[Humakty]

I like your idea of using personal lifeforce to 'start' an effect, but I don't think it would be appropriate to wound yourself, in a medieval context it most often than not implies your death after all, but rather a drain effect that is spread to all your body at once, which can kill you if your clumsy or if you want too much at once. (Shadowrun RPG, and others, use this idea quite well)
This effect should alter your potential lifetime, and your physical might permanently. (not really in any RPG I played so far... Or I can't remember...)