[@BurningCold]
Not to sound like i'm the smartest guy ever or anything but your description just gave me a fucking idea.
What about a power that makes you a silent, nigh-invisible and murderfuckerous super-monster [b]when nobody sees you[/b] BUT when someone manages to catch a glimpse of you and concentrate his vision on you for some time they see through the monster and notice that that thing that just ripped ten elite soldiers to bloody shreds is just a teenager and he has to escape everyone's sight in order to transform again.