Keyblade War

The Keyblade War (キーブレード戦争, Kīburēdo Sensō) is a legendary conflict that occurred long before the events of the Kingdom Keymasters series, yet is a major driving force behind Xehanort's actions.

"In ancient times, people believed that light was a gift from an unseen land by the name of Kingdom of Hearts. But Kingdom of 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 of Hearts was swallowed by the darkness, never to surface again."
 * —Master Xehanort.

General Description
Both Jolnir and Xehanort revealed that when the World was whole and full of light people desired the power of Kingdom of Hearts, many creating the first Keyblades in order to obtain the means to control Kingdom Hearts: the χ-blade. However, many sides clashed over who would get the χ-blade, causing a massive war that consumed everything in darkness as the ancient weapon was shattered into twenty pieces, seven of pure light and thirteen of pure darkness, and the "true" Kingdom Hearts disappeared into the darkness.

However, the light within the hearts of children kept the darkness at bay yet separated the worlds from each other. The remnants of the battlefield on which the Keyblade War took place became the Keyblade Graveyard, littered with Keyblades of fallen warriors. The remaining Keyblade wielders resolved to protect the World from further destruction, becoming guardians of the World order.

The story of the Keyblade War was passed into legend, and the Keyblade was depicted both as a bringer of chaos and destruction and as a weapon to save the World. However, ages later, Xehanort came upon the story of the Keyblade War and resolved to recreate the χ-blade to cause a second Keyblade War.

Truth about the War
As revealed in the Kingdom Keymasters X, the war was actually predicted in the Book of Prophecies. Before the events of the players start their journey, the five Foretellers and one other Keyblade wielder were the apprentices of his creator, the "Master of Masters." Shortly before he disappeared, the Master of Masters assigned his disciples new names and gifted five of them with a tome from the Book of Prophecies, which taught them of the event that would doom the world.

However, for Luxu's case, instead of given the tome, he was made privy to the secret contents of the Lost Page detailing the intentions of the Master of Masters—intentions that are not concerned with the survival of the world, and instead foretell how the conflict between Unions will eventually erupt in the Keyblade War—and was given the mission to observe these events to ensure that they will occur as prophesied. Due to the Lost Page, he watches over and manipulated the events and Foretellers, just to ensure that the prophecy of the war will come true.

Due to this, there are conflicts in between the Foretellers, who have mixed reactions and thoughts of preventing the world's end. Ephemer, a Keyblade wielder in Daybreak Town discovered the truth much faster than the Foretellers. To ensure the survival of the children and wielders, under Foreteller Ava's instructions, he went to the outside world to wait for the dandelions. When the time comes for the original world to be destroyed, the children will create new worlds through their hearts.