“Demon” is an extremely broad term that covers a vast number of supernatural entities ranging from the relatively harmless and even docile to the unspeakably evil. Some are incredibly intelligent. Others are as stupid as cattle. Some are massive and others are tiny. But all of them share a physiology that is distinctly different from all organic life on earth. In fact, a demon’s physical form can change drastically depending on its will. The more powerful ones can take almost any form, but its true shape is always something shadowy and elusive for the human mind to comprehend.
There exists a complex class system for demons that is also virtually impossible for humans to understand. At its base, less powerful demons are subordinate to more powerful demons. However, a demon’s intelligence also plays a powerful role, as does its breed. So do countless other factors. It is not uncommon for a much weaker demon to hold power over much stronger ones.
Modern religion has made many cultures quick to blame any unnatural occurrences on demons, but true demonic activity is exceedingly rare. Most people who experience supernatural activity are usually dealing with ghosts, projections or other far less dangerous manifestations. Modern religion has also created the myth that all demons are tirelessly dedicated to the corruption and ruination of humans in the name of some great personification of evil. But they possess free will and individuality. And to put it bluntly, most of them have better things to do with their time than tempting you into sin. In fact, it is not unheard of for a demon to be benign. Although it is, admittedly, fairly rare.
Some of the more common and noteworthy breeds of demon have their own entries in this compendium.
(Note: I have come to understand that demons do not, in fact, originate from hell. Nor are they necessarily connected with hell in any way. They are much older than any concept of hell.)