In my story, there are two kind of magic powers: White Magic and Dark Magic. White magic are powers such as Telekinesis and its limitation is the consumption of vital energy: The more the user does white magic, the more vital energy he loses, and in the end he could die.
Dark Magic is more offensive, such as Throw Energy Balls or Throw lightnings. My idea is to have a different kind of limitation instead of Vital Energy consumption, but I am not sure what kind of limitation could be. Can anyone think on some kind of possible limit about this?
With Vital Energy I mean what in some oriental religions mean as Ki, so a human has some vital energy and when it is consumed, the human stops living.
EDIT 2: I would like to share some idea I have recently thought, so you can help me to find any flaw. One idea is to, instead of user Ki, Dark Magic consumes Ki from the surrounding living beings, such as animals, humans, plants and even planets and stars. The problem is that with this idea, the user can use the power he wants without being worried.