Pretty straight forward, you get this for finishing the game. I'm posting this only to aleviate potential worry from people who may see the credits start rolling and think the achievement glitched. It doesn't unlock until AFTER the credits roll (and the first set are worth watching anyway as there's plenty of story-related content rolling during the credits anyway. Following the first set of credits, there is a little mini-scene that gives you one final action to execute, then a second set of credits rolls. Following THOSE, the achievement pops.