Where Do Demons Come From? ~ Michael S Heiser

https://blog.logos.com/2015/10/where-do-demons-come-from/

Excerpt:

Everyone familiar with the Bible knows it talks about angels and demons. But most would be surprised to learn that there’s no verse in the Bible that explains where demons came from. Christians typically assume that demons are fallen angels, cast from heaven with Satan (the Devil) right before the temptation of Adam and Eve. But guess what? There’s no such story in the Bible. The only description of anything like that is in Revelation 12:9—but the occasion for that whole episode was the birth of the messiah (Rev 12:4-6), an event long after Adam and Eve. The idea of a primeval fall of angels actually comes from church tradition and the great English poet John Milton in his epic Paradise Lost.

Continued at the link above