Keyblade War

The Keyblade War (キーブレード戦争) was a mysterious conflict in which many Keyblade wielders fought prior to the events of Kingdom Hearts Birth by Sleep. According to Xehanort's research, the conflict arose when the Keyblade wielders of the Light and the Darkness clashed with each other to create the χ-blade and, with it, control Kingdom Hearts. The war itself was so massive in scope, that worlds that were not originally involved were drawn into the conflict. The war is what caused the Kingdom Hearts universe to be remade into what it is today. The failed attempts also resulted in the scarred remnants that would someday be known as the Keyblade Graveyard.

Several years later, Master Eraqus and Master Xehanort do discuss the events of the Keyblade War, having heard legends of it. It is during this encounter between the two Keyblade masters that Xehanort's goal to unlock the mysteries of what took place at the Keyblade War's end through creating the χ-blade become clear. In order for this to be done, however, Xehanort would need to join two hearts, one of pure light (Ventus) and the other of pure darkness. The one of pure darkness, whom he named Vanitas, would later become his apprentice and seek to open Kingdom Hearts through the χ-blade, thus allowing Keyblade wielders from other worlds to reassemble in the Keyblade Graveyard, resuming the Keyblade War.

Master Xehanort was foiled, however, by the combined efforts of Terra, Aqua, Ventus, Mickey Mouse and the Lingering Will, and the Keyblade War was prevented.