Master Xehanort was a Keyblade Master, the first master of Ventus, and the creator and master of Vanitas, the original incarnation of Ansem and Xemnas and the future self of Young Xehanort. He manipulated almost every event in the series, and devised many backup plans that involve dividing his heart among 13 different beings created from his power and take over the universe. He is the original incarnation of Xehanort from which Ansem, the Seeker of Darkness, Xemnas and all others spawned. Master Xehanort is the main antagonist of the Dark Seeker Saga, the first phase of the Kingdom Hearts franchise. ~ Master Xehanort describing his long term goal.
The χ-blade needs to be forged and with it, the door to the Keyblade War unlocked. So what, then would another Keyblade War bring? When the darkness falls, will we be found worthy of the precious light the legend speaks of? I must have these answers. It is a curious tale and one worth exploring. Amidst that crisis, a precious light was found. We know so little about the Keyblade War, only that it was just the beginning.
But, darkness did cover the world once, in legend.