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

Three years. Three years it had taken Celestia to finally be able to see the real Sunset Shimmer hidden beneath all the greed, disgust and hate. But now that she saw the real Sunset for what she was, she was determined to finally help the girl. It wouldn't be easy; Sunset had many skeletons in her closet and the path before was marred and twisted with her past mistakes. But perhaps, perhaps if she plays her cards right, Sunset could have the one thing she always dreamed of. A family.

Inspired by every fanfic where Sunset gets adopted by Celestia and/or is Celestia's kid.