I think evil spirits exists. But, i think how it all works is the evil spirit gets stronger the more we feed it. We feed it by fearing it and by fighting its temptations.
In rare cases an evil spirit is so strong that it takes "posession" of the person. This comes with spectacular menefestations. These are rare.
I knew 3 people who told me they saw demon posession.
One guy told me he saw a little girl chained up. She grabed his friend when he came close and she threw him accross the room.
Another guy (misionary) told me him and his crew went to this guys house. The guy was crawling on the cealing like a spider. The guy kept screeming in a monstrous voice "we are not comming out!"
Him and the crew tried to call on Jesus to cast it out, but it was not working.
The misionary told me he got his laptop out in order to type out every detail he was witnessing.
Then he said the room felt very oppressive mentally. Then he heard a voice say "delete that! My power is more then the power of evil!" He said after he deleted it, the oppression off his mind lifted.
After that, they left, they could not help the man.
The other guy told me he knew a pastor who went to this house. A child was talking in a monstrous voice, blaspheming God excessively. The pastor had a side kick with him. The pastor told the side kick "make sure your heart is right with God, because when this demon comes out, its gonna want to jump in somewhere else."
Interestingly, after he ordered the demon to come out, it did jump in his side kick. It picked him off the floor and pinned him to the wall.
These 2 testimonies i got direct from others. The third, was a second hand.