His powers are clearly protection based, and we're fairly sure powers negate or deal with their weakness in some way. So I'm putting my money in the weakness being somewhere in the field of seeing people come to harm. And seeing as how his weakness has never been activated during the book (to our knowledge), it is probably seeing a very specific group of people come to harm. Next question we must ask ourselves is how did Prof overcome the Rending and regain his senses. Conflux was the only other epic able to do that, and since his powers were limited to solely gifting, I wouldn't be surprised if he never had a rending. My guess is prof experienced his weakness for a long enough time that he couldn't use his powers for a while and was able to become good again. And I think he met his weakness when he destroyed his school on his rendinf