chapter 11 and 12 (warning: spoilers)

[CowboysFromHell]

The item costs 2 million gil from the store and is called Trepezohedron. It also drops from monsters, but it is a rare drop. You know those really big adamantoise with gear on them. They drop the item.

Can I use trepezohedron on any lighting's weapon even if it's at lvl 1 and it transforms into the ultimate weapon? What about other characters?

[Andromeda]

No you have to go through the steps it'll only work on the tier 2 version of any weapon. All characters weapons to go to their final 3 tier form requires the item, but all tier 2 weapons will change into the ultimate weapon regardless of which weapon you choose. The only difference is the stats depending on which weapon you use. So you could end up with a weaker ultimate weapon if you choice poorly.

[CowboysFromHell]

No you have to go through the steps it'll only work on the tier 2 version of any weapon. All characters weapons to go to their final 3 tier form requires the item, but all tier 2 weapons will change into the ultimate weapon regardless of which weapon you choose. The only difference is the stats depending on which weapon you use. So you could end up with a weaker ultimate weapon if you choice poorly.

I don't even know how to upgrade it to tier 2 weapon. I upgraded to the star lvl for lighting's weapon which she started off with but it sucks.

[Andromeda]

Once a weapon or accessory is at the max level (the star) all of the components in the upgrade list will be grayed out except for the specific transformation item that is used. So if you're not in the upgrade menu, go in select the weapon you want to transform like you would for upgrading. And then if you have your list sorted by category the transformation items will be at the bottom. If you don't have any item that is not grayed out, then you don't hold the item need to transform it. Though to go from tier 1 to tier 2 you probably already have a couple of the items.