After Luna and the citizens of Ponyville defeat the Tantabus, Twilight writes a letter to Celestia regarding the events, including an explanation of why the Tantabus came to be. Celestia is devastated at this news, and decides to confront her sister. Little does Luna know that she's not the only one facing so much guilt.
Based on Season 5, Episode 13. Told from Celestia's point of view.
(Also, rated Teen because I put, like, 3 cuss words in it.)