Long before Ponyville was founded, there lived a changeling who called himself Margil Apple. He wore a cutie mark of a heart, and most mares didn't figure out what it meant until he left them behind in search of someone new to love. He's long gone, stabbed by a spurned lover, but his legacy is alive and well--a fifth of the ponies in Ponyville have a blood connection to him.
Two months ago, a changeling posing as Rainbow Dash tried to abduct Twilight. Nopony was hurt, but Twilight became a bit paranoid--how could she make sure that wouldn't happen again? The best thing she could think of was to invent a spell, empowered with her full might as princess, that would forcibly remove the disguise of any changeling in the vicinity.
It's testing day, and Ponyville's about to get twenty percent buggier . . .
AU taking place between seasons 3 and 4. Trigger warning for one scene of implied dubcon.