Just as angels are the agents of God who act with divine authority to do good works among men, so demons are the agents of Lucifer who act with infernal authority to do works of evil. Indeed, in Jewish, Islamic and Christian mythology, Lucifer was himself once an angel of heaven who defied God.
Lucifer seeks to harass God by plaguing mankind with a multitude of troubles, and by inciting human beings to defy God. He uses his fallen angels as his agents, sending them abroad across the face of the world to incite and commit evil. Every time he succeeds in inducing a human to defy God, Lucifer gains another soldier in his rebellious army.