How about, now here me out, it has the similar mechanic to Ludens, Dead Man's or Statikk Shiv. They build up to their extra damage by either moving or attacking, right? In the case of Duskblade, it charges when they are 'unseen'. So normally you'll be wandering around with it full charged anyway but in combat you have to 'hide' to get it to charge for an extended trade, this is the most likely case anyway, it allows assassins to use it properly, but adcs and others trying to use it who have to stay in combat to get all their damage off can't just dip into a bush and out again to proc it again.
Or is that not a good idea?