Ultima Spell
To gain access to the Ultima Spell, give three augments to Palom and Porom (they must each have one, and one of them will have two). After they leave, go back to Mysidia and you will get augments from the Elder. Two of them will be Twincast. Give one to Cecil and the other to Rosa. When both of them use the Twincast command in battle, they will cast Ultima, the most powetful spell in the game. This will only work with Cecil and Rosa, no other combination of characters can cast Ultima.
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