For years, the creatures of Tartarus have been planning war on Equestria. They have planned, observed, scouted and made alliances. Little did the ponies know that the Changeling attack was merely the start of something much larger… The story is about the war between the Equestrian Royal Army and the Tartarus Empirical Brigade.
Also, there will be a minor season 3 spoiler, so if you don't want spoiled, then don't read it until the episode is released. It's nothing big though, most people already know it by now.