With the advent of medieval Christianity, the term "demon" was applied to any evil spirit in general. The term was especially applied to fallen angels, or those that were banished from heaven and thrown into the fiery pits of hell, as opposed to angels, who sided with God and remain in heaven. Lucifer, the most beautiful angel of all and God's favorite, incited a heavenly rebellion long ago because he either claimed to be more powerful than God or because of his jealousy towards Adam, who was permitted to name the animals of earth. A cosmic battle ensued, where Lucifer and the angels that had sided with him were defeated and subsequently banished from heaven. They were thrown into the fiery pits of hell to suffer until the end of time for their insurrection. After having been thrown into hell, the fallen angels were referred to as demons.

Demons were reputed to tempt humanity to evil so that people would share the same eternal fate as them. They were also thought to be the causes of madness and disease, especially epilepsy, causing such ailments by possessing people. Exorcising the afflicting demon would remove the ailment caused by it.

According to medieval theological tradition, a demonic hierarchy was established in the nether realm, with Satan (Lucifer) as the chief and other angels presiding in offices below him.