When, where, why, and from whom did the concept "demon" originate?
I don't recall any "demon" amongst the Greek gods; there is no concept of demon visible within that cradle of civilization where -- according to Homer -- all souls went to Hades.
When the damned things pop up into our perception, at many varied places over time, because they felt like it and as fallen angels they were originally created by God with independent volition. Go attempt to explain physical phenomenon such as a chest of drawers moving on its own across the floor or knickknacks being tossed about without proximate cause. Rare events, but nonetheless very obviously discernible when they occur.