My group ran a fairly successful corruption system in the D6 Star Wars RPG. It was cobbled together from the core rules, and the rules laid out in The Jedi Handbook v1.3.
To help give a sense of scale, here's the core mechanics of the system we were using:
Players have skills and attributes rated as a number of dice (i.e. 3D). To use one, roll a number of six-sided dice equal to the rating of the skill and total them.
One die is always a different color from the rest. If it shows a one, remove it and the highest other die from the roll. If it shows a six, it explodes and is rerolled until it doesn't show a six (adding to the result each time).
Player characters typically have skills in the 2D to 6D range.
Character points are a type of experience/meta-point hybrid (lose one to add a die to a roll). Force points are large meta-points (use one to double all rolls for a turn).
Our Dark Side Rules
Whenever a character commits a dark side action, they receive a Dark Side Point (DSP). Some Force powers automatically award DSPs, other actions are up to GM discretion (players are usually warned).
When a character reaches six DSPs, they turn to the Dark Side and become an NPC.
Players with DSPs may choose to feed on them, and add bonus dice to their Force powers equal to the number of DSPs they currently have. If they do so, they are much more at risk of gaining DSPs from the Force use (i.e. using an attack power directly will likely now result in an additional DSP).
If players do not wish to feed on the DSPs, they must fight them. This increases the difficulty to use any Force power by one level (approximately five to ten points).
Whenever a character with Dark Side Points is in a position where he may commit evil, the GM rolls a d6 and if that number is less than or equal to the number of Dark Side Points then the Dark Side demands some action for the Jedi to perform (note: the D6 is wild, thus a roll of one is "less than" one DSP).
When a character has 1 to 3 Dark Side Points, they are tempted to commit actions that involve the immediate situation. For example, if an enemy was subdued, the Jedi fighting him may be tempted to kill the villain, even though he has surrendered.
To resist this temptation, a Force Sensitive character must make an opposed roll of Willpower, or Perception, vs. the GM's roll of one die for each Dark Side Point the character possesses. Non-Force Sensitives add their Willpower and PER die codes together(with only one wild die) for their roll to resist, since they are not affected by the Dark Side as much.
When the character has 4 or 5 Dark Side Points, their temptations turn to causing unnecessary harm and destruction, or initiating unprovoked attacks. The character is still entitled to a Willpower/Perception opposed roll, as above. If they succeed in the roll, then they may act normally, but the Dark Side enacts a punishment, according to the following table.
PC rolls >
GM roll by Character Loses
0 - 5 1 Force Point (or Character Point's equal to # of Dark Side Points x3)
6 - 10 Character points equal to # of Dark Side Points x2
11 - 15 Character points equal to # of Dark Side Points
16+ Character points equal to # of Dark Side Points x.5 (rounded up)
If the character is unable to pay off the cost he must “take” 1D from the attribute or Force Skill of his choice. Naturally, all skills based on that attribute will be reduced also. If any attribute ever reaches 0D, the character is consumed by the Dark Side and dies.
When you have 1-5 DSP you may begin to atone the cleansing of the corrupting influence of the DS is a long & difficult process & you must be of serious mind while attempting to atone (GM discretion). You must choose your way of atoning based on spiritual belief. (the Jedi usually fast, reflect through ritual & meditation & renew their commitment to live by the Jedi Code & the ways of the Light). When atoning you must actively work against evil from occurring, & follow a base code in creating your personal spiritual atonement process, with the following guidelines:
Reaching each through non-violent solutions when possible
- Preserve the existence of Life
- Preserve the rite to gain knowledge & enlightenment
- Preserve the rite of peace & harmony
- Defend the defenseless
Time required: of approximate atonement, all atonement must begin again if a DSP is received during atonement. Increase time required for characters who have removed DSPs through atonement in the past.
1 DSP = 40 days
2 DSP = 60 days
3 DSP = 80 days
4 DSP = 100 days
5 DSP = 120 days
These rules worked out pretty well... There was always the temptation to get just one or two DSPs in exchange for doing something evil but expedient. But thanks to the wild die mechanic, even a small number of DSPs could occasionally spiral wildly out of control.
Likewise, players who were corrupted had to choose between more power, at the cost of an increased chance of more corruption and the inability to atone, or more difficulty using their powers.
Case Study: Marcus Orion
The most successful use of this system was a Jedi character named Marcus Orion. The system played out like this:
Marcus received one DSP as a result of sacrificing himself to save others, then being captured and trained by a dark jedi.
Marcus managed the first DSP, but quickly received a second one by using it to fuel a particularly deadly attack power.
From two, Marcus spiraled upwards to three and four by failing temptation rolls. This reset his atonement timer each time.
At four, Marcus finally had enough invested in Willpower that he could resist the temptation rolls. With some careful management to minimize opportunities for rolls, he was finally able to fully recover.
This entire process played out over the course of several real-time months, and felt like a truly epic addition to that campaign.
Adapting to your Game
In your case, you would probably move away from the fiat-based moral judgement, and attribute DSPs directly to using the "dark" version of the various powers.
Because the granularity of the system is low, I would recommend either increasing the number of DSPs (and reducing the individual effect of each one) prior to turning, or allow the players a roll to avoid receiving one when activating a dark power. Tune based on how quickly you want a dark user to become corrupt.
Alternatively, you could scale the roll to have a lower impact on corrupt characters, i.e.:
- No DSPs -- Automatically receive a DSP.
- 1-3 DSPs -- Difficult roll to avoid receiving a DSP.
- 4 DSPs -- Easy roll to avoid receiving a DSP.
- 5 DSPs -- Trivial roll to avoid receiving a DSP.