Insidious Offers
This gets used in Wraith: the Oblivion, as well as in Earthdawn with the Horror known as Tempter.
Essentially the critter lurks in your mind, whispering offers of power and aid for your cause.
Need just a little more strength to save that child from the falling rocks?
He'll be glad to give it to you. Of course, the first taste is free.
All the while this critter is manipulating things to put you in situations where your inate power isn't going to be enough. You can choose to fail, or you can accept the insidious offer.
Sooner or later though, you pay the piper. Sometime you're going to need that strength boost, and it isn't going to be there. And the little voice will act so sorry and apologize. Of course you can have that awesome godlike power again, just do a little something for me.
It's nothing really, just a little white lie ... this time.
Of course next time it's just a little theft ... or a little murder.
And of course you're totally free to refuse these gifts ... I just thought I should offer ...