Demons Cause:

Seizures (Matt 17:15; Mark 9:18)

Deafness (Mark 9:25)

Blindness (Matt 12:22)

Disease (Matt 10:1 Mark 1:23-26; 3:10-11; Acts 5:16;8:7)

Harmful and Erratic Behavior (Matt 8:28; Mark 5:2-5; Luke 8:27-29)

Loss of Speech (Matt 9:32-33)

Life Threatening thoughts and actions (Mark 9:22)

Physical defects (Luke 13:11)

False Teachings (1 Timothy 4:1)

Demon Possession Stories Found In The Bible 

A Man possessed by Legion and Jesus healed him Mark 5:1-20, Luke 8:26-39

Man of Capernaum healed by Jesus Mark 1:21-28, Luke 4:31-37

Man that could not talk was healed by Jesus Matthew 9:32-34

A slave girl healed by Paul at Philippi Acts 16:16-18

A girl from Tyre and Sidon healed by Jesus Matthew 15:21-28, Mark 7:24-30

A blind and deaf man healed by Jesus  Matthew 12:22, Luke 11:14

A boy at Mount Hermon healed by Jesus  Matthew 17:14-21, Mark 9:14-29, Luke 9:37-43

Woman That suffered 18 years was healed by Jesus  Luke 13:10-13

Judas Iscariot was possessed by Satan  Luke 22:3, John 6:70-71;13-27

Mary Magdalene had 7 demons that Jesus healed Mark 16:9 Luke 8:1-2

Sceva's sons used the name of Jesus to try to rebuke demons out of man, but the man ripped the clothes off them. Acts 19:11-20

Demons

Demons Can:

Teach false ideas and beliefs (1 Timothy 4:1)

Speak (Matthew 8:29, Mark 3:11;5:12; Luke 4:34; Acts 19:15)

Experience Fear (Luke 8:28; James 2:19)

Have great strength (Mark 5:2-4; 9:17-26;Revelation 9:15-19)

They know their fate (Matthew 8:29)

Have names (Luke 8:30)

Know Jesus and are afraid of him (Luke 4:3)