I have to ask this question.. did you get this idea from the hobbit?! because if you did i ahve to say thats pretty awsome.
and for a catch, i would go with what landothern said(i think i spelled that right) or go with somthimg pretty obvious, like a specfic part of your body, your achellies cord, shoot that you cant walk and bye bye powers, and just for funsies you could copper bullets. but thats all i can think of really. you have bamboozled me