Creatures hailing from another world. Demons have an affinity for corrupted magic and cannot be tainted or harmed by it. Because of that, some people accuse them of having caused the Great Corruption, which they deny vehemently. Despite what one may think, the extinction of humankind would be a disaster for them, which is why they took as many humans as they could back to the Demon World to prevent their total extinction. Their reasons for that are simple: without humans, demons and angels alike would stop existing because both species are born of corrupted and blessed human souls, respectively.
Demons don’t have any ruler, they rule themselves just fine, although some demons do have followers who willingly or unwillingly follow their orders. The “Demon King”, despite what the title might suggest, doesn’t rule, it is simply a title to indicate that this particular demon is the strongest in all categories. That title cannot be inherited in any other way than through beating the current Demon King in a series of duels of force, intelligence, and agility. Only by succeeding in every single trial does one obtain the title of Demon King.
Every demon, with no exception, is intersex. They cannot reproduce naturally; new demons only appear if a deceased human’s soul is corrupted enough.