War, another aspect that Equestria is not used too. Six years after the changeling invasion, Queen Chrysalis had started an all out war with Equestria, and with the Equestrian military not ready for invasions in other areas besides Canterlot, there numbers dwindle. With no other option, Celestia summons who she hopes is the best war commander in the universe, and she ends up... with a roadie.
My own spin on a Brutal legend Crossover.
Big thanks to DoctorDerpy on Reddit for the new cover image