You’ll recall, in your first journey, that you brought many worlds back from the darkness― but some never returned completely. They still sleep, cut off from all outside channels. Not even the Heartless can enter. But these Sleeping Worlds are said to have their own manner of darkness. They are called “Dream Eaters,” and there are two kinds― “Nightmares,” which devour happy dreams; and benevolent “Spirits,” which consume the Nightmares.
We must be ready for Xehanort. Which is why you, Sora and Riku, are to be tested for the mark of a true Keyblade Master. As there are seven pure lights, there are seven “Sleeping Keyholes.” Unlocking these will both grant you new powers and free the worlds from their wakelessness. Complete this task, and return here safely, and I will name you both true masters.