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The crystal is at the end of the game. Garland tells you about it before he disappears, "...A presence that presides over all life and memories. The crystal..." and he tells Zidane to protect the crystal. Kuja wanted to destroy it because he thought that would end everything. I had to refresh my memory on exactly what the crystal was for, but I remembered it was at the end and you have to protect it. I really need to play this game one more time
