This story is a sequel to The Alicorn that Time Forgot

Midnight Requiem has big shoes to fill. The new Princess of the Night has a long list of royal duties, one of which just so happens to be dealing with Luna's less-than-savory hobbies from the distant past. She'll discover that regardless of a pony's social standing, their upbringing, or their lot in life, everypony still has dirty laundry they'd rather not air out.
When a posthumous letter appears, Midnight Requiem is thrust into an epic journey to alleviate a demon from Luna's past, and relieve herself of the same burden. Along the way she'll solve cheesy puzzles, encounter the ghost of Princess Luna herself, and find out more than she ever knew (or wanted to know) about the former Princess of the Night.