With the coming of the Codex Infernus, by D. Jarvis (on Kickstarter). I've been thinking more and more about a fairly simple ''Corruption'' system. Something to indicate the gradual descent into darkness, the slippery slope to Evil.
In fantasy tropes, certain classes or character archetypes can be stripped of all their powers/benefits if they stray even a little form their path or tenets of faith. But what if the descent into darkness was more gradual and relied on the character personal convictions and his own certainty that he's actually doing good, that he believes he's still advancing his faith's tenets.
Sometimes our heroes may be forced to make hard choices, decisions that may be questionable. Maybe they had good intentions, perhaps it was for the greater good. For example; Churchill once made the difficult choice to let an english town be bombed by the Germans, in order to avoid the enemy from discovering that they had broken the Axis' code. Did he let people die when he could have saved them? Yes. Was it evil? Maybe (it was definitely a cold and calculated decision). Would it have merited a ''Corruption point''? Yes, maybe, most likely, depending on the GM of a game with such a moral dilemma as this.
But in the kind of game I'm thinking of, a fantasy setting, things are more black and white. You might have a few shades of grey. That's where my idea of a dark benny / devil's benny comes in. When a hero does something that can be considered bad, he will earn a dark benny.
I firmly believe this is a campaign element that need to be mentioned to the players before gaming starts, since it can have a serious impact on their characters and their fate later on in the game. And the players should be on board for this to work and you don't want to spring this on your players mid-game. They will most likely feel like your punishing them and trying to take their character away, or their player agency. Of course, if they are aware of it in advance, and you hand over that devil's benny after they've left some goblin children to die, then they can't really complain.
If your campaign has a heavy infernal genre and where demons can have some influence over some mortals who attract their attention, then this dark benny mechanic can come into play. Not all acts will be seen by powerful Demon Lords and not every mortal will be of interest to the Princes of Hell. But that's what the minor devils are for. Imps, Quasits, etc sometimes spy on some regular mortals to corrupt their souls in little but cumulative ways, in order to fill the pits of Hell (Think of the satirical novel ''The Screwtape Letters''). That being said, some mortals will attract the attention of more important, more powerful demons. And their actions will be observed more closely. The Paladin or the Priest of Light, for example, would make a tempting addition to their hellish ranks and would be a severe blow to the forces of good. Of course, a powerful Mage or Warrior would also be substantial gain to the forces of evil.
THE MECHANICS SIDE;
So when a hero does something evil (not just mean or because he's a little ticked off, but really evil, nasty, cruel, vicious, etc). then he gets a dark benny/devil's benny. This benny is always there and available for the player, to tempt him to more/greater evil (i.e. it actually does carry over to following sessions. He doesn't have to use it, but it will be a constant reminder of the possibility). He could perform acts of atonement or purification appropriate to the setting, in order to remove the devil's benny.
He can use this benny just like any other, except he gets a +2 to the final roll. But once a player has decided to use this devil's benny, he also gains a mark upon his soul, even if he used it in a selfless act. This act is tainted by his previous actions and choices.
Call this mark what you will; corruption, taint, black stain, dark side, etc., but these can be impossible or extremely difficult to remove, depending on the mood and grittiness of the setting. Can someone be redeemed or not? If there is a chance to remove these Marks, then after such an act of atonement or purification, the hero must make a Spirit roll at -2 (with an additional -1 per Mark gained). If successful, he removes one Mark (optional: two Marks with a Raise maximum).
GMs are encouraged to use their own judgement based on the act of contrition (sacrificing oneself in order to save many innocents from death or worse, might save his immortal soul from eternal damnation).
But once a hero has accumulated a number of Marks equal to his Spirit die (maximum 12), then he's irrevocably damned and corrupted and the Demon Prince will show up soon to claim his prize.