The Elements of Harmony find themselves facing an old evil united with one from beyond their world entirely, bringing magic so alien that Equestrian magic cannot defend against it. Yet, long ago, when the Elements existed in a different form, they summoned aid from beyond the world to defeat a similar threat from beyond.
Out of desperation, today's Elements repeat that ancient summoning, and they get – a hillbilly with a silver-strung guitar?
MLP:FiM (c) Lauren Faust and Hasbro; John the Balladeer and Rowley Thorne (c) estate of Manly Wade Wellman
And due to the grace of several fans, primarily Jordan1179, this story now has its own Tropes page: Tv Tropes My Little Balladeer