Twilight Sparkle has retired from her role as ruler of Equestria after a two hundred year reign. Expecting to enter a peaceful retirement, she is instead confronted by the return of King Sombra, who once again seeks to enslave the ponies of Equestria and bring darkness to the land. Her friends having grown old and passed away long ago, Twilight must face Sombra alone.

Art by Adreamera, http://derpibooru.org/images/2600332