A Shadow Across Time

Professor Dumbledor often thought fondly of his old partner Gellert. He some times felt twinges of guilt that he wasn't able to stop the man that Grindelwald became. When the guilt becomes unbearable, he decides to bring a teenage Grindelwald to the future and into Hogwarts. To have another chance to prevent the dark wizard from rising. He didn't think, he'd get close to Tom Riddle