I don't know about you guys but I got a serious "LOST" vibe when that black mist started swirling around Aerith and Cloud.
We now know that the mist is called "Keeper of Fate", and we have no idea what that means. But that's good, because we can speculate.
Personally I believe that the "Keeper (Keepers?) of Fate" will be an expansion on the Cetra storyline. Somehow, somewhere the "Keepers of Fate" are witnessing the destruction of the planet and are forewarning Aerith about upcoming events.