Demons are malevolent creatures strongly associated with negative emotions or dark and evil places. Demons are generally human shaped, but their features are grotesque and horrifying, and they typically possess some terrific physical adaptation as well. For instance, the classic depiction of demons have them owning leathery wings, cloven hooves, or gruesome horns. True as these illustrations may be, demons can come in many different forms, from unassuming shadowy figures to flesh-eating monsters to completely incorporeal.
All demons are born from darkness, both physical and spiritual. Being born from physical darkness is not dissimilar to how an elemental is formed by high concentrations of elemental energy. Demons would form from negative energies, the same energies that raise the dead or sap life, which happens to be dark. However, if this was all that took to form a Demon, they would lack the drives, desires, and intelligence that are characteristic of them. That is why spiritual darkness is necessary; spiritual darkness is conjured by intelligent creatures who possess a particularly wicked or vile will.
As terrible as Demons are, mortals can breath easy as demons nowadays are hardly as plentiful as they used to be. Because Demons are beings of negative energy, they are undone by focused positive energy, a feat The Church of the All Mother and Great Father (CAMGF), as well as other popular divine institutions, are well practiced at. On the flip side, demons can only ever be truly slain by positive energy, otherwise their essence is merely dispersed and will reform after a while, as is the case with outsiders. Once upon a time the mortal world was ill equipped to deal with demons, so they were allowed to run ramped and unchecked; traces of their foul blood can still be found in the far reaches of the world to this day.