The true Divinity, if indeed there is such a thing. Even the most righteous Powers have flaws, according to the Athar, but that doesn't mean that they completely rule out the idea of there being a being that is worthy of worship. Such a being is simply unknown.
Despite its unknowableness, some Athar, such as Factol Terrance still receive spells from somewhere. So there has to be something to the idea, right?