After introducing nearly every faction and nation into Crusader Kings 2, it appears that Paradox is now getting a lot more experimental with their expansions and DLCs. Just recently they released The Reaper's Due, a DLC that brings with it the black plague and a variety of other maladies, and now they're working on a DLC that brings forth monks, mystics, and secret societies into the mix.
If that sounds like something you might be interested in, then have a look at the recently posted teaser trailer. It doesn't show anything in terms of gameplay features, but it does a rather good job of setting the mood. Here's the video:
As far as the gameplay details are concerned, there is unfortunately not much information available just yet. What I can offer you, however, is the list of features that came with the announcement. While not exactly detailed, it should give you a general idea of what the Monks & Mystics expansion is all about. Here's the list:
• Monastic Orders: With lay offices available to non-clerical characters, monastic orders will increase your character’s piety - but they have expectations.
• Cults: Gnostic heresies may flourish and its is rumored that the worship of Satan continues in the darkened fringes of the world
• Climb the Ranks: Characters work their way up the ladder of Societies, unlocking new abilities and triggering new event chains
• Hunt for Heretics: Send your chaplain out to find secret societies that undermine the holy church and your realm. But what if the hunter is guilty himself?
• Other New Councillor Actions: Your councillors have a fourth action that will make them more powerful and integral to sound rulership
• And much more: Assassins, archaeological expeditions, holy relics, great treasures and more
There is currently no release date set for the Monks & Mystics, but according to the official website you can expect to see it arrive rather soon. For comparison's sake, The Reaper's Due took about a month to go from announcement to release, so expect the same to apply here as well. In other words, my educated guess is that Monks & Mystics will arrive in early January.