Demons{1} can sense and are attracted to magic. For only through magic can demons enter our world to feed and corrupt. One of the easiest "doors" to open is the mind/soul of magic casting mortals. Thus they are constantly tempting caster's with power beyond their means and more than happy to provide it. This magical power offered by demons is called blood magic.Anyone capable of casting magic may use blood magic. No special training or preparation is required (the demons helpfully "explain"/handle all the details). The only requirement is that the caster must draw some of their own blood. So, they must have a knife, other sharp implement, or the willpower to bite into their own flesh. They may then cast any spell they know regardless of whether they have it memorized{2}. And they may continue to cast spells via blood magic for as long as their blood is flowing.
After casting a spell using blood magic, regardless of whether the spell was effective or even successfully completed, the caster takes Xd6 hit point damage. Where 'X' is equal to spell's level. For each '6' rolled one point of damage is permanently lost hit point. If the damage is enough to reduce caster to 0 hit points (or whatever is dead in your game) they become an abomination{3} possessed by the "demon" who fueled the foolish caster's blood magic.
For Example: Malcazaar, after a busy day of sorcering, is fresh out of memorized spells when gang of interlopers interrupt his evening repast. With a carving knife he slashes his left wrist. Then with blood splattering everywhere traces the pattern for Davros' Fiery Damnation, a 3rd lvl spell. After thus incinerating his foes Malcazaar rolls 3d6 (6,2,6), suffers 14 points of damage and his hitpoint total is permanently reduced by 2. A costly result, but a better one than being dead.
Notes:
{1} Exact nature of these demons (cthulhu elders, Type I-IV, evil spirits, etc) depends on campaign.{2} Or whatever floats your boat.
{3} What abomination means is left up DM. Should definitely be NPC/Monster, be hard to kill, lvl based on former caster's lvl and level/nature of spell that killed them, be able to cast former caster's spell repertoire using blood magic, and pay for blood magic out of their hitpoints or a blood pool. Blood pool being a running sum of all damage Abomination has inflicted or the like.
I adore risk/reward and "enough rope to hang themselves" type mechanics. This is fairly powerful but limited by hit points addition. And permanent loss of hitpoints is a significant risk. Still for thematic as well as mechanic reasons I believe the non-permanent damage suffered should be resistant to magical healing and require time/natural healing.
What do you think? As a player would you take the risk?
