Luna feels sorrow, pain, deapir, and is lonely. She feels fear and pain, and slowly watch as this eventually turns her into a Nightmare Version of herself. This is the stroy of the transformation and change of Luna into Nightmare Moon.

(Note: This story does take place right before her rebellion against her sister.)

(And yes, I know there are several hundred if not thousands of stories and videos about this change and her redemption and reformation, but please, this is my own so please support it, like it if you want, and please leave a comment if you have any questions, comments, concerns, feedback, etc.)