One Thousand years ago, the saints Celestia and Luna drew upon the power of the Red Stone of Aja to defeat the demon Discord. Now one thousand years later the Stone is destroyed, beginning the tale of six ponies and their quest to redeem the world.
When the Mane Six get fragged, they try to carry on as if nothing happened. The living, however, don't appreciate that. Princess Celestia's solution is not at all what the Mane Six expected.
Luna has tried to prevent herself from becoming Nightmare Moon, but she has failed. This is her last action as Princess Luna, her last action before the Nightmare comes to life.