What does the bible teach us about demons or possession by evil spirits? And does this happen today?
The bible is clear that there are evil forces and powers at work in the world. The whole letter to the Ephesians is about helping the church understand who they are in Christ so that they take their place in the battle with evil that rages.
The gospels also show us demon possession (Matt 4:24, 7:22, 8:16, 8:28, 9, 12. Luke 7:21, 8:2, and so on) as does the book of Acts (Acts 19). So demon possession is real. However even in the bible there is an increased occurrence of this during the gospels. Jesus is victorious over them and they fear him because he is the son of God.
Does it happen today? Yes it does. Though we can be confident that Jesus will be victorious.