It begins hundreds of years before the current episodes, during the imprisionment of Princess Luna. At the time the crystal empire is currently still in existence and King Sombra is preparing for war against Celestia. Later on in the war all the magic used in battle causes Lord Tirek to escape Tartarus and soon the war becomes even more intense. The ponies soon realize its beginning to become a hopeless fight except for a small group, the resistance who track down the elements of harmony and have to smuggle them to Tartarus, the only place where Lord Tirek doesn't keep an eye on (because he would never want to go back) plus all of the ancient evil creatures have been released causing the war to have another side. (I know facts may be off but thats so I could make it fit into my story)