After Twilight returns from the Human World, Celestia is desperate to know of her former student, Sunset Shimmer, and what happened to her. Upon learning of the horrible things her former student has done, Celestia breaks down in tears as Twilight asks why she was so important to Celestia and why she never told anypony. Celestia tells the story of her and Sunset. How they were so close, and how one secret ruined everything between them.

Spoilers in comments.

Thanks to Biker_Dash, ugugg93, and The Abyss for edit and Pre-reading

Thanks to SilFoe for letting use the art