Question: "What does the Bible say about demons?"
Answer: Revelation 12:9 is the clearest Scripture on the identity of demons: "The great dragon was hurled down-that ancient serpent called the devil, or Satan, who leads the whole world astray. He was hurled to the earth, and his angels with him." The Bible indicates that the demons are fallen angels – angels who along with Satan rebelled against God. Satan’s fall from heaven is described in Isaiah 14:12-15 and Ezekiel 28:12-15. Revelation 12:4 seems to indicate that Satan took one third of the angels with him when he sinned. Jude verse 6 mentions angels who sinned. So, it is likely that demons are the angels who followed Satan in sin against God.
Satan and his demons now look to destroy and deceive all those who follow and worship God (1 Peter 5:8; 2 Corinthians 11:14-15). The demons are described as evil spirits (Matthew 10:1), unclean spirits (Mark 1:27), and angels of Satan (Revelation 12:9). Satan and his demons deceive the world (2 Corinthians 4:4), attack Christians (2 Corinthians 12:7; 1 Peter 5:8), and combat the holy angels (Revelation 12:4-9). Demons are spiritual beings, but they can appear in physical forms (2 Corinthians 11:14-15). The demons / fallen angels are enemies of God – but they are defeated enemies. Greater is He who is in us than those who are in the world (1 John 4:4).