"Though my soul may set in darkness, it will rise in perfect light, i have loved the stars too fondly to be fearful of the night"
The story sets when Tirek striked again stealing all the magic he could find. The princesses were worried, what they could do, there wasn't an ally with the level of magic to face both Tirek and Discord…except from one, but it was too dangerous.
In an act of sacrifice, Princess Luna put to sleep Princess Celestia, and sent her dear sister away along with Princess Cadence and her husband. She summoned Nightmare Moon again, and offered her body in exchange of Nightmare Moon defeating Tirek, Luna's soul separated from her body, and Nightmare Moon was born once again.
Nightmare Moon instead, offered Tirek and Discord to rule at her side, and they agreed, the other villains were summoned too, and this is how began the darkest era of Equestria.
The mane 6 were captured and were imprisoned in the Canterlot castle dungeons, they could not access the elements. The villains made an arrangement, the mane 6 could not longer be together, at least not all of them. Pinkie and Fluttershy would be sent to the Crystal Empire with Discord and Sombra, Applejack and Rainbow Dash would be sent with Chrysalis to the everfree forest (Starlight joins in as the "guardian" of Applejack later), and Rarity and Twilight would stay in Canterlot with Tirek and Nightmare Moon.
Nightmare Moon made an arrangement with Twilight, Nightmare wanted her powers, she admired the amount that she had when she defeated her, and well, it was rumored that the original posesor of her body and the purple alicorn had a relationship, so she figured this could be something she wanted. Twilight accepted to marry her, but with the condition that she became less violent with her subjects, and that her friends would be safe.
This is their wedding, with a heartbroken Twilight and a triumphant Nightmare Moon.
I got a creative rush and wanted to draw pretty dresses qUq so…