War. War is a powerful word, and a dangerous thing. When all else fails, and we are not able to reach the standards we desire, the things we want, feel that should be ours some turn to war. And while Equestria itself does not face war, an alternate world does.
King Sombra's return, was more detrimental than any could imagine. Controlling ponies was a catalyst that brought about Sombra taking over a large section of Equestria, but controlling other species was what started a grand battle on Equus of which the likes Princess Celestia never imagined could ever happen. And right now, the moment to end that war, lies on the shoulders of a pegasus, a broken pegasus. General Rainbow Dash of The Resistance.
Rainbow Dash has lost a lot, her dignity as a pegasus, many of her friends to the war and Sombra's antics. But Sombra is strong, he is powerful, he is confident. He has made calculations and has prepared for this very moment.
What he didn't prepare for, was a certain Stormtrooper to join the battle and aid Rainbow Dash in the fight, that would ultimately decide the world's fate.