a question about X

[Moogle]

To get the treasures with the distruction spheres, you have to get bevelles one at the time of the original trial, because you can't go back there again. The others can be got afterwards, though once the airship is got, you have dark aeons to contend with:
Dark Valefor outside of Besaid
Dark Shiva outside of Macalania
Dark Yojimbo also appears in the cavern of the Stolen Fayth, but that may be only after you have aquired the aeon and return.

[Snaggle_Tongue]

What moogle said. Also, There is no way to remove the Dark Aeons other than defeating them, even after your traitor status has been revoked.

[VITG]

Originally Posted by Moogle

you have to get bevelles one at the time of the original trial, because you can't go back there again

Just a note about the Bevelle treasure, the treasure for Bevelle is automatic. There are two chests. One is right at the end no matter what, at the top of the stairs will be the Knight Lance. The extra treasure you can get with the destruction sphere, the HP Sphere, is just a bonus, and doesn't affect the unlocking of Anima.

Useful tidbit.

As for the Dark Aeons, the only ones standing in the way of any trial treasures are so laughably weak. You shouldn't have a problem if you just get your strength and agility up to scratch.