Note: As the Fan Fic this page is about is still in progress, this trope page is currently under construction. Anypony who has read this fic should feel free to add any tropes we seem to have missed.
Only one can wear the 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 its secrets and magics. The future of Equestria may well depend on that one mare being Rarity Unicorn.Can be read here or here.
Anti-Villain: Antimony may be a bitch, but she was legitimately interested in using her position as Baroness for the good of all, even if her family is brazenly corrupt. Ditto for the other mares vying for Rarity's position. They were played up to be villainous, as bad as Antimony herself, but they're all quite nice and share Antimony's goals towards helping their people.
A Party Also Known as an Orgy: Lady Euporie, as a titled hedonist, is renowned for throwing these. Chapter 26 sees Pinkie Pie attending one with her sort-of coltfriend, Pierce, though come the next chapter, she finds she doesn't quite like this sort of party. At least, not the way Euporie throws them.
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.
Baker Of Iron: Pinkie skills during the Black Wedding arc have consisted of weaponizing baker utensils and goods.
Battle Couple: Cruciger and Twinkling Star Light. Twilight Sparkle and Alpha Brass as well.
Big Bad Ensemble: After Antimony's defeat, Alpha Brass and Queen Chrysalis take over as the Big Bads. Though ostensibly working together it's no secret (at least to us) that they're both prepared to sell each other out at the drop of a hat.
Big Screwed-Up Family: Lord Cruciger and his children Polished Jewel, Chalice, Alpha Brass, and Antimony. Goes back to Arsenic, who was more than a little bit loony.
Black Magic: Quite a few spells are mentioned that are officially banned in the modern era. "Old Magic", in general, seems to be purposefully forgotten... and for a reason. Chalice, for example, uses some Old Magic spell to become "Sagittarius", a sort of giant pegasus archer made of living stars (similar to the Ursa Minor and Ursa Major) which has an ultra-lethal Fantastic Nuke arrow that never misses its target and returns upon striking.
Ritterkreuz: When I see a cloud, I want to fly up to it and kick it. When I see a wall, I want to break it down and see what’s on the other side. When I find a pony with a strong body and a strong will, I wanna fight them. Everypony and everything in this world is just another cloud or another wall to me.
Ritterkreuz doesn't call hers aloud, but being an ex-Wonderbolt all of her attacks are named like airshow stunts, primarily the Galloping Grenadier. Dash, of course, has her Sonic Rainboom.
Cargo Ship: Invoked and Played for Laughs. When discussing Twilight's engagement to Alpha Brass Pinkie comments that she always thought Twilight would bring a book to life and challenge Equestria's marriage laws regarding civil unions.
Cover Identity Anomaly: The Changeling that impersonates Blueblood doesn't know about the conversation that he and Twilight had about his family's archives. It's not enough to break his cover, but it does make her suspicious.
It quickly becomes apparent to Rarity that the Changeling is not Blueblood, as it lacks many of Blueblood's Character Tics. What ultimately proves to be its undoing is lack of knowledge of what Blueblood first said to Rarity on the morning after the Gala: "You're still here."
Culture Clash: Between the Unicorn nobility and commoners. The most obvious is Rarity's insistence on becoming Blueblood's wife, rather than mistress, but it's hardly the only one.
Drill Sergeant Nasty: Raging Storm, Vice-Captain of the Wonderbolts. She's serving with an active unit, but the constant abuse she showers on her subordinates places her closer to this than Sergeant Rock. Per chapter 30, this is why Spitfire was made Captain instead of her.
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).
Ritterkreuz expects that this is how her fight with Rainbow Dash will end. It doesn't.
Evil Can Not 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.
Evil Power Vacuum: Sort of. Not really evil per se, but when Rarity defeats Antimony, this allows all of the other mares that Antimony defeated previously for the right to marry Blueblood to remake their claim by challenging Rarity.
Faux Affably Evil: Alpha Brass is extremely suave and likeable, able to easily manipulate those around him, but he is seriously screwed up. Even Queen Chrysalis finds him disturbing and crazy, noting he's so messed up his emotions are completely inedible.
Author: The tricky prospect with Brass is that his weapon is his charm and the fact that he is, or appears to be, likeable and reasonable. His strength is literally in the ponies he surrounds himself with.
Foil: Quite common. The honorable Shining Armor is a foil to the charming but a bit unscrupulous Prince Blue Blood. Rarity, who wants to marry Prince Blueblood for love, is the foil of Lady Antimony is trying the same, but solely to acquire power. Pinkie Pie is the foil of Lady Euporie; both like good jokes and fun, but Euporie's frequently delve into cruelty and depravity.
Go Mad from the Isolation: Antimony mentions having done this to others before. When she tries it on Rarity, Rarity turns it around on her by pointing out that Antimony had created an entire illusionary copy of Equestria completely empty of any other pony, where Antimony herself poses as an Alicorn.
Heart Is an Awesome Power: It's implied that the Mane Six's levels in badass are a result of their connection to the Elements of Harmony; more directly, the Magic of Friendship is what allowed Rarity to defeat Antimony's illusory world.
Rarity's time as a seamstress has given her an eye for detail, skill at illusions (to aid in the presentation of her products), and helped her develop her telekinesis to very precisely manipulate many objects at once. It turns out that these are all very useful skills outside her boutique, too.
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 just maybe it was humans.
I Am Not Left-Handed: Chapter 30 displays that Ritterkreuz was using only one wing for her Galloping Grenadier the whole time.
Kill 'em All: Alpha Brass's main scheme is to exterminate the Changelings forever. The invasion of Canterlot? It's a trap.
Lethal Joke Item: Unicorn duels are fought with pastry in lieu of actual weapons. Sounds crazy at first, but they can use their magic to modify the pastries, effectively turning them into mind-controlled cannonballs smeared with paralysing venom (a numbing agent is an official ingredient in duelling pastries, to ensure that a hit duellist can't feign that they weren't).
Mighty Glacier: Shigure's primary fighting style is to just sit there and tank the shit out anything his opponent throws at him, including (at least in Pinkie's Imagine Spot) a Giant Mecha. He meets his match against Chalice's alternate form, "Sagittarius", though.
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.
Apparently, this tends to differ in parts of Equestria; its mentioned that Southern Equestria, such as where Sand Dune comes from, illegitimate foals are legally just as valid as legitimate foals. This is one of the reasons why she's willing to form an alliance with Rarity, without striving to be Blueblood's mistress, if Rarity will let her conceive a foal by Blueblood.
Mythology Gag: Twilight's freaking out about being sent back to "Magic Kindergarten" in "Lesson Zero" is explained here as a reference to one of Celestia's kookier training ideas; Twilight was wrapped with the illusion of being a kindergarten-aged filly and sent to a unicorn kindergarten, where she had to sustain the illusion perfectly over the course of the day or else be humiliated when it broke. This was Celestia's way of helping Twilight hone her stamina.
Twilight then does something similar to help Rarity train for her duel with Antimony, though in Rarity's case she just has to blend into Cheerilee's school.
Never Gets Drunk: Cruciger, despite spending an entire night drinking with Pinkie Pie, remains totally lucid throughout. He claims to be extremely hung-over but shows no sign of it.
OOC Is Serious Business: Twilight, despite having her copy of Dueling for Dummies on hand, prefers to use a demonstration to show just what Rarity has gotten herself into.
Power Incontinence: Antimony seems to keep her eyelids at half mast because people who meet her gaze get caught in her illusions.
The Power of Friendship: It's implied that the Mane Six's levels in badass are a result of their connection to the Elements of Harmony; more directly, the Magic of Friendship is what allowed Rarity to defeat Antimony's illusory world.
The Power of Love: An effect of Cadance's magic is that she can increase the powers of a pony that she loves, and who loves her in return.
Proud Warrior Race Guy: The Griffins are presented as this, though they don't have much active involvement in the plot.
Red Baron: Cruciger is also known as the Black Duke.
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.
Something Else Also Rises: Scootaloo has trouble folding her wings after reading some of Rarity's more adult-rated love letters.
Squick: In-Universe. Neither Rarity nor Sand Dune takes it at all well when they confirm that Blueblood has been replaced by a Changeling in chapter 34.
Space Is Magic - Taken quite literally. The medium of outer space is essentially condensed magic.
Taking the Bullet: Or in this case, the arrow. Shigure throws himself in front of an arrow from "Sagittarius" to save Lady Yumi, which sees the previously Nigh Invulnerable and immovable Earth Pony blasted away into a hillside... which is then the site of a Fantastic Nuke as the arrow explodes inside of him.
Throwing Down the Gauntlet: Sure, there's the obvious one when Rarity challenged Antimony to a duel, but for sheer gumption it doesn't come close to looking into Antimony's eyes during the duel, after Rarity knows what that means.
Time Master: Lady Sand Dune and her clan have access to time magic.
Time Stands Still: She is said to be able to stop time over quite a large area for a very long time, which she once used to stop a fire from spreading until the ponies inside a burning building could be rescued.
Save Scumming: Using time magic, she creates an anchor point that she can then travel back to, having experienced the intervening future and going back to the anchor.
Trickster Mentor: Rarity is initially excited to be trained like Celestia trained Twilight. But the Princess operated in the fine tradition of Zen masters everywhere, either by giving Twilight impossible tasks for the day so she'd come up with ways to succeed, or by throwing things at her when learning to teleport. Twi pays the confusion forward to Rarity.
"Well Done, Son" Guy: Antimony's major reason for making her claim. She's very put out when her father, Lord Cruciger stops observing her duel with Rarity. May or may not be tied to Alpha Brass' motivations as well.
What the Hell, Hero?: In Chapter 38, Sweetie Bell calls out Cadance for arranging for Blueblood to be captured by the Changelings (long story).
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 and treated with a contact sedative. 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.
Xanatos Speed Chess: Alpha Brass has a general outline for what he wants to accomplish, but not anything set in stone.