The secret societies are a fun edition to the game and add new interactions and flavor to the base game. One of the better expansions in my opinion.

It would be full on 5 stars if the "secret religion" didn't bug me off the wrong way, having most of your family secretly adopt a pagan heresy and knowing about it without being able to do much unless willing to accept becoming a kinslayer and burning them at the stake like all such worshippers of false gods should.
Anyway, joining any religious cult definetly adds something for those dull peacefull eras and bring with them both events and perks. The worsts cults like Satanism, the Indian death cult or Assassins do come with quite the high price however, but definetly provide quite the colourfull experience, where as most christian cults reduce your fertility but add to stats and provide you with abilities like telling the future (can be bad) and making medieval anti-depressants by throwing bunch of... Interesting items in a pot.

It's among the better DLCs. The societies mechanics is pretty cool, but there aren't too many events and it can get stale a little too soon. Artifacts are fun and pretty OP, I find myself rarely joining anything else than the Hermetic society so I can get artifacts. Devil worshipping got nerfed, but it's pretty fun as well.