Titled The Story of Aislinn. This is part of the Homeworld Series of worlds, following Aislinn as she goes through her homeworld during its final moments.
The story opens with Aislinn speaking with her sister, the two discussing words of parting. It appeared that Aislinn, a wielder of the keyblade, had been tasked with a mission and destiny to go eradicate the darkness that had been encroaching upon her world. To that end, she was to go on a journey that would take her far from home. Knowing this time was upon her, Aislinn went to say goodbye to those she held close to her, the first being her sister. Next she went to visit her master, the one who had taught her all she needed to know about the keyblade, saying goodbye to him as well.
As the woman made her way into her hometown, she admired the peace and cheeriness in the air. As she was saying goodbye to her home for one last time, disaster struck. The barrier that kept her people safe from the encroaching darkness started to wain as the heartless began to attack. Rushing to the aid of her people, the girl ran past the barrier to help the soldiers fighting the heartless.
Despite her best efforts, though, there seemed to be no end to the flooding tide of heartless. As such, the other soldiers who had been fighting were starting to get overwhelmed, and were on the verge of death. Rushing to their aid, Aislinn managed to stave off the attacks, allowing the soldiers to retreat back beyond the barrier but leaving herself vulnerable.
Taking advantage of this, the heartless continued their relentless pressure on Aislinn, eventually overwhelming her. Unable to fight back for much longer, she was eventually consumed by the tide of darkness and forced into a black abyss. When next she awoke, she had no idea where she was, only surrounded by darkness and on a singular path. Following this path, the woman had found herself at the entrance of a large, foreboding castle.