Okay here is the first draft of "pain magic" I'm Currently calling it Sadicas, which i've blatantly stolen from CJ Carellas Witchcraft all rights are his yadda yadda.
Sadicas base -5 +1 for evocation and thaumaturgy. If you do not reliably have a source of pain add +1 to the cost of this quality, if you have a reliable source of pain and thus always have access to this then do not. Constant Pain counts as a source for this purpose so you can only gain the +1 from Constant Pain or from not always having access to this power. not both.
Cost: To Reliably draw power from pain and sacrifice you must already be a wizard or sorcerer of some stripe, well-versed in Evocation or Thaumaturgy (with at least some specialization in Biomancy or Psychomancy), and have Learned how to channel magical power from pain. Therefore this is available only as an upgrade to those abilities.
This power is much more then the occasional use of temporary pain or anger to fuel your magic, it is a complex and comprehensive understanding of how to use emotions, pain, suffering, and sacrifice. Theoretically any emotion could be substituted in for pain and you would have a similar form of sponsored magic, but as pain is much easier to come by then say love Sadicas is far more common, if you where going to adapt this power to another emotion the bonuses it provides would have to be changed to better reflect that concept.
Benefits:
Standard sponsored magic benefits (page 288).In the same way that blood calls to blood, so to dose pain call to pain. If using pain as a power source on any spell intended to inflict stress or consequences add +2 to the power of that affect, you may also use evocations speeds and methods for any such spell as well as for any Spell that would logically gain strength from pain as a power source[This is up to your story teller, howver this form of magic is about as far from subtle or delicate as you can get, so generally any "brute force" type affect should qualify and anything finicky or delicate should not.] coloring such evocations with the agonizing Caress of pain and adding a +1 to the spells complexity threshold. It should be noted that you must find a source of such strong pain or suffering to draw on before being able to claim this benefit.
Downside:
Every three times [or so] that you Channel pain in such a way You must change one of your aspects To reflect that you draw so much of your power and self image from pain/Suffering. Once all of your Aspects have been changed such you do not need to keep changing them unless one changes to something else through the course of play and you continue to use Sadicas after such a change. This is in addition to the standard debt mechanics of sponsored magic. While there is no sentient force corrupting you all uses of magic require the magician to believe that that spell is Right, channeling pain in such a way leaves a mark on your soul forever changing who and what you are.
Upgrades
Constant Pain [+1]
Your character has Fibromyalgia or a similar condition that causes you constant, sometimes overwhelming pain. Take a -1 Penalty to any use of the Athletics skill. At least one of your aspects should reflect this state. This dose qualify as a "source of pain" for Sadicas and is one of the only ways to use such magic in a morally neutral way.