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That's one way to put it. If the ability is especially powerful, there's only so much the individual can use it before they become very fatigued. The Wardens can suppress them at will, no matter how strong the person is.
[Purebloods included. So far as he knows Yuuki has only experienced it to a point, seeing how she really doesn't use much of her powers at all.]
Oh; this entire thing is like a joke for them. We're nothing but an experiment in a cage for them to toy with us in any given number of ways. Just ask anyone else here about the issue with time. You'll hear the same answer from everyone.