Talk:My Little Pony: Friendship is Magic (comics)/@comment-26973802-20130611005027/@comment-26973802-20130618051304

As to the hints that Nightmare Moon is separate: In the comics, Nightmare Rarity speaks of Rarity as a separate pony, saying why she chose Rarity as her vessel, and talking directly to Rarity when the Mane 6 were fighting her off (something like "You will be remembered, Rarity"). And she hints at being the original Nightmare Moon by saying the Mane 6 had "defeated her once".

The Nightmare Forces thing is not elaborated on (but can't say that because of the in-universe thing, grumble grumble), but from what I'm guessing, it's an evil force that magnified or merged with Luna's bitterness and told her things (probably saying ponies hated her) to possess her as Nightmare Moon. Destroyed by the EoH, revived by the cycle of the new moon in a one-time thing, corrupted Larry, Jerome, and the others to do its bidding, possessed Rarity, and destroyed for good by the Mane 5 and Spike.