Χ series

The χ series (χシリーズ) is a series of titles in the  series that are primarily set in the ancient past, during the "age of fairy tales" and up to Xehanort in his youth and his journey to becoming a seeker of darkness.

The series is followed by the Dark Seeker Saga.

Beginning
This storyline covers all games that canonically take place before Kingdom Hearts Birth By Sleep, sometimes referred to as "the age of fairy tales". Long ago before the Keyblade wielders, a war was fought between the physical manifestations of light and darkness for countless eons to finally end the conflict Darkness freed its physical form; however, in doing so, its will couldn't be maintained nor survive without a host. By infecting others, the darkness gave birth to creatures of darkness the Pureblood Heartless. To combat them, it began the rise of the first wielders.

The Early Keyblade Wielders
This era was a golden age for them where the world was whole and light prevailed. One of the eldest Keyblade Masters, the Master of Masters, trained apprentices of his own known as the foretellers forming their own guilds with their own roles. With Ira taking over as the leader, Aced as Ira's supporter, Invi as the mediator, Gula to find the traitor, Ava to gather wielders devoid of unions into a separate group and travel to another world to preserve light to bring order the increasingly chaotic and fragmented world and Luxu the far observer. The five were armed with a special book created by their Master known as the Book of Prophecies. To aid in their endeavors further the Master created the first Dream Eaters the Chirithy. However, thanks to the Nightmare Chirithy this era was soon wrought with turmoil and destruction due to the introduction and influence of darkness on the young and impressionable warriors, which culminated in the Keyblade War. Many innocent souls lost their lives, though the art of Keyblade wielding continued for some time after the dust settled. As it was the master's ulterior motives to set everyone else up first using himself and his apprentices to seal seven of the thirteen darknesses away from within.

The Union Leaders
Those that fled from the war became a separate group called the Dandelions. Five of them became Union leaders as they were the only ones to remember the past. To further bury the past the leaders created new dream eaters called Pet to aid the wielders themselves in their new adventures reuniting with old and meet new friends and enemies. However just like the Foretellers, the Master knew that the remaining darknesses continued to follow them and it was his intentions to trap five of them within the young leaders and the last one in the Data World itself. But thanks to Darkness interfering and yet the brave sacrifices of valiant friends most of the Union leaders escaped to parts unknown and the remaining dandelions eventually met a grim fate and were forced to sacrifice themselves to become Dream Eaters.

Xehanort's Early Years
We learn of Xehanort's life before he became a dark seeker he was born in Scala ad Caelum and with the residents of the world believing him to be the fabled "Child of Destiny" as a baby he was sent away for his safety to Destiny Islands being cared for by the reincarnated Player. As a boy, he had the power to connect his heart with others growing up, he was plagued by dreams of Player's memories and wished to under there meaning and thanks to his future self; Xehanort returned to his home world Scala ad Caelum, trains to become a Keyblade master alongside with Eraqus and his other apprentices. During his time as an apprentice, Xehanort and his classmates discover that their upperclass brethren went missing and journeyed across various worlds searching for them. However, this was Darkness' handiwork again this time striking down Baldr's sister Hoder and using him as the next dark vessel because of his sensitive heart.

As such Baldr struck down his comrades causing the remaining ones especially Vidar to enact a plan of revenge by the stealing worlds most precious artifacts to find the world's light summoning Kingdom Hearts to purge the Darkness. But when Vidar has a change of heart Baldr finishes the job leaving Eraqus, Xehanort and Master Odin as the last survivors and soon it was time for their Master's exam to which he and Eraqus gain the title.

After the many deaths of his friends, Xehanort's heart was broken and corrupted by darkness. After meeting a time traveling Ansem, he is explained about his future plan to summon the seven pieces of light and thirteen pieces of darkness to reforge the χ-blade and summon the true Kingdom Hearts. He spent the next several decades study about the history and workings of Keyblades, as well as the Heartless, Nobodies, and the process of recompleting. This all culminates in him discovering Terra, Aqua, and Ventus, as he wishes to use them and their insecurities to do his bidding and forward his master plan.