Story-driven games are the heart of what makes gaming so appealing to me. The narrative that is co-authored by the GM and the player naturally unfolds into something unique and awesome, but I've noticed over the years that one aspect of the narritive - that is, character development - usually lacks the dynamic of the rest of the story. As characters are played they are often fleshed out as the player explores their nuances, but rarely do characters undergo meaningful and interesting changes that aren't forced upon them. Worse, sometimes characters' alignments arbitrarily change suddenly as an attempt to inject character growth by a player or DM that isn't very good at structuring narratives. (continued after the jump)
Make sure the PCs victories are downplayed, and seem to have very little effect on the organization as a whole.Breed distrust in the ranks of your PCs. Give the motivations that are not necessarily aligned with each other. Then, you can bring in your corrupting party with a special offer - whatever it is, it's powerful and definitely a game changer. But after the party has used this new power, they notice that it's got a catch - perhaps it's only capable of controlling the minds of those that the control will bring suffering to, or needs to drink the blood of innocents to power it. Players may not be able to sacrifice innocents - perhaps it draws on the strength of the wielder itself, corrupting and twisting their flesh and soul.
The key is making the power amazing. Every fight it's used in should be a spectacular use of its power, and it should be available at all times. Perhaps the corrupting party has merely opened up a conduit to nefarious energies in the PCs, and they need but activate the supernatural abilities at any time. This will lead to characters using it in their dire times of need. It could even be healing! Eventually, characters will succumb to its allure.
While everyone may not be comfortable with the change (and even speak out against it), it's likely that one character will be drawn in by its promises. You will notice a dramatic change in their behaviors as they struggle to come to grips with using these dark energies. If they seem happy to just accept mechanical penalties, be more drastic - perhaps they wake one night to find themselves in an unknown house, a dagger in hand, and a corpse violently sacrificed in front of them.
All the while, the main threat is ever-present and the focus. If the characters can shrug off the influence of this power, it is likely that they will have grown as characters for the positive by their experiences. If they succumb to the benefits and use evil to destroy evil, the public views them as just another villain and they may outright accept this mantle with justification - having slowly been twisted by their victories, thinking that it was the only way.
Next time, I'll give a specific example of how to use these tools. What about you? How do you promote character growth (whether it be good or evil)?