Demons or unclean spirits are hardly mentioned in the Old Testament. But they are mentioned many times in the four gospels as beings with evil powers. Often, demons controlled people and caused them to do things they would not ordinarily do. What is most important about these evil spirits is that they were always shown to be weaker than the all-powerful Son of God.
We do not know for certain where evil spirits came from. But we do know that some wicked angels rebelled against God and were thrown out of Heaven (read 2 Peter 2:4).
Today, there is no need to worry about being “possessed by an evil spirit.” The Bible says that there would come a time when God would cause the “unclean spirit to depart from the land” (Zechariah 13:2). We know, like those who saw Jesus cast out demons in the first century, that He is in control. And, as Christians, you can be sure that “the One who is in you [Jesus] is greater than the one who is in the world [Satan]” (1 John 4:4).