Originally Posted by
Fable Wright
Third—We don't know what Tharizdun is exactly. It could be something beyond a god that can only be understood in terms of deific power. He does not come from the same cloth as other gods, as anyone else should have starved by now for lack of worship. I find it possible that Tharizdun and the Lady of Pain could be the same kind of entity. But it should be emphasized Tharizdun is very, very different beast than a normal deity. We categorize him as a god because he can grant spells and has power that can dwarf divine abilities without worship. The Lady has the latter, and we don't know about the former.
If you want to do a campaign on the Lady being a deity, or... whatever Tharizdun is? Great! But canonically, none of the Factions in Planescape would dare consider her a deity. That's just the canon stance for the time being.