Note that I have read just the first two books. (yesterday I gave my orders to the fellow publisher, who is bying foreign books to me:). I know that it's late, but i wanted the new edition of KE4). So what do you mean? I don't remember this moment?
I have a crazy theory. In my opinion Sathar is IN Vilmos. Remember the dreams of the boy, when something evil was trying to conquer his mind. This evil, I think, is Sathar. That's why Xith told Vilmos, that the Dark Lord is not real. He is not in flesh, he is an evil spirit, and he is trying to possess the first human magus in a thousan years. Doest it make sense, what do you think?