It's cool that you're listening to suggestions this early in the game's development :D
Here's some ideas to take into consideration, possibly for that Jester player character you mentioned.
Shift ideology: A ritual (or card) that relies on randomisation. The ritual targets an entire society and (to a certain degree) randomizes the ideologies of its pawns. The cost would increase for larger societies, either leading to a destabilized kingdom ripe for the civil war treatment, or you could leave the society's bonds stronger than when you started.
Peace bonds: More on the political side, if the darkness takes interest in stopping a kingdom from assimilating another, it can orchestrate a peace agreement between the two societies. It would simply be decided by voting, if both societies are made to have a majority vote in favour, neither society can attack the other for a number of turns. Pacifists support, militarists oppose.
Plague warding: Occasionally a seemingly well placed plague of discord can unintentionally spread towards cities that the darkness would prefer to go uninfected. The plague totem would be given a world power that protects a selected city from plague for a number of turns. Of course, there's a chance others in the society will notice the city's uncanny immunity, and possibly begin investigation.
Cleansing light: Armies from cities that are part of a kingdom of light are taught a small amount in the lightbringer's ways, giving them a noticeable advantage against human armies that are enshadowed.
Crushing darkness: Similarly, if a kingdom of darkness has been established, armies under enthralled lords take a piece of the shadow's will into battle, evening the playing field against lightbringer armies, and giving them an advantage against regular humans.
Both of these buffs can be improved by buildings exclusive to lightbringer/enthralled towns, an increase in power for either side is declared in an event.
Poison Soul: Essentially a slower version of assassinate, the darkness slowly nibbles at its target, instead of outright devouring them. This can be made to look like a death by regular human espionage and costs slightly less power. This means that less evidence will be created and less courtiers will become suspicious, but removing the pawn takes an unpredictable amount of time, and is by no means swift.
Influence stars: The shadow can only operate for such a short time due to the position of the stars (If I remember that right), but creating an astrology based building can change its time, in both directions. It would have access to two rituals, one which would increase how many turns the game lasts, and another to reduce the amount. The ritual that adds turns would debuff the shadow in some way, while the one that shortens its precious time would give it some kind of advantage.
Cultists: An enthralled character could be ordered to form a cult. This cult could then be sent to a city that is not under the control of the shadow. Depending on which type of cult was selected, the city would be debuffed in some way (weaker armies, poorer relations with other pawns etc). Cults would generate evidence in the city they occupy, and can be discovered by investigators sent by other lords. This suspicion could also be used to turn members of a society against a specific pawn by ensuring that the investigator does not return with news as normal. If the cult is uncovered and a lord receives knowledge of them, it is traced back to the enthralled lord that founded it and gives them a large amount of evidence, as well as making both the investigator and the lord of the cultist afflicted city aware of the darkness.
Torment: If orchestrating a society to disagree with a ruler's views is the scalpel, Torment would be the mallet. An expensive ability that can be used on a lord to confer the "tormented" trait, lowering his general likeability the same way "hideous" does. The Torment of mortal minds is a common tool of the darkness, causing worldwide evidence to increase at a steady rate for each turn the tormented pawn is alive as memory of its use is sparked.
I realize this is just a sea of half baked ideas and I don't know much about game balancing, but maybe there's something usable here.