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 itself from 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, the first Keyblade Wielders came into existence.

The Early Keyblade Wielders
This era was a golden age 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, who formed their own guilds with their own roles. With Ira becoming the leader, Aced as Ira's supporter, Invi as the mediator, Gula attempting to find the traitor, Ava gathering wielders devoid of unions into a separate group, and Luxu observing from afar. All but Luxu were armed with a special book created by their Master known as the Book of Prophecies. The Master of Masters secretly bonded each of his Foretellers, including himself and Luxu, with seven of the original thirteen darknesses. This knowledge was known only to the Master of Masters and Luxu.

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.

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 who remembered the past. To further bury the past, the leaders created new Dream Eaters called Pets to aid the wielders themselves in their new adventures. 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 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 understand their meaning. Xehanort travelled to Scala ad Caelum and trained 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 upperclassmen peers went missing - they 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 stealing the 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. Later, Xehanort and Eraqus would take their Mark of Mastery exam, which both would pass.

After the many deaths of his friends, Xehanort's heart was broken and corrupted by darkness. He spent the next several decades studying the history and workings of Keyblades, as well as the Heartless, Nobodies, and the process of recompleting one's body. 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.