mountain for the Final Selection.

Demons are not born, they start out as humans and once they get demon blood in their systems and then they become demons. Once a human becomes a demon they lose all their memories and develop a murderous instinct. They still retain their personality but it turns into a twisted, dark version. When they are killed, their memories flash before their eyes. However, they are a few demons who can retained their humans emotions and memories such as Nezuko Kamado, Tamayo and Yushiro. 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 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 the demon’s body.

A demons power is dependent on how many humans they eat, this can become from physical changes, as seen with the Hand Demon, or in their Blood Demon Arts, as seen with Rui. 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 thread and Daki can turn into and manipulate special scarfs.

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.

List of known Demons
L - Lower, U - Upper, M - Moon
 * 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
 * Kokushibo (UM1)
 * Kyogai (LM6)
 * Nakime (UM4)
 * Rui (LM5)
 * Spider family
 * Mother Spider Demon
 * Father Spider Demon
 * Older Brother Spider Demon
 * Older Sister Spider Demon
 * Susamaru
 * Swamp Demon
 * Tamayo, the only other Demon aside from Progenitor that was able to turn a human into a Demon
 * Tamayo's cat
 * Yahaba
 * Yushiro
 * Rokuro (LM2)
 * Wakuraba (LM3)
 * Mukago (LM4)
 * Kamanue (LM6)