Nightmare Rarity

Nightmare Rarity is the main antagonist of the second arc (sometimes called the "Nightmare Arc") of the My Little Pony: Friendship Is Magic comic series. She is the evil form of Rarity after she is taken over by the Nightmare Forces and a member of the Nightmare Mane Six and one of the main antagonists of Rise of the Nightmare Mane Six.

Appearance
Nightmare Rarity is significantly taller than regular Rarity. Her coat is dark purple and her Cutie Mark is a modified version of Rarity's cutie mark, featuring stars and glimmer. Just like Rarity, her mane is groomed in an elegant hairdo, but hers is much longer, multicolored and somewhat etherealized, star-like glitters can be seen within her mane. Her eyes are a pale cold blue, with vertical pupils, and diamond-like glimmers can be seen within.

She sometimes wears crowns, usually incorporating crescent moons and diamond symbols. During her invasion of Ponyville, she wears a golden and very tacky version of Nightmare Moon's armor, featuring a high collar and engraved with a lot of jewelry.

Personality
Nightmare Rarity's personality is very reminiscent of Nightmare Moon's. Very little is left of Rarity, though she keeps her love of fashion (as can be seen with her battle armor) and is very vain. Nightmare Rarity is a villain through and through, cruel and sadistic, delighting in bringing enemies down and humiliating them.

While she can be manipulative, it is mostly only to toy with enemies - primarily, she relies on her army of Nightmare Forces to get her way. She does not often engage in battle personally, though when she does the results are usually devastating for her enemies.

Powers and abilities
Nightmare Rarity wields a great amount of dark magic. Offensively, she is so strong that a single blast from her was enough to take down Princess Luna (although it should be noted that Luna was at that point holding herself back out of fear of becoming Nightmare Moon again).

Nightmare Rarity, true to her name, has the ability to manipulate the dreams and nightmares of others, creating vivid illusions. She was almost able to trick Spike by posing as regular Rarity and offering him a place by her side as King of the Nightmares.

In the Arc
Early in the arc, each member of the Mane Six is disturbed by a particular nightmare. Rarity's hits her particularly hard when she witnesses everyone else admiring a far more generous pony while leaving her behind. Rarity is abducted in the process and carried off to the moon where she is deemed the perfect host. She resists at first, but eventually relents due to the creatures exposing her fear once again. In her weakened state, she is transformed into Nightmare Rarity.

After the remaining Mane Six, Spike, and Luna arrive on the moon, they are accosted by Nightmare Rarity and the shadows. Nightmare Rarity shrugs off their attempts at bringing Rarity back and has them detained after the Mane Five convince Luna to return to Equestria to aid in its defense. Nightmare Rarity appears once more in an illusion as "Queen Rarity", making it appear as though she had returned to normal. Spike, who was previously knocked out, regains consciousness and sneaks in to rescue Twilight and the others. He crosses paths with "Queen Rarity" and is almost fooled by his love for her. However, when she fails to recognize the special gem Spike carried for Rarity, the little dragon exposes her illusion. Nightmare Rarity nevertheless taunts him with the allegation that Rarity is gone.

Princess Luna returns to Ponyville to inform her sister of the ponies' capture and of the Nightmare Forces' impending attack. Celestia says they'll be ready, and Luna is resolved to help. In the dungeon, Rainbow Dash throws a fit over her and her friends' situation, annoying Applejack and spurring an argument. When Fluttershy reminds them of what good friends they all are, a warm glow envelops their bodies; Twilight thinks the glow is their friendship lighting them up from within.

On the castle balcony, Shadowfright continues to taunt the restrained Spike, telling him that Rarity's transformation will soon be permanent and breaking his fire ruby into pieces. As Nightmare Rarity, now clad in gold jewel-studded armor, and the Nightmare Forces commence the attack on Ponyville, Spike, using the camouflage slugs from before, manages to sneak away. He gathers up the pieces of his fire ruby and goes to free Twilight and her friends from confinement. Guided by the light of the ponies' friendship, he sneaks past Shadowfright's sidekick and into the dungeon. Spike tries in vain to set the ponies free from their cages; luckily, Pinkie Pie has the sidekick's key, which had fallen out of his pocket earlier and she'd forgotten about afterward. When the sidekick catches wind of the group's escape, he blocks the exit and prevents them from leaving. However, it's clear to the ponies that the rest of the Nightmare Forces don't respect him—Shadowfright doesn't even know his name. Nonetheless, he refuses to let the ponies leave.

Meanwhile, in Ponyville, Rarity and the Nightmare Forces begin their attack, and the village turns into a warzone. Princess Luna faces off against Nightmare Rarity, and an exploitation of Luna's weakness gives Rarity the upper hand. However, her efforts to turn the other ponies against Luna are futile; in spite of her past as Nightmare Moon, Celestia and the citizens of Ponyville accept Luna as one of them. With renewed confidence, Luna resolves to protect Ponyville and defeat Nightmare Rarity, and her mane becomes a rippling, translucent blue.

As Nightmare Rarity spits at the sentimentality, Shadowfright's sidekick, apparently named Jerome and now no longer smoky in appearance, arrives with Spike and the other Elements of Harmony. Princess Luna, channeling the Elements' magic, purges the moon creatures of the nightmare energy as well, including Shadowfright. Luna and the Elements then shower the still defiant Nightmare Rarity in magical light. The nightmare energy's grip on the real Rarity is momentarily loosed, but due to Rarity's lack of certainty in herself, the energy's too powerful to destroy. Twilight rallies her friends to remember the things about Rarity that they love, to show how special she is to them and how she'll never be forgotten. The power of the love they hold for their friend restores Spike's shattered fire ruby and banishes the nightmare energy from Rarity's body completely. With the Nightmare Forces' defeat, Rarity tearfully reunites with her friends and family.

Later that night, Princess Luna goes to escort the moon creatures back home, promising to visit Twilight and her friends soon. Fluttershy leaves the rabbit she befriended in the dungeon in Jerome and Larry's care. Luna bids goodnight to her friends and thanks them for freeing her of her nightmare.

The story ends with Twilight, Spike, their friends, and Princess Celestia looking fondly at the full moon.

Trivia

 * Nightmare Rarity makes an appearance in one of Darkness' illusions to try distract Courage would break him and turned into madness.