My taste in magic for RPGs runs more toward my sources of inspiration, which are often the darker sword and sorcery works – especially Robert Howard and Karl Edward Wagner. Magic is dangerous and mysterious, its forces governed by beings inimical to man. The practice of magic carries the risk of turning the practitioner progressively inhuman.
I’m thinking one way to do this is to have a Hubris score that goes up with each successfully cast spell. Hubris represents the character’s growing pride and readiness to call upon the supernatural, his disregard for the human costs – e.g. sacrifices, casualties, suffering, collateral damage, etc. – and the likelihood that he will overstep himself.
The rate at which Hubris increases depends on the spell and its effects. Flagrant violations of natural laws results in major increases in Hubris. Killing something directly with magic results in a huge increase in Hubris. More subtle uses of magic results in a lesser increase in Hubris.
In D&D terms, subtle magics add the spell’s level in Hubris. A spell that flagrantly violates the laws of nature, such as Fly, adds 2x the spell’s level in Hubris. A spell that violates the laws of the cosmos, such as Resurrect or Summon Demon, adds 3x the spell’s level in Hubris. And lastly, spells that instantaneously killed any intelligent creatures add the slaiin victims’ Hit Dice to Hubris.
So if you cast a level 7 attack spell like Delayed Blast Fireball and killed 20 goblins with it, that’s 7x2 + 20 = +34 to your Hubris.
So what does Hubris do? Every time a spell above a certain level is cast, roll d100 and check the result against current Hubris. If the result is lower than or equal to current Hubris, a complication occurs. Summoned creatures may not obey the caster. A damage-causing spell may strike an unintended target, or do less damage than usual. Spell durations may be unexpectedly shortened. Spells may even be twisted, for example a shapechanging spell may not just turn you into an animal form, it makes you a lycanthrope as well.
When Hubris is thus triggered, current Hubris is halved.
Hubris is also removed by spell failure or when the target successfully saves against it. Hubris can also be voluntarily removed by rendering homage or service to one’s power patron – the gods for clerics and some wizards, or to one’s master in the arts, etc. etc.
Another way to remove Hubris is to have a Terrible Revelation. A Terrible Revelation happens when the character indulges in some excess due to his pride or decreasing humanity, and then realizes what he’s done after. Players can collaborate with the GM to set up Terrible Revelations for their characters.