Level: MediumYou have to win a civil war for this one. Get a strong ally and make some vassals hate you. The easiest way is trying to imprison one of your Vassals and if he gets away he will start a liberty faction war or you can wait until some of your Vassals start an independence faction. When they declare a war on you, you have to WIN this war, no white peace.
Credit to
Bayek von Kuwaq on Steam for his
guide.