Well, a while after we sealed him, Princess Celestia brought her to us in Ponyville in an attempt to get him to reform and start using his powers for good. How she came to the conclusion that such a thing was even possible, I'm not sure.

We all put in an effort, but in the end it was Fluttershy who pulled it off. It was remarkably simple, too. She managed to befriend him, and when he realized that if he went back to his world-wrecking ways he'd lose her as a friend, he admitted defeat.