Demons are malevolent spirits created by Lucifer. They are created from corrupted human souls that have endured extensive torture in Hell. They become corrupted and extremely evil, and also very powerful, in this process. They require a vessel to walk the earth, though are able to roam in their smoke form.
Demons are notoriously difficult to kill, with Brandon, using psychic powers, and Ollie, using a special knife, being the only main characters to actually kill a demon. Hunters usually having to rely on trapping demons using devil's traps and exorcisms.