A demon is a supernatural being, often malevolent prevalent in religion, occultism, mythology, fiction and folklore. In some religions they are considered unclean spirits often fallen angels or deceased humans. In other religions or occultism demons are spiritual entities that may be conjured and controlled.
Like most of the things I am writing about for the challenge this year I have always had an intense fascination with, not necessarily demons themselves but with all the mythology and lore surrounding them. I think they are a great subject for stories much like vampires and witches because there is so much there to work with.
Some interesting things about demons are:
1) The Ancient Greek word daimon refers to a spirit or divine power with no connotations of evil or malevolence.
2) Popular Hebrew mythology has demons that are believed to come from the nether world and have various physical ailments ascribed to them such as headaches, epilepsy and nightmares. (suffering from migraines I totally see how they could come from the nether world)
3) There was a demon, 'Shabriri' who rested on still water at night and blinded people who drank from it.
4) Demons are often shown to be under divine control despite typically being associated with evil.
5) Grimoires contain that names and abilities of demons as well as instructions for conjuring and controlling them.