You can join a variety of societies, including monastic orders, secret organisations and devilish cults.
Ostensibly, Monks and Mystics is all about opening up new roleplaying possibilities in regards to faith. Crusader Kings 2 has always had a bit of a dark side, but with the new Monks and Mystics DLC, it’s been cranked up to 666. Last night I ate my cousin because the Devil told me to do it. I’ve been hanging around with a bad crowd, staying up all night, attending weird orgies and torturing rivals until they embrace the teachings of Lucifer.