Demons

The term "Demon" can refer to multiple creatures, all connected innately to darkness.
The first kind of creature are demonic spirits, which is how a demon begins its life. These are spirits of darkness; mostly unintelligent, mindless, ethereal creatures which are personifications of fear, lies, anger, and death. These spirits wander the realms until they find a creature to latch onto and corrupt. This can grant the creature dark powers, and in more extreme cases transforms them into a cambion, or a physical demon, which is the ultimate goal of all demonic spirits. 
A human or humanoid who is fully corrupted by these spirits becomes a proper demon; a terrible mindless beast of fury and fear. The appearance of these creatures varies wildly. Most commonly, they are vaguely humanoid beasts with black skin, strange proportions, sharp claws and red eyes. They also often bear horns, tails, and wings. However, some of the more powerful demons can have truly monstrous appearances, with enormous toothy maws, many arms, spiked tendrils, and other strange features.
Arch-Demons, or Demonlords, are greater, more powerful creatures, which have retained their intelligence through the transformation process, and become a powerful entity of darkness. These creatures are also known as Dark Ascended, and they often command groups of other demons, cambions, and dark magic users.
Demons are often, but not always, tied to an Arch-Demon.