The Dark Quickening is perhaps what every immortal fears. If he even knows it exists that is. Few do, of course. The Dark Quickening has been illustrated in an episode of Highlander: The Series. Duncan kills one too many purely evil immortals, and he becomes evil. (He goes on to a magic pool and fights himself, yaddy yaddy ya, we will ignore that however for the sake of gaming and realism). Here are my rules:
Every immortal has a gauge as to how much evil he can have enter him through the quickening. When this gauge is broken, he obtains the dark quickening and will act according to the rules to come later in this section. The gauge works as follows:
EVIL -10- EVIL
NEUTRAL -5- NEUTRAL
GOOD -0- GOOD
This is by no means a way of describing the character himself but a measure of what flows through his body.
A character at 0 on this gauge is, regardless of how he acts, good internally...a character at 5 is on the fence....and a character at 10 is wretched and evil inside.
How does a character get this way now? All characters begin at 0, sorry to those of you evil-mongers, you can change where you start if you like. As the character goes through fights, and gain quickening, the storyteller must have pre-decided how evil a character they are facing is. If the character killed is above the character alive in rating, the winner must make a willpower test against the loser's rating. If he fails, he gains the opponents rating divided by 3 rounded up to the nearest half on this gauge. Scary huh?
Example: Vincent is facing Lars, an immortal with a rating of 8(not a nice guy). Vincent ends up defeating him and gains his quickening. Along with the power comes the other end... Vincent has a rating of 2(on the inside, he's nice). Vincent now must make a willpower test against a target of 8(Lars rating). All he needs is one success(that's all that is ever needed). He attempts to resist, but uh oh...he doesn't. He now must rise in rating on the gauge. Lars rating divided by three is 2.5... Vincent now has a rating of 4.5... getting worse.
Now how does one battle this? If he desires, he can attempt good deeds to bring this down... the effect of the deed is entirely upon the game master's head.......
What happens if the dark quickening is gained? The immortal is completely unpredictable... he may act decent one moment to a friend, and then try to kill him the next. The immortal is almost possessed by a demon. The Game Master can decide how the Dark Quickening will have an effect on the game...perhaps making it similar to rage... making the character make willpower tests to resist evil doing.
How does one overcome the Dark Quickening? There is no known way...like I said before, Duncan fought himself in a magic pool that Methos brought him to, perhaps that is unrealistic...but isn't the thought of two people trying to chop each other's heads off for power? This will be up to the Storyteller, and should entail much roleplaying, and the character fighting an internal battle, if not one outside as well.