It's been a thousand years since Nightmare was defeated by Twilight. Nightmare is back, but hasn't embodied Princess Luna. This time, she has her own body. And she rules as a third princess along with Luna and Celestia. And she's fallen in love with the leader of her own group of Pegasai, the Shadowbolts.

The 'Sad' tag might seem bad for the story right now, but it gets sad later on, trust me.