First, you're going to want to make sure that the game you are going into has the rise and fall dlc activated. Then load into the game.
Second, now that you are in the game try to do as little as possible to getting era score. This is to ensure that you get a dark age.
If/when you get a dark age, go and change your policies for your government. Now all you need to do is assign a dark policy to a wildcard slot and confirm your selection.
Remember to read each dark policy card carefully as they all have a negative to them.
The achievement is now yours.