Twilight the princess of Ponyville has the fear of losing two things. 1) Losing the love of her brother, Shining Armor or hurting him in any way. And 2) Doing the same thing as 1 to her beloved assistant Spike. Discord and another someone has a BIG surprise for her then....

Source for cover art: https://derpibooru.org/10332?scope=scpe4d86d8caeb3694ed37123b990207a55b3266f2f9