A demon (also called a devil in the King James Version) is a spiritual being. There are Christians who believe that demons were once angels that were in heaven with God, but chose to rebel against Him by joining Satan and thus were cast out of heaven.
These beings now work in what Scripture calls "the sons of disobedience". However, most demons shown in Left Behind operate at God's behest.
There were demons that appeared throughout the Left Behind books that were working with the forces of the Antichrist and the False Prophet. One of them was a wonder worker named Orcus that appeared outside of Petra in "The Remnant" that lured unbelievers away from the place of refuge with powerful miracles, only to kill them in the wilderness. Three other demons, named Ashtaroth, Baal, and Cankerworm, inhabited the bodies of Carpathia clones in order to gather the armies of the Antichrist to the Valley of Megiddo in an attempt to overthrow God and Jesus Christ. These three demons were later destroyed by Jesus Christ in "Glorious Appearing".