During her entrance exam, Twilight's powers goes far out of control, leaving Celestia a shell of herself and Equestria in ruins. Now Nightmare Moon is free, but even she can't stop the Tyrant Filly. She flees through time and space, finding herself in an Equestria familiar to us but odd to her. Can the Mane Six help her stop a Twilight out of control?

Fanfiction.net link here!
Weekly featured fic in Authors Helping Authors group

TV Tropes Page, with sincere thanks to DrTempo.

Cover art from here.

Luna is no longer Nightmare Moon, and has returned to rule Equestira once again, but not everypony is so ready to forgive. When a secret organization kidnaps the Elements of Harmony and threatens to murder Luna, Rainbow Dash must go back into the memories of her ancestor Firefly, to learn the ways of the Assassins to fight this new terror and save the new princess, as well as her friends.

Credit to he4rtofcourage for the title image.