A character cursed with demonic powers, able, but not willing, to use them. A fire mage who always risks creating a fiery hell when using the powers. A mage who can use his lifeforce to power his spells, but who risks death or worse when doing it.
One way of representing such characters is to give the player access to powers, but with a hook: Every time they are used, there is a chance of something very bad happening. In the end, it will be inevitable.
Player, or character, can receive a significant benefit by bringing the character’s doom closer. When the power is used, roll a die. If result does not exceed the number of times the power has been used, Bad Stuff happens. Otherwise the risk has just increased a bit.
The size of the die determines how often the ability can be used. A d4 means that even the first try is quite risky (1/4 chance of bad stuff). It can be suitable for a one-shot where the power should be used no more than few times. d6 may work with a game the length of a session or two. Increase as appropriate, but a d100 may be a bit excessive and anything larger than that missing the point.
The chance of being able to use the power n times without bad stuff, given a die with s sides, is (s-1)/s*(s-2)/s*…*(s-n)/s, or zero once n at least equals s.
Player-controlled destiny is much more appealing than a demonic power usable once a day and GM-assigned side effects every now and then. It is up to the player and the game group if something the character could have little to no control over is still given to the player. A traditional example is a destiny of some sort: The character is destined to die by drowning, say. When the bad stuff happens, the character will die next time it will be possible and not utterly silly (GM’s or player’s call). This might kill the suspension of disbelief or break immersion for some players.