Before finishing the game, I decided to go and beat those giant bosses (Atlas and the Flan). I went straight for the Flan, beat him, and unlocked a new world.
This world features some kind of story
narrated by Mog, with Noel and Serah disguised as monsters. Everyone looking for ingredients to poison the flan king. And ending with a rather bizarre laughing Mog.
... and I got a new fragment... and that's all. There is no gameplay in that world, or any mission given to me etc. So my question is: am I supposed to do something related to this world?