In an Equestria where the Sonic Rainboom never happened, Rarity never truly discovered her talent for finding gems that should have led to her Cutie Mark... Instead, she simply left that giant rock to topple off the cliff face, unopened... While on her way back through the dangerous mountain pass, however, Rarity tripped and fell down a deep hole, landing on a mossy slide that took her straight into a pile of the most beautiful treasure she had ever laid eyes on! As she regained her composure, all she could do was marvel at the gold, silver, gems, and trinkets before her, not even noticing the three-diamond Cutie Mark that had appeared on her flank; unfortunately for her, they already belonged to someone... or something...
She had fallen into the hoard of a massive and hungry dragon, who was currently looking over its newest meal... Without warning, Rarity found herself scooped up onto the dragon's warm wet tongue, drawn into its gigantic maw, and swallowed in one powerful gulp... The dragon, satisfied with the delicious little filly, laid down on its pile of treasure to sleep, leaving Rarity to digest in its massive belly...
But, in what can only be described as a miracle of Celestia, Rarity survived!
Left to stagger home, covered in dragon filth, her fur and mane melted off in some places, Rarity spent the next day huddled and crying in her shower, utterly traumatized by the entire experience... After that, Rarity became cold and distant, shutting herself off from everypony, even her family... Nopony had been there to comfort her after what she had been through, so why should she be there for anypony else??? She would continue down this path of isolation for many years, until the coming of Nightmare Moon...
On the eve of the Summer Sun Celebration, Nightmare Moon broke free of her prison and sought to bathe Equestria in eternal night forever. The residents of Ponyville were fleeing and screaming in terror at the mere sight of this legendary monster, all except for one... Rarity stood in the center of the room, alone after all the other ponies had fled; just her, and the beautiful dark alicorn standing before her... As she looked into Nightmare Moon's eyes, she didn't feel an ounce of fear; on the contrary, she saw somepony who was just as lonely and isolated as her, and would allow her to express it in ways she could never imagine... She would continue to watch as Nightmare Moon loomed over her...
"You... Are you not afraid of me? Are you not aware of whose eternal shadow you stand in???"
"I am..."
"Then tell me, little mare... Why do you not tremble in fear before my hooves???"
"I do not fear beauty... I worship it..."
The alicorn goddess' eyes widened in shock at such a compliment, a hint of blush appearing on her face as the white mare kneeled down and began kissing her dark hooves.
"You are quite an... interesting mare... What is your name, devoted one?"
"Rarity. But you can call me what you wish... I am here to serve and obey you... my queen..."
"And quite a rarity you are indeed... Very well. You have intrigued me, Rarity... Not many ponies can say they have done that... I will definitely have plenty of uses for you..."
"Thank you... my queen..."
Things Worth Noting
--Rarity loves her queen deeply and would do anything to serve her.
--Rarity despises dragons and will rarely pay them any mind; usually when she's looking to use them as playthings.
--The only members of the Mane 6 Rarity knows in this timeline are Guard Captain Rainbow Dash and Grand Inquisitor Pinkamena Diane Pie.
--Rarity's sister Sweetie Belle has been given to Nightmare Moon as her special filly plaything.
--Rarity's cat Opalescence has been grown to a gigantic size by Nightmare Moon and now lives in a huge room in the dungeon, where she enjoys being fed useless prisoners and servants. She also loves to cuddle with her Mama Rara and Mama Moony.
--Princess Twilight Sparkle is #1 on Nightmare Moon's Most Wanted list. If she ever comes back to this timeline, she is to be apprehended and brought to Nightmare Moon immediately. Personally, Rarity would rather just feed the upstart to Opal and be done with her...
--Celestia was not banished to the moon in the sky... The rest is up to your imagination...
--Nightmare Moon and Rarity both have incredibly realistic-looking model cities in their bedchambers; Nightmare Moon has one of Manehattan, and Rarity has one of Ponyville. These cities are where they keep prisoners who have been shrunk by Her Majesty and sentenced to live in them. The day Moony gave Rarity her own model city, Manehattan and Ponyville had disappeared off the map...
--Rarity loves being petted, especially by her queen.