Hey so I believe I mentioned this before (like 5 years ago) but thought I'd start it up again. Jim has brillaint way of grabbing an idea, and putting a fresh spin on it. I think when it comes to TWG, and the Lord of slowest terror, he may have used some of the lore from Diablo.
I think that the way things formed is that in the beginning there was one. Just one being in all of existence. However this being wanted to improve iteslf so it purged all of the negative from itself. In doing this though the negativity took on it's own persona. It's own self.
So that's how I think the Outside, and Reality came to exist. One being of light, the other of dark. One is about free will, the other is about slavery. One creation, one destruction. The Perfect Being, and the Prime Evil