Lost Master Arc

The Lost Master Arc is the 2nd Phase of the  series, beginning from Kingdom Hearts IV.

Chronologically, it is preceded by the χ series and the Dark Seeker Saga.

Defeat of Master Xehanort
Following the defeat of Master Xehanort and the Real Organization XIII, Sora resolves to find the heart of Kairi, who was struck down by Xehanort in order to forge the χ-blade moments before the final battle. Despite misgivings from King Mickey, since Sora previously failed his Mark of Mastery exam, Sora successfully manages to locate and restore Kairi, though his own heart is lost in the process and he fades away. Sora's friends later try various methods to locate him, but all are unsuccessful until the Fairy Godmother intervenes and suggests that they use the dreams of Kairi and a Nameless Star. Kairi is confronted in her dreams by an illusion of Xehanort, who mentions a world "on the other side", which the Nameless Star goes on to identify as a world named Quadratum. Using the power of waking, Riku opens a portal to Quadratum to search for Sora. Meanwhile, in Quadratum, Sora is accosted by a young man named Yozora, who he narrowly defeats.