There's yet another alternate dimension in XI too called Abyssea, which is a straight up parallel world. In that world, reality splintered because your party died during the end of the 2nd expansion (Chains of Promathia) so instead of you defeating the Big Bad, the calamity happened and the very fabric of reality started to get eaten away by the Emptiness.

So your world's version of the self has to dimension hop to that dimension and do it again, but stronger (level 99 instead of level 75) so you can kill the Big Bad properly this time. (And the Big Bad got worse because he ate one of your party members who contained an avatar and is now Bigger and badder.... yeah. oops.)