What are demons according to the Bible?

from the old testament

• Demons are evil/fallen angels who rebelled against God. When Satan was cast out of heaven, the demons fell with him instead of staying in their place as God's ministers of good. Somehow, evil was found in them (Ezekiel 28:15). Satan's fall is shown, symbolically, in Isaiah 14:12–15 and Ezekiel 28:12–15.

from the new testament

• A description of the fall of Satan and his demons can be found in Revelation 12:9, which says "And the great dragon was thrown down, that ancient serpent, who is called the devil and Satan, the deceiver of the whole world—he was thrown down to the earth, and his angels were thrown down with him."
• Revelation 12:4 possibly indicates that a third of the angels fell with Satan, but this is not explicitly stated.
• The sin of the angels is also mentioned in Jude 1:6–7. Because verse 7 compares the angels' sins to the sins of Sodom and Gomorrah (i.e. "pursued unnatural desire") this passage can be interpreted as a reference to those demons who came to earth and engaged in sexual activity with human women, producing the Nephilim (Genesis 6:4).
• Just as God's angels are sent to praise Him and aid believers, Satan and his demons are bent on the besmirching of God's name in the earth and the destruction and deception of all those who love and serve Him (1 Peter 5:8; 2 Corinthians 4:4; 11:14–15).
• The Bible calls demons "evil spirits" and "unclean" (Matthew 10:1; Mark 1:27; Acts 19:15) and asserts that they are indeed the angels of Satan (Revelation 12:9). :
• Satan and his demons also attack Christians (2 Corinthians 12:7; 1 Peter 5:8) and do battle with the holy angels (Revelation 12:4–9).
• Demons, like the holy angels, are spiritual beings that can sometimes take on physical form (2 Corinthians 11:14–15).
• Demons are powerful, and the Bible indicates that any human being who trifles with them is entering foolish and dangerous territory (2 Peter 2:10–12; Acts 19:13–16).
• Demons/fallen angels are enemies of God and humanity (Ephesians 6:12; Revelation 12:9). But it is a battle that is no real contest. Demons/fallen angels are created beings with far less power than their Creator—God. They do not match Him and could never overcome Him or His purposes. God allows demons continued power at this time, but ultimately He has defeated them and their nefarious purposes (Colossians 2:15; 2 Peter 3:8–10). Satan may be the prince of this world, but greater is He who is in us than those who are in the world (Ephesians 2:2; 1 John 4:4).
• Demons will be cast into the lake of fire with Satan when Christ judges the earth (Revelation 20:10, 14–15).

implications for today

Though demons are indeed a real and dangerous presence, intent on leading us astray and causing harm, their power is ultimately defeated by God. We must remain vigilant and aware of their schemes, as they seek to disrupt our faith and sow confusion. However, our confidence and hope rest in the fact that God, who is infinitely greater and all-powerful, has already triumphed over these malevolent forces through Christ’s victory on the cross. This assurance empowers us to resist and overcome their influence, knowing that God’s protective power surrounds us and His strength is far superior to any threat they pose. In our daily lives, we should lean on God’s promises and seek His guidance, trusting that His sovereignty and grace ensure our ultimate safety and victory.