Now, before my DM reads this and thinks that I'm bashing on the execution of this plot device in his game, I should clarify: Brad, your game is fun!
So we're going to start with what Brad is doing, as our jumping-off point. The base-line corruption model is to create a situation where the corruption occurs automatically as a result of something the character does as a matter of course. In Brad's game, the weapon found its way into the hands of an avenger which, for those of you who aren't playing 4th edition, is a high-damage class which tends to finish off a lot of enemies. So the +3 lifedrinking fullblade? It corrupts its user when it is used to deliver a killing blow. This sort of scenario works best in two situations: the first is with a player who revels in the corruption of their character, and therefore relishes the implications as they continue doing what their character is designed to do with reckless abandon. The second is when the player is someone who enjoys adding complexity to the play of their character. "Should I do 'X' or should I not, because the price is too high?" Even without either of these types of players, a little corruption is still fine, but these two types of players will cause a little corruption to explode into a great deal of fun for the group, either as they try to control the spiraling decent of their darkening comrade, or as they cajole their friend into occasionally paying the price of further corruption in order to overcome challenges that seem insurmountable otherwise.
I've flagged this as a player option as well as a DMing idea because I feel that players should feel free to approach their DM and say "I want to play a good guy who has to fight against some darker tendency" and have their DMs support. While our previous baseline model works just fine for this type of situation (the player-instigated corruption plot point) the next model works especially well for a situation where the player wants to explore the idea of fighting against corruption.
The next option harkens back to the days of West End Games' Star Wars Roleplaying Game. For those who have never tried WEG's Star Wars, let's talk about Force Points. If you have played any Star Wars RPG, you know that generally Force Points are a mechanic to simulate a character using that all-powerful energy field from the Star Wars universe to generally be better at whatever they are trying to do. In WEG's version, if you used a Force Point, you multiplied the dice rolled on the test (WEG was a d6 system where you had a target number you tried to hit above on as many d6 as possible). If the test resulted in "a bad thing," loosely defined as the subversion of the free-will of a sentient being, death of a living organism, or selfish personal gain, then the character who called on the Force while doing such things received a Dark Side Point. Too many of those and you became a NPC, and lost your character. The idea, modeled here by WEG's Star Wars RPG, is to offer a character something good, but charge them a price. At it's most simplistic, you can tell the character that the evil wizard's staff will allow them to double their damage dice once per session, for example, but they move one step on the corruption track whenever they do so. As a rule of thumb, the power, or enhancement should be available twice as often as it can "safely" be used. If the player starts to ignore the cost of using the power, you might have to scale down its availability to keep game balance intact, but you might also just ramp up the cost per step on the track.
A slightly more labor-intensive version (for the DM) would be to make the corrupting powers "limited time offers." This means that the DM has one more thing to keep track of, which may not really be feasible for some DMs who have lots of players, or prefer running large numbers of monsters, etc. However, in a very "RP-heavy" game, or one where there aren't a ton of monsters and/or players to keep track of, this sort of corrupting influence can create all kinds of RP opportunities. Instead of giving the player carte-blanche to do something awesome whenever they are willing to pay the price, create true moments of temptation: When the character scores a critical hit, but discovers the creature has high damage resistance, describe time freezing for a moment and the evil warpick whispering an offer into the character's mind: "I can bypass that damage resistance, all I need is a trifle, a meager portion of your energy..." Now the player not only has to choose whether they are willing to pay that price, but if the offer is truly well-timed, it could be a choice between easy success, or less-assured victory. These moments of temptation need to be timed well by the DM to provide the player with the most engaging decisions possible. Offered at the wrong times, and the choice may seem too easy to pass up, or impossible to refuse. And remember, this system of corruption should only be used with a player who is interested in playing out a fight against darker tendencies.
So we now have three different ways to handle corruption in your game:
- Cause the character to suffer corrupting influences whenever they do something their character is designed to do anyway.
- Offer the player an additional power, usable at least once per session, which costs the player, moving them one step closer to total corruption.
- Create situations where a corrupting influence offers the character special powers in times of need, but at a cost.
*Special Note* All of these systems of handling corruption require some sort of "condition track" for the worsening of the corruption. Make sure that you have one in mind before attempting to implement one of these systems. Also, make sure that the corruption influences the character in some significant way. Don't make the corruption of the mighty fighter who used Charisma as his dump stat a penalty to Diplomacy checks. He just isn't going to care. Now, the glib bard? He's going to feel that sting a lot more, especially when the party relies on his Diplomacy skill to get them through most social encounters. As the DM, you know your party best, and you should spend some time making sure that the cost of corruption is high enough. If it's too cheap, it cheapens the fun as well, and can quickly unbalance your game to boot!
All of these should be used carefully, and with a special sensitivity to your player's interest and sense of fun. But with a little extra effort, and a player who is interested in playing this sort of thing out, you might just find that one of these systems will draw in everyone at the table just a little bit more, and keep them that much more interested in what is going on.