Devils and demons often play an antagonistic role in fantasy. They take on many different forms and come about in so many different ways. Some are malevolent spirits, others are creatures summoned from another realm, others come from the ground and have nightmarish bodies. All seem to be evil or at least neutral, and are often dangerous when summoned, kept barely under control, if controlled at all.
The concept is mostly associated with Christianity, where the devil, Satan or Lucifer, is a fallen angel opposing God's plan, and trying to lead men to hell. Some say he is the master there, others that he just wants people to be miserable with him. Some say he works alone, others with his demon minions. Most other religions have a devil or some other sort of adversary as well, although now people seem to want to focus on goodness and leave the devil out of it, ignoring him or not believing in one.
The image of the devil came in the middle ages, when priests thought that their followers were not scared enough by the devil, so they made an image meant to frighten people into righteousness. The devil had horns on his head, hooves like a goat, claws, and a tail. Kind of like a satyr, but red. Sometimes they gave him batwings. He used a pitchfork as his weapon. Other demons had similar characteristics. Usually ugly, they might also be beautiful, to seduce you.
Usually in books, these devils or demons don't look like the classical description. The most common is the spirit, since that is what devils are, if they exist (you don't see demons walking around, at least not how they are shown in pictures. People you know, however, like your in-laws if you're unlucky, might be personal demons for you). Sometimes they will possess bodies, a common idea in fantasy.
In games, sometimes they will be shown in a physical form. In Heroes of Might and Magic 6, the demon faction has 7 creatures, each representing one of the deadly sins. In literature, they represent the dark side of humanity, the nightmares, the fears, the depraved acts humans can commit. In this way, Pandora of mythology was the mother of all demons. They also often represent fire, or are represented by it, which, with Prometheus, acted the same way as the forbidden fruit of Adam and Eve: it brought knowledge to humans and the ability to act for themselves, separating them from animals. This knowledge burned with the pains of sin and the light of knowledge.
Since they are rarely, if ever, the good guys, they are a bit more one dimensional than other fantasy creatures. Because of this, and because people are more uncomfortable with religion, they are less and less common as characters. If thinking about using demons, be aware of what they represent and if they go long with the story, and use them in a creative way.
Reblogged from http://sageeyesblog.wordpress.com/2013/01/30/devils/
