The most well known possession by a demon took place when Satan the devil was allowed to influence and control Judas Iscariot. God permitted this act in order to insure Jesus would be betrayed to and arrested by those who hated him (Matthew 26:3 - 4, 14 - 16, Luke 22:3, John 13:2, 27).
More than one demon, according to Scripture, can take possession of and bring torment to an individual. For example, Jesus spoke of an unclean spirit finding seven other fallen angels in order to increase the suffering of a person (Matthew 12:43 - 45). Mary Magdalene had seven devils cast out of her (Luke 8:2). The Lord, in one of his most famous miracles, cast out not one demon but several thousand (a legion) from a Gadarenes' man (Luke 8:26 - 30).
A demon, if it chooses, is capable of possessing animals. Satan, in the Garden of Eden, personally took over a creature with the ability to speak and stand up in order to tempt Eve (Genesis 3). Several thousand demons preferred entering pigs rather than having nothing to torment (Luke 8:31 - 33).
Those who are the victim of demon possession sometimes are endowed with unusual abilities such as superhuman strength (Luke 8:29, Acts 19:14 - 15). Other times, they are deceivingly led to believe that the disobedient spirit within them can contact the dead or foretell the future (see our article on the witch of Endor). Usually, however, a demon's primary task is to afflict their host.
In the gospels, demon possession is frequently mentioned in relation to Jesus' earthly ministry. Many of his miraculous healings involved freeing a person from such unwanted spiritual guests (Matthew 4:23 - 24, 8:16, Mark 3:11, Luke 6:12 - 18). The recording of such evil activities underscored the Lord's goal of destroying the efforts of Satan's kingdom (1John 3:8, Hebrews 2:14).
The Bible warns us not to take any demonic activity lightly (1Peter 5:8). God's adversaries have dedicated themselves to undermining, in any way possible, the advancement of his kingdom. In fact, according to Christ, their unity against God is such that they have agreed not to fight amongst themselves or hinder each other's evil schemes (see Matthew 12:25 - 26)!
Jesus had great compassion toward those suffering due to demon possession. He initially gave his twelve disciples (Matthew 10:1, 8), then later seventy believers (Luke 10:1, 17), the authority to cast them out of people. He gave future Christians, as part of his great commission (final instructions) to his church before his ascension, the authority to cast out a demon (Mark 16:15 - 18).