Demon

»But why are the demons involved in this?« she inquired. »Demons are never major characters. They care nothing about human events, unless they wish to interfere with them.«
»Will you make a princessly oath of secrecy?« the Professor asked. »The question has an answer, but it is not given for mere humans to know.«

Piers Anthony, Demons don't dream

Clinically speaking, demons/daemons/devils are personifications of personal weaknesses/fear. To the perciever/believer they are quite real. Generally the demon is a symbol/metaphor for a personification of fear, they are made up entirely of fear. Their characteristic as demons points to a refusal to take responsibility for the demon, i.e. the devil made me do it.

Computers these days have daemons, as did Socrates, and surely these are related to demons. I have a hunch, in fact, that they are etymologically no different from demons, and perhaps there are no computer viruses, no hackers, no crackers, but only demons who have wheedled their ways into the favor of programmers and have been set free in the cyberworld to work their mischief. (I blame them, too, for my errors of typing.)

demons swarmed around her head like a cloud of angry gnats.

Demons come from our mythology, which spring from our imaginations. How many colorful stories and beliefs have sprung from our imaginings?

Demons are the devil personified, or creatures from a nether world bent upon our destruction, or creatures from another plane that are summoned and captured on this plane to be slaves to mages. There are many stories about or involving demons – if you've read any of today's fantasy novels, you've been introduced to many of them.