In this game of EU4, I was playing as the Polish-Lithuanian Commonwealth was got Austria as a vassal through a personal union. As a result, the Palatinate became the new leader of the Holy Roman Empire.
After all of this, I decided it would be the perfect opportunity to get the Ruina Imperii achievement, where any nation must disband the Holy Roman Empire. To do this, I allied as many electors as I could (I was already allied to Bohemia) got into a war with Brandenburg and then declared war on Saxony.
After a few tries, occupying all the capitals of the electors I was at war with, I disbanded the Holy Roman Empire in 1557, thus gaining the achievement.