What does the Bible say about demons?

Are demons merely myths, or are they fallen angels, allies of Satan, active spiritual forces of evil in the heavenly realms at work today? Find out why, despite the reality of the powers of this dark world and demonic activity, believers holding fast to Christ have nothing to fear because, as 1 John 4:4 states, "greater is He that is in you."

Source: www.gotquestions.org/demons-Bible.html