Nightmare Moon is not just some evil version of the night princess, she is a culmination of malice, darkness, jealousy and pure unfettered burning HATRED for her sister. So why does she feel so alone?
Starlight won. She prevented the sonic rainboom. She stopped the mane six from getting their cutie marks. Unfortunately she did so at the cost of her own life. Twilight managed to tell Celestia and a younger Sunset everything before fading away. AU.
When Celestia banished Nightmare Moon, what if she had to pay a price? Nothing so easy as death, no. Just something that carried her down into depths far deeper than Luna ever went.