Heroes of Redcoast City
Demons originated as inmates trapped in the Demonic Plane as part of the Pocket Dimension Private Prison Plan. The life-sucking nature of the demonic plane caused the inmates to mutate into magical abominations, which soon took over the plane.
Demons cannot reproduce, but after they are killed they are reborn anew in the rivers of the Demonic Plane.
Demons vary wildly in power and form and are generally non-sapient (altough exceptions have been recorded, as JUSTICE has in fact fought off what seemed like primitive Demonic warlords in the past). Some earthlings can even manage to pull them out from their realms and bind them so that they execute their bidding.