The Epilogue. Just the entire epilogue. every single word in it seems designed to make you squee and make your heart feel warm as you bask in genuine and hard earned emotion
The fact that Twilight's unflinching love and devotion for Celestia, watching how much they meant to each other and how the separation was hurting them, moved (emotionally) the Sun to hand her a punishment that, while both symbollic and meant to teach a necessary lesson, would allow Celestia to be with Twilight for the rest of her life, as a mortal, until she was ready to rise again. This is the same bitter being who had been gleefully mocking Celestia the entire fic, and she was so deeply touched by it, she admitted she was basically doing it for Twilight, in acknowledgement of that love.