Thanks Illathid, glad to help out with bringing the awesome world of Warhammer to CK II
Besides making a basic plot line for the Von Carstein invasions, I've been working on chaos cultist mechanics. The basic "fall to chaos" system was already there, where based on certain traits, you might get tempted by one of the ruinous powers (for example lustful, hedonistic characters might get tempted by Slaanesh). Other traits, such as diligent, brave, and especially zealous help you resist the powers of chaos.
For an example, I console-cheated my way to demonstrate the life of a new Slaanesh cultist real quick:
We have the one of the Imperial claimants who (if I hadn't used the console to trigger this event) would eventually get this if she had certain slaanesh-inclined traits. Mean time to happen is 40 years, but this decreases drastically based on traits of course. If you reject chaos once, you get a character flag, meaning you will not keep getting spammed with these events - you've shown that you can resist the lure.
The AI will make the decision based on certain traits, as mentioned, but the player can always choose to join or not. In this case, I join.
Now our character has the hidden_cultist_slaanesh flag, and has access to a decision menu where you can donate to get cult favor and do other various vile deeds, and can also access a new plot to try to convert others to your cause. We try with one of our Dukes:
Eventually, the plot power will be big enough or enough time passed to present an opportunity. Mean time to happen for the trigger depends on the plot power; so far by default it is 20 years, but can happen much faster if you get enough unwitting helpers. We decide to invite him to a special party, hoping that we can corrupt him.
Now as you can see, this guy does not have any Slaanesh-friendly traits, and is in fact Zealous, so he'll probably be extremely difficult to convert to Chaos.
Our target gets the invitation, however, and since I'm console-cheating and switched to play him for a while, I decide to accept the invitation (though as stated, due to his pious/zealous traits, the AI would almost never choose this option):
He now becomes a lustful hedonist, making it more likely that he will accept when approached directly next time.
After a while, the plot event fires again, and this time we choose to approach directly:
Our target gets the message:
We decide to accept, since I'm once again console-cheating to make the choice for him, though the AI would almost certainly refuse due to his aforementioned traits:
The Empress meets him, and he joins the cult, and she gains in Slaanesh favor:
Slaanesh favor is used as a local character variable tracker to determine how you advance in the cult (I plan on making 3 levels of each hidden cultist), but you also have to be careful of a "suspicion" variable tracker, which increases each time a plot does not go as planned. For example had you invited him to the party and he refused, your suspicion factor would go up by 1, and if it reaches a high enough level, you might get a visit from a witch hunter.
Likewise, if there are enough chaos cultists in a court, the ruler can get the decision to invite a witch hunter to investigate; depending on the witch hunter's traits, however, you might end up regretting this

A cruel, proud, zealous and impaler witch hunter is likely to burn half your courtiers in his quest to root out corruption, and possibly even point the finger at yourself or your family! (These events are not written yet, however, so far I just have the convert to chaos, and convert another to your cause plots done for all 4 ruinous powers).
On my immediate to do list:
Make 2 or 3 cult-specific plots/decisions for each power. For example Nurglers might try to spread disease, while Khorne followers would instigate an attack by beastmen or something.
Create the Witch Hunter events/system to keep chaos in check.
Intermediate to-do list:
Track the waning/waxing of chaos via a global variable, and when this gets high enough, some cultists might declare themselves openly weakening the empire from within as a chaos invasion starts brewing in the north. This means that if you have a neighboring realm that has fallen to chaos, you might very well have to seriously think about intervening, as the longer his realm persists, the faster the chaos-doom-clock (variable) ticks
Oh, and as for the name of hte plot itself, obviously localisation is not done yet. We have 2 options:
1) Make all plots "secret plots" so you have no idea who is working for Chaos or not
2) Make a harmless duplicate plot for each one; for example make a "become friend" plot which is perfectly harmless, and then call the convert to your cult plot also named "become friend" so you can't determine if one is perfectly innocent, or perfectly evil.