This Platinum Crown

After the time-looped events of The Best Night Ever, Rarity's relationship with Prince Blueblood seemed to be a fairy tale come true. Perhaps so, but it was a fairy tale soon cut short. Her dreams and future position quickly earn her the attention of powerful enemies. Only one mare can bear the Platinum Crown of the Duchy of Canterlot and be heiress to it's secrets and magics. The future of Equestria may well depend on that one mare being Rarity Unicorn.

Can be read here or here.

Tropes used in This Platinum Crown include:

Affably Evil: Lady Antimony is supremely agreeable and oftentimes downright amiable, at least when she doesn't have a metephorical (or not-so-metaphorical) dagger at your throat.
Arranged Marriage: Because Princess Celestia promised to reunite the Terre Rare and Blueblood family lines, Blueblood has suddenly become engaged to Lady Antimony. Antimony tries to soften the blow to Rarity's dreams of marrying Prince Blueblood by saying she'd make a fine First Mistress for the Prince.
Duel to the Death: When a duel between unicorns ends in a tie (with neither one being humiliated) and the two sides still cannot let go of their grievances, they can choose to duel à outrance (or in English, to the death).
Evil Cannot Comprehend Good: Though More like Ambition can not comprehend love, During their duel, Antimony can not grasp why Rarity would want to marry Blueblood for reasons other than power.
Humanity's Wake: Lyra came in to Twilight's Library with a torc that most definitely does not look like a pair of human hands, as humans are just fairy tales...with a disturbingly large amount of corroborating evidence.

Lyra:Now I'm not saying it was humans, but it was humans.

Master of Illusion: Antimony
The Mistress: Since there's such a high ratio of female to male ponies, it's apparently a quite acceptable practice for stallion to take one wife and have any number of mistresses. The children of the wife are the only ones allowed to inherit anything from said stallion, though.
Royals Who Actually Do Something: Blueblood's responsibility as Grand Veneur.
Sealed Evil in a Can: It's been hinted that there's some sort of evil unicorn who was impossible to kill after Princess Platinum gave her life sealing his magic. What the pre-classical ponies did to him after that would have qualified him for a Rasputinian Death, if he had actually, y'know, died. Eventually they just ended up giving him something that amounted to cement shoes and diverted a river onto him.
Took a Level in Badass: Rarity learning from Twilight the fine art of unicorn dueling.
Wizard Duel: Unicorn nobles have these when they have a dispute they cannot otherwise peacefully resolve. First a time, a place, and a type of weapon (a variety of baked good) are selected. Then, each unicorn attempts to cream the other with said baked good until either one hits the other, humiliating them, or each pony runs out of ammo. If both run out of cake, cupcake, pie, or what have you, then they can either decide to walk away amiably with their honor sufficiently defended, or go for a duel à outrance