Rockhoof has returned to Equestria after a thousand years in Limbo. He's also severely depressed. Princess Luna pays him a visit in his lonely dream to find out what troubles him.
It's been two years since Applejack died and Pinkie Pie took Applebloom in. But being a "just found out we're related" cousin isn't the same as having a sister...
Do you remember the three nights of darkness when Luna refused to raise the moon and bring out the stars? It was a thousand years ago, but her heart will never heal nor forget.