Discovered this while doing a little research on the subject.
I really hadn't thought of him as an end of campaign boss. Just a malign pain that is a constant pain in the side of those try to tackle evil and end cult plot after cult plot. Being as he is not bound to a single cult, as many different worship him I felt he would be an almost never ending antagonist. But I do see you point.
*updated per CM's remarks.Go to Comment