In a twist of fate, Twilight Sparkle never applied to Celestia's school for gifted unicorns. Turning to solitude and study, she fully engrossed herself into the mysteries of the arcane. In her solitary tower at the edge of world, she prided herself in her mastery over magic.
Then one day, she receives a letter from her brother Shining Armor. He is getting married and has requested she be there. Though she hasn't left her tower in years, she decides to make an exception for her brother.
Unfortunately, when she arrives, she finds that things have changed greatly in Equestria. There are three rulers now. Three alicorns of great, dark power.
The first, Daybreaker.
The second, Nightmare Moon.
And the last, Heartache.
Her brother is marrying the last one. A bit of important information that she was not told. (Thanks Shining Armor).
Content to get through the proceedings of the wedding and leave, Twilight tries to make her presence as obscure as possible. It goes about as well as expected.
Things are about to get a lot less lonely, and not in a good way...