Shadow priests have the balance that Duskworn has been talking about. However, when they loose that balance and go *completely* to shadow, both times I know of that happening, the priests have insane and become servants of the Old Gods (Archbishop Benedictus and Natalia Mar'alith, High Priestess of C'Thun).
Listen <mister/missy>, I'll have you know that whatever attacked us was no priestess of nothin'! Her eyes were aglow with something evil and she was genuinely frothing at the mouth! She came in, did some kind of crazy scream that sent us all runnin' in separate directions and then started flayin' our minds one by one. I'd be hearin' shadowy words in my head, watchin' the other's periodically black out here and there. It was madness! We would have been done for if not for the damned monkey.
Shadow priests have the same abilities as Old God priests and varying degrees of "goodness/evil", which is probably why they're often rolled into one package by some players. However, we can still use the Light and rationality (I'm assuming Benedictus is an evil Shadow priest who looses all control over base instincts/the Shadow and turns into an Old God priest on the second phase).
Nobundo's story also vaguely mentions what could be the void/shadow for something that doesn't involve Old Gods. Or it could be gravity.
And there was more. Out here in the void was another element, one that seemed to bind the worlds together, one composed of unspeakable energy. If he could call upon this one--but he knew immediately that he was far too inexperienced at this stage of his journey to commune with this mysterious new element. This was just a glimpse, a gift of understanding...
But that's really just interpretation, and not solid fact. That would be a good cdev question.