There is no being more universally believed in across the world than demons. Even in the modern, industrialized nations there are many who claim the existence of demons as true.
The really interesting thing is that all of these nations and regional versions of demons are very similar. That is, cruel spirit-beings who delight in tormenting mortal creatures. Many times they are seen as monkey-like in built (long arms, stooped posture etc), and often resembling deformed humans. The classic gargoyle is a good example of this design.
From the Hindu Asuras and New Age Flyers to Christian Demons, these monsters are seen as having any number of horrid animal features, such as birds feet, horns, wings, tails and the like. But the most disturbing are the immaterial forms of smoke, of a formless terror in the night.
Some reports of demons say that these monsters literally prey upon mortal creatures. Seeking out men to either kill them directly or to "feed" on blood or less material essences such as breath, dreams, hope or less defined energies. Whether or not this "feeding" gains demons nourishment, or is just part of a depraved need is up for debate.
Demons are reputed to give mortal creatures many kinds of diseases and to "posses" men and women, possibly turning them into werewolves or "just" murderous madmen. Their apparent obsession with these activities appear to be out of an unbridled hatred of men, and in the case of possession, to indulge in fleshly pastimes not allowed to them in their "natural" form.
More on this subject later.