Nightmare Rarity is the main antagonist of the second arc (sometimes called the "Nightmare Arc") of the My Little Pony: Friendship Is Magic comic book series. She was the dark form of Rarity after she was corrupted by the Nightmare Forces into becoming the new host for Nightmare Moon, transforming her into their leader.
In Issue 6, Nightmare Moon is depicted as a separate entity from Luna who originated from the moon (as evidenced by her speaking to Rarity while possessing her and speaking as if she were the original Nightmare Moon). It is said she convinced Luna to let her possess her and that she promised the Nightmare Forces a kingdom of their own. When the Mane 6 restored Luna, Nightmare died, but is revived through the cycle of the new moon, and corrupts the inhabitants of the moon into new Nightmare Forces. Nightmare herself convinces Rarity to let her possess her, becoming Nightmare Rarity.
In Issue 7, Nightmare Rarity imprisons the ponies in her dungeon once the Nightmare Forces defeat them and resists Spike's attempts to get through to her. She tries to trick him with an illusion but he realizes her deception. She has Shadowfright and his sidekick capture him and says the nightmare has only begun.
In Issue 8, the ponies escape the dungeon and de-corrupt Shadowfright's sidekick, Jerome, with the power of friendship. Luna does the same to Shadowfright and his army, turning them back into the denizens of the moon. The ponies then use this power of friendship on Nightmare Rarity to weaken her and, with them thinking of memories of Rarity and Spike's love for her along with his restored Fire Ruby, the Nightmare Energy is destroyed once and for all which restores Rarity to her normal self.