The Eternal Calm is a special movie that bridges the gap between Final Fantasy X and Final Fantasy X-2. On the main menu, when you choose which game to play, you will see an option called Final Fantasy X Eternal Calm. This is the movie and it will run you around 10-15 minutes to watch in its entirety. The trophy/achievement will pop at the end of the movie. Note that there is no requirement to view this, but it’s a good idea to play the game first if you haven’t.