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.

Overview
This storyline covers all games that canonically take place before Kingdom Hearts Birth By Sleep, sometimes referred to as "the age of fairy tales". This era was a golden age for Keyblade wielders, where the world was whole and light prevailed. However, 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. A group of guilds run by the foretellers all led by the Master of Masters formed to bring order the increasingly chaotic and fragmented world, though the master has ulterior motives unclear to everyone else. At this time, the players themselves go on adventures and meet new friends and enemies, and eventually meet a grim fate and are forced to sacrifice themselves to become Dream Eaters.

Many years later, Xehanort, who Player connected hearts with before their demise, trains to become a Keyblade master alongside with Eraqus and his other apprentices. After many tragic events, including the deaths of many of his friends, his 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.