The Trooper

This is accidental magic.  You have to spill blood and have your blood spilled.  The blood you spill and the one who spills yours need not be the same person.  It must be in a skirmish or a battle where more people die in the course of a day than the number of people you have ever known by name.  It is more likely if the battle is indecisive or a massacre, though this is not a guarantee, and some have come away from well-fought and decisive battles with the mark of this working on their souls.

When this happens, and other things that people cannot fully understand, let alone engineer align right, the survivor or survivors in question see the face of evil.  Every culture that fights in war knows something of how witnessing battle changes and damages a person.  This is different.  When you see the face of evil, evil sees your face as well, and it's the nature of evil to never, ever let you alone.

Evil chooses a guise for you, a beautiful disgraced servant of the Powers, a grinning skeleton in a dark robe bearing rusty tools, a beast whose sweat steams and muscles strain on chains that fade into the distance, a sweet-faced young king with bloody footprints and a dark, dark shadow.  The face you see is the face you will always see, when you are quiet, when you are alone with your thoughts, when a conversation falls into lull.  In those spaces, evil will speak to you, and you may choose to speak back.

No one has ever gone into this bargain willingly or with foreknowledge.  If you are desperate for power, there are better ways, but, when evil speaks and you reply, there is an opportunity to force your will on evil.  If you win, it will do a thing for you, anything but leave you.  If you fail, evil gets a free hand to manifest in your world.  One man so cursed kept a journal of his intercourse with evil and discovered, at the end of his life, that he had won out in the will against evil five times for every eleven.