Nightmare Rarity... scion of darkness and once the Element of Generosity had been defeated thanks to the efforts of the rest of the Mane 6.
This happened three months ago and Rarity still can not get sleep. She falls asleep everywhere, her work has suffered a similar blow and she gets small thoughts that she knows are not her own.
Finally, after a shocking encounter, she contacts Princess Luna. She believed the former Nightmare Moon would be able to help her plight but she discovers far more about the entity known as the Nightmare.
Note: This takes place after the IDW comic in question. There are discrepancies between the events in the fic and the show since it seems to have taken place between seasons 1 and 2. Luna doesn't use Olde Equestrian which is one such discrepancy. The need to read the IDW comic is not required as the gist will be explained and as a result, it will contain spoilers.