A beautiful and wise high ranking angelic being was once given a position of power and influence, but due to his pride he wanted to take over the position of God. For his rebellion he was cast down out of heaven along with a third of all the angels in heaven. Since then the enemy has been in a state of perpetual warfare against God. Through the old testament times his plan was to destroy the messianic line, and once the Messiah (Jesus) came the enemy's plan was to divert Him from fulfilling His mission: dying on the cross. Having failed that the enemy has fixed his focus firmly on us who were created in God's image to steal (our joy in God), destroy (our relationship and trust in God), and kill every single Christian on earth if possible.
This being of course is Satan and the fallen angels are his minions, the demons. One of the greatest things ever written about the enemy was written by CS Lewis in the Screwtape Letters;
“There are two equal and opposite errors into which our race can fall about the devils. One is to disbelieve in their existence. The other is to believe, and to feel an excessive and unhealthy interest in them. They themselves are equally pleased by both errors, and hail a materialist or magician with the same delight”