Long ago, before her transformation into Nightmare Moon, Princess Luna struck a deal, a deal meant to save Equestria. A deal that she didn't share with even her beloved sister. A deal that was meant to save the nights she had worked so hard to create.

Princess Luna reached a compromise with a species of monster that had once stalked her nights: The Vampires. A promise that both sides kept for centuries...until one night, in Ponyville, it's shattered, and a pony is found dead.

At Twilight Sparkle's insistence, Luna brings her into the most closely kept secret of Equestria, and unwittingly, into the center of a conflict ready to boil over once more.