https://www.gamespot.com/articles/civ-6-expansion-rise-and-falls-new-features-detail/1100-6456131/

Civilization VI's first big expansion, Rise and Fall, is coming soon, and today developer Firaxis showed off more of what you can expect from the DLC. Rise and Fall adds a bunch of new systems, and a new video details what those will do and how your experience will change.

These new features include Dark Ages, a Loyalty System, Governors, and Emergencies. Golden Ages have been a mainstay of the Civilization series for a while now, but Rise and Fall adds a counterpart with Dark Ages. If your Era Score isn't high enough, you can enter into a Dark Age, which reduces citizen happiness and can cause your cities to revolt or secede. However, Rise and Fall also adds Dark Age policies which can affect your progression positively; in addition, it is easier to achieve a Golden Age when coming out of a Dark Age.