This story is a sequel to Sunset Shimmer is MAD about EVERYTHING

The mysterious Table of Harmony has called on Starlight Glimmer to solve a friendship problem at Canterlot Castle, but wait! A second cutie mark belonging to a magically inclined alicorn has appeared, and unfortunately for Twilight, it’s not hers.

Can Sunset Shimmer help Starlight Glimmer untangle this mysterious problem in Canterlot, or will her own personal past prove more a hindrance than anything?

Spoilers for 'A Royal Problem'

Sunset vector by Sunsetmajka626.
Starlight vector by Cloudyskie.

Why does Princess Celestia leave Twilight to face all the villains?
Sunset Shimmer has returned to Equestria, to confront her former teacher.
The solar princess decides it's time to answer Sunset's questions.

Losing a child is the most terrifying thing that can happen to a parent.

Sunset Shimmer had been gone for years. Celestia had never really given up hope that she would see her again, but the doubt had always been in the back of her mind: what if she never returns? The echoes of their last fight followed her all those years.

And now that Sunset Shimmer has returned, Celestia wants to see her again. She has an offer to make, to a filly she raised as a daughter.

Princess Celestia has something to tell Sunset Shimmer.

Something about her parents.

It's nothing strange at all.

Yep.

Rated Teen and Sex for a very brief mention of how babies are made.

Sunset Shimmer, twice savior of Canterlot High who earned her wings by using the combined magic of the Earth elements of harmony to save the world from Twilight Sparkle (you know... the one with the glasses). She's offered to help Twilight's (you know.. the alicorn Princess of Friendship) new student learn about friendship while the whole crew embarks on a trip to the Crystal Empire. This should be a fairly relaxing and stress free trip. Right? RIGHT?!

It should go without saying, but MAJOR spoilers to the season 6 premier.

Sunset vector by mighty355. Other cover art stuff by Novel Idea!

When Sunset was just a filly under the tutelage of Princess Celestia, she developed a powerful crush on her mentor.

It didn’t end well.

Years later, Sunset is older, wiser, and ready to finally reconcile. But as she’s prepared to put her past behind her, Sunset finds out with growing horror that some feelings aren’t so easily forgotten.