The Old Gods are physical manifestations of the Void. They are nightmares incarnate: mountains of blighted flesh and writhing tentacles that grow like cancers within the worlds of the Great Dark. Living only to transform the worlds they infest into places of despair and death, these malignant entities serve the void lords.
The void lords are monstrous entities composed of pure shadow energy. These beings are cruel and merciless beyond mortal comprehension. Driven by an insatiable hunger, the void lords seek to devour all matter and energy in the physical universe—including the energy of the Old Gods if they have any.
Professions of the Cosmos
Shadow priests are priests who derive their power from the Void, straying dangerously close to the domain of the Old Gods. To truly understand such ancient, corruptive influence is to be driven mad. This is the state in which these dark priests thrive, embracing insanity and feeding off of the minds of their opponents to reach terrifying new limits.
Are Old Gods Demonic or Demons?
Old Gods are evil immortals who feed upon life and magic. As such, Old Gods fit the definition of demons that is provided by the Warcraft Encyclopedia and might, technically, be demons. If they aren't, they are demonic if anything because they relate to demons in that they're both, ultimately, a result of the Light and the Void.
Can - Or Do - the Old Gods Have Fel?
The Old Gods are creatures of chaos and so the force of the Old Gods would be - or is - "of chaos" by extension. The primal force of chaos is fel and, as creatures of chaos, it's possible that the Old Gods have fel.