What if the Nobodies didn't come back? Sora would be lost. And everything else with him. I don't own Kingdom Hearts.
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As Xehanort's glowing heart grew closer to Sora, Riku screamed, his eyes welling with tears as he violently struggled against the unyielding grip of one of Xehanort's incarnations. Sora had been his tether to the light, his shield against the darkness. As Mickey and Riku looked on, Sora shuddered slightly, while inside, unknown to them, Xion, Roxas and Ventus desperately fought against Xehanort's dark heart. But even their combined efforts could not halt Sora's corruption. Xehanort laughed cruelly as they shattered, each having failed to protect what was most important. Sora's slumbering body on the lofty throne was cocooned in darkness as his spiky hair turned silver.
Mickey himself was sobbing loudly. Memories of Ventus, Aqua and Terra flooded through him. He had valiantly fought for too long. Black energy surrounded the mourning king, who could no longer absorb the onslaught of despair brought about by one ambitious man. Mickey quickly dispelled the darkness, resolving to go onwards. However, his heart was forever tainted, his grin a little less bright, his eyes duller than before. Sora – no longer Sora – opened his eyes, now golden. The sight caused another round of heart-breaking cries to ensue from Mickey and Riku. So much had relied on Sora. So many had relied on Sora.
In the Realm of Darkness, Aqua felt a keen sense of sorrow. The Heartless swarmed around her quailing body, at last devouring her. Terra, deep within Xehanort, at last stopped resisting, his hope crushed along with the faint light of his two friends.
"The thirteen darknesses are united at last." Sora's voice was now deeper, but still distinctly childlike; reminiscent of corrupted innocence. The sound was so wrong that Riku dry heaved, his body under more strain than ever before. Mickey at last managed to break free, grabbing Riku and leaping through a portal to Master Yen Sid's tower. Not-Sora's laugh hung in the air as the black-coated figures dispersed throughout time. He himself would cautiously watch and make certain that the seven lights united.
Inside Yen Sid's tower, Mickey's hastily breathed news before fainting had sent the other occupants into chaos. Yen Sid had quickly summoned Kairi, telling her of the news. She was now unconscious next to Riku, her cheeks damp. They could never prevail against Xehanort. Yen Sid knew they would die; either in the creation of the X-Blade or after long torture by a triumphant Xehanort.
Sora's heart, so long bolstered by the bonds of his friends, brokenly dreamed of beaches and races and sea salt ice cream with all the stars in the sky above him. He would never wake up.