Serenity can be empowering. Serenity can only come from within you - which is the same place, the only place, where you have absolute control.
Think of the "serenity prayer" that has been everywhere from greeting cards to posters to coffee mugs and back again. It isn't something that brings power and control, as symbolized by the Emperor card, to mind.
To change the things you can is an act of leadership, discernment, and courage.
To "accept"...in other words to deal with, to cope with, to rise above... the things we can not change or control takes a different kind of courage, but it is courage just the same.
To rule wisely, an Emperor must be in the moment, the here and now, see things as they are, act both compassionately and effectively. Many of these same things are found on the path to serenity.