This is what I pictured what happened 1000 years ago when Luna and Celestia defeated King Sombra, and the origin of Nightmare Moon was born.
As you see, Luna has stared into the door of fears, and Sombra mocking the subjects, and teasing Luna that she will always be a shadow of her sister ever since they both took the throne.
Sombra: Tsk, tsk, it seems that your subjects don't care about your night. Luna: No, no they care! They do. Just as much as they do for my sister's day! Sombra: Do you actually believe that, dear Princess of the Night? You hide these fears from your sister, do you believe them?
Days has past since then, Luna just stares into her fear, until Celestia finally finds her way to her. Taking Luna away from the wall, reassuring her that he's just trying to hurt her.