Although there are a few different beliefs where demons came from Biblical evidence points to the conclusion that they are fallen angels.(Ryrie p.181).
Satan was the first ever to sin in the universe. Satan's sin was a direct challenge to the authority of God. He was cast out of heaven, and it is believed that one third of all the angels rebelled with him. (Swindoll p.585). Therefore, these fallen angels work with Satan to bring about every type of sin and rebellion in the universe. Two ranks of demons described in the book of Ephesians are rulers and authorities. Demons in Ephesians 6 :12 are also called "unclean spirits."
Demons have personalities and knowledge just like angels. Demons have observed man for long periods of time and are aware of how they will respond to particular situations. There are two types of locations of demons, first there are demons who work with Satan and his evil plans. The second location of the other demons are those there are temporarily confined and demons that are permanently confined in Tartarus. (Ryrie p. 183). In 2 Peter 2:4 the Bible states, "For if God did not spare angels when they sinned, but sent them to hell, putting them into gloomy dungeons to be held for judgment-."
Demons work in conjunction with Satan to oppose the purposes of God in this world. ...