Kingdom Hearts Birth by Sleep: The Novel (light novel)

Ten years before Sora fought to save the worlds, three young Keyblade wielders named Terra, Aqua, and Ventus trained in the hopes of becoming Masters one day. But everything changed on the day of Terra and Aqua's Mark of Mastery exam, and the three each set off on their own journeys that would test their friendship-and alter the course of their destinies forever.