Although widely cursed by superstitious folk, demons do not in fact exist. Certainly, nefarious beasts of many kinds, supernatural or otherwise, do exist. But there is no nether realm where demons feast on the blood of mortals and plot their days in sun where they can defy the gods and rule the lands of humanity.
My research on the matter has led me through thousands of impressive tomes, hidden scrolls, reliable first-hand accounts, and spurious rumor, and with all of this I have reached my certain conclusion. The tales of horrible demonic possessions and attacks have almost exclusively been the work of forbidden gods.
In reality, there is nothing truly "forbidden" about these gods, except that their worship is generally banned in polite society. However, throughout history they have only been very rarely worshipped willingly, with or without these laws. Instead, they are worshipped subconsciously by every person with impure thoughts and feelings - therefore every person alive.
As a result, while these gods are ignored and made illegal, they are nonetheless startlingly powerful. We will list a few of them here for educational purposes, but, even in the chapters of history, we must be careful not to give them too much breath. Technically, they are Lesser Gods, but they are often some of the most powerful of them because of the amount of time our corrupt hearts spend on them.
Belgot, god of disease
Akrinex, god of greed
Yalliope, goddess of torture
Hylene, goddess of shame