Long ago, in the "age of fairy tales," the world's light is protected by seven individuals who reside in Daybreak Town and wield unique weapons called "Keyblades": the Master of Masters and his six apprentices. A devastating conflict has been foreseen, and the world's end is imminent. In order to heal it, the Master of Masters gives his disciples unique roles and instructs them to form Unions comprised of other Keyblade wielders. Soon afterwards, the Master vanishes.
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(Source: KH Wikia)

Kingdom Hearts χ Back Cover