This story takes place after the episode "Do princesses dream of magic sheep"

Big Macintosh is a simple earth pony who one day runs in the misfortune of learning he's the next in line to wielding the Amulet of Malum. With this artifact, he can transform into Princess Malum, a powerful alicorn who is burdened with the duty of protecting Equestria from Nightmare Moon and her minions. Being inexperienced and forced to share his body with the spirit of the former user of the amulet. Big Mac now has to learn how to use his new body to protect Equestria; while keeping his identity a secret and performing his duties of running Sweet Apple Acres. Luckily for him, he's not alone. Along with Twilight and her friends, Luna will help him deal with his everyday life and hero life. In the end, Big Mac will have to choose one or the other. But, before then, he is...Princess Malum!