We. The Revolution

In Act 1 (on day 6 iirc), you'll eventually judge the former king, Louis XVI. You have to either send him to jail or acquit him for this achievement. If you try to do this the first time (possibly if you still decide to execute him as well), the game will stop you and reveal more evidence of Louis's guilt. After a speech and coming back to the court room, you'll need to select jail or acquit again. Doing this significantly lowers relations with everybody, so you may want to reload the day after you get this achievement and execute him before continuing on.