Starlight and Twilight drink a potion which reveals the downward spiral one of Starlight's alternate universes took. Meanwhile, Twilight watches Starlight take a downward spiral of her own.
They say Luna turned into Nightmare Moon soon after she and her sister had taken down the Tyrant King Sombra. What they don't know is that it was more than just a coincidence. He was once the pillar that held her up. She was too much for one to bare.
After the Crystal Empire is freed by Twilight and her friends, Luna realizes she must say a final goodbye to someone she thought had died long ago. Deep in the caves beyond the Empire, he waits for her.
A few years after Luna's return it seems that Equestria will finally know an era of peace and appears to be on the verge of a new renaissance. Ponies are happy. Luna is recovering. Celestia is miserable.