Demons are agents of Satan from Hell who manage to get upon the Earth and get resituated. This is part of the conspiracy of the Hell dimension to embed sleeper agents, infiltrate our organizations, and subvert our moral paradigms. They usually appear as red horned goatse motherfuckers (literally) who have pointy arrow tails, leather bat wings, sharp bad teeth and cool fashion sense. They often sport twisty beards and carry pitchforks (they love to drive a point). They almost always have red eyes, even when shapeshifted into other forms. Contrary to popular belief, albinos are not usually demons. Gingers may be, however, as are anyone without eyebrows (not including cancer victims), sharks, some bears, bats, Christian fundamentalists, ISIS, L Ron Hubbard, 17-year locusts, certain aliens, and hockey hooligans. Demons can take many forms with their oddly malleable, boneless bodies of flesh and smoke. When they transubstantiate as rocks or plants, they can get stuck that way for ages.
Demons have been known to infiltrate human society and influence them via a system of secret societies like the Illuminati, greed via the 1%, or an army of trolls using the power of LOL. They also take a more serpentine route (they would) by seducing youth using music, selfies and tabletop games. These are merely conduits to a demon's main goal, which is to take over human bodies themselves, since they are inexplicably comfortable and desirable to demons (as opposed to itchy and disgusting to humans).
In Myth Edit
Information surrounding demons is entirely confusing, every piece of writing contradicts the other. No one knows exactly which demons do what or where they come from. Ask anyone and they won't be able to tell Lucifer and Satan apart or differentiate Beelzebub from Asmodeus. Are they the same or are they different? Is Lucifer an angel still or a demon?
Certain video games have been known to be portals to the realms of hell, and cause people to flip out and kill everyone.