Keep Calm and Flutter On, epilogue

Princess Celestia did have some misgivings about leaving Discord in the care of Twilight Sparkle and her friends. (Luna had even more misgivings. That was a less than pleasant conversation.) Still, she had faith that they would, in time, be able to show Discord the importance of empathy and friendship, and convince him to use his power for good. It would be a long and difficult trial, but ultimately a rewarding one.

Imagine her surprise when, a mere three days after leaving Discord in the ponies' care, she received a letter from Fluttershy saying that Discord was officially reformed.

Celestia wasted no time in making arrangements to return to Ponyville. This she has to see for herself.