What does the Bible say about demon possession?

In Mark 5:1-20, we see the account of a man possessed by a legion of demons. This man could not be controlled or tamed by anyone, signifying that demon possession can render individuals unable to govern their own actions. The New Testament reveals that such possession is a manifestation of the spiritual battle that exists in this world, where evil spirits have power over those who are outside of Christ. Ephesians 2:1-3 further emphasizes that all unbelievers are under the dominion of Satan, fulfilling the desires of the flesh and, thus, are, in effect, under a form of demonic possession. This does not only refer to explicit cases of chaos and unrest but also encompasses the religious who, without the gospel, are still captives of sin and works religion, which can be seen as a more subtle form of Satan's control.