"In ancient times, people believed that light was a gift from an unseen land by the name of Kingdom Hearts. But Kingdom Hearts was safeguarded by its counterpart, the χ-blade. Warriors vied for that precious light, thus beginning the "Keyblade War". The violent clash shattered the χ-blade into twenty pieces—seven of light and thirteen of darkness. And only the real Kingdom Hearts was swallowed by the darkness, never to surface again."
-Master Xehanort, to Ventus in the Badlands/Keyblade Graveyard around 1260 N.E
The Keyblade War, was a conflict manipulated by Hitt'lon Zizz'lii as a means to free himself from his prison, he started by filling peoples heads with the promise of a "Great Light". there is little known of the war, but it became a legendary conflict and became the major driving force behind Xehanort's actions of which is equally manipulated by Zizz'lii to free himself from his prison.