A Demon, sometimes also called a Yokai or a Spirit, is a manifestation of destruction spawned from the abyss.
True to their chaotic nature, there is no right way to create a demon. Many lords of the abyss create legions of demons in their image, but other demons are spawned haphazardly from cursed or shunned souls. Soul-shunned demons are often born in the Ethereal Plane and spend much of their life there, causing mischief and danger from behind their veil of etherealness. Sometimes, a demon under stress can manifest other demons, manifested as copies of their madness. These summoned demons often times live very brief lives, symbiotic to their summoner, but others grow in strength and perpetuate through the planes.
The more chaos a demon causes, the more powerful it becomes. The quickest way to disrupt life in the Material plane is to kill something, and murder is often a high priority for demons who make themselves known to mortals. Many times demons who grow in power also grow in size and shape, even mimicking the form of stronger demons they may have encountered. They do not do this as a compliment, but as a challenge, as demons almost always war with each other when crossing paths.
Demons can be destroyed only when killed in their home plane, The Abyss. A demon vanquished in any other plane or realm will be reformed and trapped in the abyss, where they must wander until they once again find a portal to the outside.