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The moment Castiel looked Dean in the eye and all he saw was eyes as black as coal, is the first moment Castiel has ever felt true pain. He just lost his best friend, the love of his life, the only thing he lived for. the person he gave up everything for, was now a demon. At that moment it felt like someone had ripped open his chest and tore out his heart. Castiel then yelled “we can fix this Dean!”
Dean then looked Castiel straight in the eye, with his evil black orbs and said with a horrible smirk upon his face “oh angel, its fare past broken.” And with that, the demon, once called Dean was gone.