Power is a most persistent taxman. As interesting as those examples are of the bodily tariff that attends tapping into extreme power (see Rock Lee’s Lotus taijutsu in his examination fight with Gaara in Naruto, the Elric brothers losing limbs in an attempted resurrection in Fullmetal Alchemist), I am much more fascinated by the intangible requisitions. Sure, you may be willing to sacrifice an arm or perhaps even your eyesight to wield the power necessary to defend/avenge your loved ones, but would you sacrifice your goodness? Would you sacrifice your sanity? In Robert Jordan’s masterwork series, The Wheel of Time, it is a question asked of every male channeler who wields saidin. Go ahead and tap into the One Power, energy powerful enough to manipulate the universe. We kindly ask that you leave your mental health at the door. That way lies madness.
(Note: That way also lies spoilers for the Wheel of Time series.)