This story is a sequel to What do Nightmares dream of?

Nightmare Moon is caught in Celestia’s bedroom, where she is confronted by the two princesses and opens up completely to them.
She tells them that she has changed, and her feelings are the complete opposite to what they used to be. Especially her feelings towards the Princess of the Sun.

Cover art from Vector-Brony’s deviantart

Nightmare Moon has spent years of accumulating enough magic for herself to materialize after her defeat by the Element Bearers. She is separate from Luna. Her own being. She does not want eternal night anymore, having realized it would doom Equestria.
She can only think of the Sun. Of how dear is the Sun for her actually. She wouldn't take it from the skies... Everything needs her.
Even dark creatures as herself.

Cover from 90sigma's DeviantART

EDIT: Wow. Featured. I didn't expect that. You guys are wonderful. There will DEFINITELY be a sequel to this now.