The power of Kairi's heart was what saved Sora. The strong bond between them became the light that pulled him back from the darkness. Promising Kairi he would come back, Sora returned to Hollow Bastion to seal the final Keyhole and save Riku.

The Keyhole was sealed, But Ansem and Riku were nowhere to be found. Ansem had left to fulfill his final plan.

Sora and company followed him to a world shrouded in darkness. Destroying Ansem could restore all the worlds taken by the Heartless, but it would also mean those worlds being disconnected, as they oncce were.

Once, every world was isolated, divided from the others by impassable walls. The power of darkness demolished these walls. If the walls return, traveling to other worlds will no longer be possible. Sora may never see his friends again.

Still, he's resolved to fight to the very core of darknesss, where Ansem lies in wait. He believes no matter how deep the darkness, a light shines within.