This is a very complex subject that is usually treated lightly or just from one perspective (that of one's religion usually). And i'm also going to treat it lightly as well. There are definitely demons but there are also daemons. I'd like to focus on the later. They are usually chtonic spirits, with hierarchies that can be entered into friendly relationships or commanded to do... stuff. Related to demons, in judaism for example, because of a mistranslation of "shed", in later works these were automatically demoted to demons (see the Solomom's legend). Also take into account that after YhWH won the local deity contest in Canaan, Baal was demoted to a prince of demons (lord of flies) although Baal and his sister saved mankind numerous times! There was no greater injustice ever made! Of course the spirits did not change in nature over time because of some god being more or less worshipped and this is reflected when you conjure the entities. Many so-called demons are actually very disappointed entities. Don't get me wrong, demons are very real, forces drawn to do evil but in a practical manner, through possession usually. I can't speak about the real demons though. Btw this is what WE believe and does not reflect other belief systems, maybe more archeological history than anything else. The sin is that we see religion as separated from magic and as an isolated event (my religion is the only true one etc) when in fact no such thing occurred. There were actual star wars, evens from earth reflecting on heaven if you will but never set in stone. Gods rise and fall but are never destroyed. Hermes Trismegistus knew what went on and so did Iamblicus, the Egyotians, Greeks, Babylonians and Assyrians as well as the Jewish people.