In pretty much every game, the developers find it useful to present your character with a trait that goes up or down to reflect how much and what kind of shit your character is in. Whether it's Hit Points, Health, Sanity, or Humanity (which go down) or Stress Tracks, Damage, or Banality (which go up), the idea is to create a sense of tension by making real, on the character sheet, the fact that BAD THINGS ARE HAPPENING TO YOU.
I even once played a game that used marbles.
This is in all ways laudable. I don't know about you, but the act of erasing or filling in a dot or a box or a number on my character sheet does in fact drive home that things are changing. Watching that number rise or fall, those boxes or dots show up or go away, can be very tense.
I'm going to call this idea a Menace Mechanic. That is, it's a mechanic for tracking threats to your character's continued health, sanity, or existence: menaces.
The only problem is that most menace mechanics don't provide a lot of room for customization. The game decides what menaces to measure, whether it's how close your character is to dying, how long it will be before he betrays his people to the crude sun-worshipers overrunning the land, or how close she is to forgetting her beautiful secret faerie nature. There's rarely any room for you to formally determine exactly where the fault-lines in your character's personality are. The Abigail and I often find ourselves discussing her characters' weaknesses, where and how they could go bad, and then leaving it at that. I get to incorporate these personality problems into the story, but I rarely get any mechanical support (unless, of course, I find a way to incorporate the character's issues into an existing menace mechanic).
Now, this isn't always a problem. I'm not the kind of designer who thinks that you always need to write a system for every thing that's going to happen in your game. That said, if you don't write a system for it it's not definitely going to happen, and sometimes that's sad.
So, what follows is a customizable Menace Mechanic that can probably be adapted to any system that tracks mental and spiritual menaces.
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The first thing you need to do is give your character's Menace a name that sums up what it is, what it means, and why it's bad. Some ideas that spring to mind: Anger Management Issues, Cold-Hearted, Desire to Give Up Human Foibles and Become a Robot Chick (actually ran a game for one of those once).
Then you need to pick a number of intermediary steps - we'll call them Milestones - that lie between a theoretical ideal state (I embrace my human flaws and love my meaty flesh!) and some kind of theoretical fallen state (Love is illogical). The number of steps probably varies based on the kind of game you're already playing and how you want the Custom Menace system to fit in with everything else. If you're using Custom Menace in place of Humanity or Morality, for example, you probably want to identify ten states (Menace 1 through 10). In other systems you might be happier with five, or even three.
Two probably won't work, though.
You should also figure out if Menace is going to rise or fall. Again, you should probably make this compatible with the rest of the game. If Menace is replacing or coexisting with White Wolf's Morality, for example, you probably want Menace 10 to be good and Menace 0 to be bad.
For each Menace Milestone, you need to pick one or two acts that could constitute a breaking point. In other words, something the character could do that might cause them to slip further down that slope towards robot-hood (or whatever). If a character performs an act that would be unacceptable for her current level of Menace, some kind of dice roll is in order to see if the character loses (or gains) Menace. The exact kind of dice roll is, again, a choice you need to make based on the rules of the game you're adding this system to.
So, for example:
Menace: Become a Robot-Chick.
10: Make any decision based on logic rather than emotion, sacrifice a relationship for any reason.
9: Damage a relationship for any reason, pass up an opportunity for physical gratification
8: Add or implant technological devices to your body
7: Damage a relationship because it is logically expedient
6: Sacrifice a relationship because it is logically expedient
5: Harm another human being because it is logically expedient, replace a damaged body part with a technological prosthesis
4: Sacrifice (end) a relationship because it is logically expedient, break your sworn word because it is logically expedient
3: Betray a close friendship because it is logically expedient, replace a perfectly good body part with a technological prosthesis
2: Alter your body with technology in such a way that you can no longer enjoy a common form of physical gratification
1: Sacrifice your essential humanity for robot-hood
This example is for a character that the Abigail played in White Wolf's Exalted. In keeping with the general themes of the system, I have chosen to have this Menace run from 10 to 1.
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I'm unlikely to ever get to actually use this system, since the Abigail gets grumpy when I add system to games (she prefers to subtract system from game), but I'm curious to hear what you may think of it, and what you may some day make of it.