The empire from the unknown universe strike Equestria with the unknown reason, their leader, queen Morte was responsible for this war. Princess Luna believe the queen's primary objective is to remove Equestria from the earth. Even now, the conflict between ponies and changelings are strong. But, when the both sides are fighting each other, queen Morte attack the both side with no mercy. that makes the both side so angry to the queen. So, they work together to take out the empire and stop the invasion, fight back is the only way to stop it.