Demons are said to be creatures comprised of spiritual energy. There are many different aspects and definitions to demonic beings which have only increased over the passage of time, though for the most part the word "demon" refers to inherently evil beings rooted to religion, mythology and occultism. Ironically, the term "demon" is derived from "daemon" which is an ancient Greek term for a good, benevolent spirit of nature or lesser gods.
Throughout most of human history, demons have been typically regarded as malevolent creatures that seek to corrupt mortal souls. Their powers and their appearances/manifestations vary greatly between different mediums but just about every society and religion today acknowledges the existence of such entities. Demons are a popular element in many forms of creative media.