There are at least three definitions of demons going around: fallen angels, daimonion (the Greek of the NT), and soul-power creations. In the third, they can even be destroyed (to tie this in with your other thread) by pure mortals, nonetheless. If I go by the one in the gospels (which is what I assume most of us would choose), according to the common use of the word in Greek, the second of my three, they are not fallen angels. In fact, fallen angels sometimes use them as "bodies" made of soul power, and in this definition they cannot be destroyed, except by God. The third definition I have learned from ex-witches, where they are created and destroyed as desired. A classic case is the discarnate entities created by the Toronto Society for Psychical Research back in the early 1970's.