Nightmare Moon returned after a thousand years of exile, and the princess of day and night vied for supremacy in the skies above Canterlot. For three days they fought, and all the ground shook with the echoes of their mighty clash. The sight of their battle was like thunder and lightning blasting through the sky, rocking the very foundations of the earth. But on the morning of the fourth day, Celestia proved the stronger. Nightmare Moon was driven from the sky and banished to the Northern Wastelands.

But something happened which Celestia did not foresee... The Crystal Empire has returned, and Emperor Sombra is prepared to do battle against Equestria once again. But this time, he has a new ally, the most brilliant general in all the world. This time he has Nightmare Moon.