Which story saga does Kingdom Hearts III conclude?

Kingdom Hearts III concludes the Dark Seeker Saga, also called the Xehanort Saga.

The saga begins with the original Kingdom Hearts and follows the long conflict surrounding Xehanort, his incarnations, and his plan to obtain the χ-blade and summon Kingdom Hearts. Kingdom Hearts III brings that storyline to its major resolution after the events of Kingdom Hearts 3D: Dream Drop Distance.

In the game, Sora, Donald, and Goofy join the seven guardians of light against the thirteen seekers of darkness. The conflict culminates in the Keyblade Graveyard and a final confrontation with Master Xehanort. The ending resolves the central struggle, though it does not end the wider Kingdom Hearts franchise.

The other options describe different concepts or story threads. The Keyblade War is an ancient event explored through prequel material, Union Cross refers to a later mobile-game title and arc, and the Lost Master arc belongs to the series’ post-Xehanort direction.