Luna's life since the banishment of the Nightmare has been better than the past thousands of years. Ponies respect her, they don't run in fear, and she has....friends.

But when she fears the Nightmare is resurfacing, she dives into her mind to find out what is going on.
Luna wasn't expecting herself.

Cover art taken from This page, who I hope is the rightful owner.

In another timeline, created by the meddling of Starlight Glimmer, Nightmare Moon's plot to return and overthrow Princess Celestia has succeeded. Now Rarity is at her mercy, and finds that the lonely banished princess has taken a liking to her beauty and fighting spirit. Rarity must fight with every ounce of her mental prowess to retain her independence and her sanity.

Rated T for torture and suggestive dialogue

Princess Luna has grown disillusioned with the elite of Canterlot. With the politics of the Night Court, dealing with keeping an eye on the hidden underworld of the aristocrats, and the lack of magical accomplishments in the last few years that weren't associated with Princess Twiilight, Luna wonders if it would be possible to have a student of her own to train in the lunar and darker aspects of the arcane. She's well familiar with the stories of Sunset Shimmer and Twilight Sparkle's training.

Unlike her sister, however, Luna has no interest in training a prodigy. What's the point in teaching somepony who learns everything so easily? She's set her sights a bit higher and opens her applications to anypony in all of Equestria, no matter their race. With her trusty aid, Night Shade and her "nephew" Blueblood helping her, it'll be a prospect to find the one that will be her first student in the years to come.