Maleficent first mention the book having the power to foretell the fixated future, especially events in the distant future. It could also have the power to create new worlds if possible.
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top 10 plot holes in the Kingdom Hearts Xehanort Saga
Xehanort hinted that the Keyblade Graveyard was the battlefield for the Keyblade War. He also said something about the X-Blade being involved in the entire war.
So far, the keyblade masters told the younger generation that the Realm of Darkness is vile, evil, and truly demonic. However, there are a few places showing beauty.