It's in their bios in the Bestiary actually. It details their origins and the story of their fall, giving a little more background to each of them. As for in-story, technically every time you face one you're breaking the seal that has them trapped and with the Occurians gone by the end, it's very likely that once that party passed on, they had free reign on the planet. Fans speculate their return and a possible counter-offensive by the Occurians trying to reclaim Ivalice is the caused of the Great Cataclysm spoken of in FFTactics.