Nightmare Rarity

"It's true, your powers defeated me before, little ponies, but what will you do now that i've taken one of you? Without all of you, your Elements of Harmony are powerless."

- Nightmare Rarity telling Twilight Sparkle and the rest of her friends that they can't use the Elements of Harmony because Rarity became corrupt by the Nightmare Forces. 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.

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

 * The comics imply Nightmare Moon to not be Princess Luna or the embodiment of her jealousy, but a completely separate nightmare entity from the moon who manipulated her into letting her possess her (as she is shown to do to Rarity). However, the episode Princess Twilight Sparkle shows Luna's transformation to occur a different way, as she is still Luna when she moves against Celestia and becomes Nightmare Moon only when she does.