You need to get Anima first.

(SPOILER)Return to ANY Aeon temple that you didn't find the treasure box in. It is impossible to miss the treasure in Bahamut's temple, since you cannot return to Bevelle. Then search around with the airship for the Baaj Temple, where you first appeared with Tidus in the beginning of the game (Don't remember the coordinates, sorry). If you gathered all the treasures, release all of the seals to find the fayth of Anima, Seymour's mother. Oh yes, you need to fight that monster you first fought again, he uses petrification attacks and is fairly powerful. It's underwater so only Rikku, Wakka and Tidus can be used.