Skinburn
It’s all the rage in modern demonology and/or necromancy. The old pacts and promises and deals are really so passe. Why sign over your soul in one fell swoop when you can do it on the installment plan?
Basically, it works like this: there’s a spell that mages can learn — very easily, no nonsense — that allow them contact with a specialized demon. It is absolutely not allowed to eat anyone, take their soul, or do anything to the mage except in self-defense (and ‘self-defense’ is defined pretty strictly, in this scenario). The demon’s just there to administer a standard Contract where a mage takes on a certain Mark on your skin.
Once the Mark is installed, the mage can draw magical power from the Mark in order to cast spells. It always works, no gimmicks, no tricks; the mage can take as much as he needs. Of course, every time the Mark is activated, Skinburn occurs: which is to say, a small dot of reddened skin appears somewhere on the mage’s body. Skinburn invariably originates on the chest and belly, and nobody’s been able to use the Mark often enough to determine whether it would eventually spread to the extremities.
That’s that! The mage never has to encounter a demon again. No questions asked, no auditing to make sure that the mage is spreading evil, no work done on the Infernal side at all except providing the mage with raw, demonic magical energy. And here’s the interesting part: acquiring the Mark is not apparently an auto-damnation event! So, theoretically, you could get the Mark installed, use it throughout your life, and still manage to avoid Hell when you die.
And, once in a very long while, somebody actually does.