Twilight Sparkle, Princess Celestia's most prized and faithful student, is being sent to Ponyville to learn about the magic of friendship.

This is going to be fun!

A spinoff of Letters From A Disgruntled Friendship Student by milesprower06.

"... at the forefront of our fandom’s writing scene ... Masterpiece." —PCaRG
"... really enjoyable ..." —City of Doors
"...worthy of high praise." —PaulAsaran
"Lots of wonderful world-building, great action, well-developed characters and an engaging story." —Hoopy McGee

Steel Song is a lot of things. Earth pony. Uncle. Professional bodyguard.

Retired.

So when he receives a mysterious package from Princess Luna a few weeks after the changeling invasion, he's understandably apprehensive. More-so when he presents himself before the Royal Diarchs to find that not only do they desire him to come out of retirement, but to take command of a most unusual position...

A position that may have more significance than any of them suspect. Because strange thefts are occurring on the Equestrian Railway, thefts that nopony can explain. Thefts that may have far more sinister intentions than their seemingly innocent nature may convey.

Intentions sinister enough to shift the balance of power in Equestria forever.

Book I of The Dusk Guard Saga.
Epic Fantasy
Has a collection of Side Stories as well, the first of which is here.
Now with a TV Tropes page!
"This is 100% Approved by Twilight's Library!"
Added to Twilight's Library 11/26/2013
Added to Canterlot's Finest 12/01/2013

This story is a sequel to Celestia

Villains rarely appear out of thin air. They start as most creatures do, but fall victim to fate, choice, manipulation... or love. Princess Luna's descent into the Nightmare is something that has been long forgotten by modern ponies, but Luna will always remember.

(Sequel to Celestia, and a part of the past that she would rather keep hidden. Reading Celestia is strongly recommended!)
(Many thanks to my prereaders Noble Cause, Metallusionsismagic, and BradTheBrony!)
(Character tags will be added as the story develops!)
(Coverart is a placeholder for the moment, until I get something better!)
(BEWARE POTENTIAL SPOILERS IN COMMENTS!)

Isolated from her friends and taken prisoner in the middle of a war, Twilight must deal with the very real dangers of being percieved as an enemy as well as the nightmares of her arrival on 'Earth'. Can she overcome her own fears and the fears of her captors? Will the wayward unicorn's assistance be a boon or a curse to the 'humans?' Most importantly, will she ever find her way home?

"In short, it's a story of friendship and plasma rifles, with the emphasis on the friendship. The characters are well written, the XCOM setting is fleshed out very well, and the story is compelling. If you love XCOM, you'll love the story, but even if you've never heard of it, it's more than worth giving the first 3 chapters a read.

There's excellent action later on, but the real strength lies in the collision between the friendly and naive world of Twilight Sparkle and the desperate world of humans fighting a losing war for survival and willing to do anything to turn the tide."

--Arzoo

(XCOM/FiM crossover)
(Cover art commissioned by a friend and created by the magnificent FoxInShadow!)
(Original cover art by the amazing Carnelian can be viewed here!)
(Gore tag for blood and some violence)
(Many thanks to the studius OminousTen, Arzoo, xXFluttershyForeverXx and finally Setokaiva for giving my rambles a once-over)
(Now with it's own TV Tropes page! Get to tropin' folks!)

This story is a sequel to Stardust

Everyone knows the story of Hearths Warming Eve. They know the tales of the three pony tribes and how their strife brought the Windigoes upon them, and how the Fires of Friendship saved them from their frozen fates. For countless generations the tale has been told, but how has it changed over those years?

Or is any of it true in the first place?

Only one pony knows, and Celestia would prefer that the past stay forgotten.

(Based on a 'What If' line of thought that was born from the Stardust storyline. While some familiarity with Stardust and Mente Materia doesn't hurt, this story can be enjoyed on its own)
(Coverart available soon from the amazing FoxInShadow!)
(Many thanks to Arzoo, BradTheBrony, Vandenbz, and Setokaiva for prereading!)
(Complete with it's own TVTropes page as part of the Stardust continuity)
(Character tags will be added as chapters are added!)
(Possible spoilers in the comments! BEWARE!)

This story is a sequel to Stardust

When Equestria’s newest member of royalty asks for aid to be offered to her friends from abroad, the best and the brightest volunteer to serve. Captain Steel Song of the Dusk Guard are immediately mobilized to help XCOM defend their world from the enemies that besiege them. However, their first joint operation with the humans takes a turn for the worse and they quickly find themselves fighting for their lives against the full force of the monsters that threaten Earth.

(Stardust/Dusk Guard crossover)
(Coverart provided by a friend of Viking ZX!)
(While familiarity with both stories isn’t required, it is recommended)
(Many thanks to Arzoo for pre-reading!)
(All screenshots can be found at XCOM.wikia.com)
(Featured on 3/13/2013. Thanks everyone!)
(Now has it's own section on TVTropes as part of the Stardust series!)

This story is a sequel to Stardust

Times are changing for Equestria as Twilight’s return brings with it news of people and cultures from beyond the peaceful world everypony knows. Aid and volunteers are freely offered to help the besieged humans of Earth because every Equestrian knows that’s what good friends do. Unfortunately, making friends can also have the unintended consequence of making enemies.

When the monsters besieging the humans strike at Equestria with a level of brutality not seen in over a thousand years, the cry for help is sent to all the kingdoms of the world. Honored allies step forth to fulfill ancient treaties and oaths. Enemies, old and new, bury their grievances. An alliance never before seen in Equestria will be created to combat the monsters from the void.

And because of these events, a grim but welcome sibling to the XCOM project is born.

(XCOM/FiM Crossover)
(Sequel to Stardust, which I recommend reading before starting this. Fortitude Amicitia and Broken aren't required reading but are supplemental to the storyline)
(Gore tag for blood and violence)
(Alternate Universe tag because of the divergence from canon that Stardust has caused)
(Uses elements of Season 4)
(Coverart produced by the amazing Carnelian!)
(Many thanks to Arzoo, BradTheBrony, Vandenbz and Setokaiva for prereading!)
(As of 03/07/2016, the prereading team consists of Arzoo, BradTheBrony, Noble Cause and Metallusionsismagic!)
(Complete with it's own TVTropes page as part of the Stardust continuity. Could use some updating!)
(Character tags will be added as chapters are added!)
(Featured 5/12/2014! Thanks folks!)
(Possible spoilers in the comments! BEWARE!)

This story is a sequel to The Best Night Ever

Final Arc - The Black Wedding
CH63 - If I Had a Heart
- The ponies of the Fourth Tribe are ascendant. Shining Armor hunts down his changeling "daughters," and even Lady Antimony won't get in his way. Meanwhile, high above the burning city of Canterlot, a long awaited reunion occurs, and Twilight confronts Alpha Brass and makes a final decision.

--- --- ---

It began with a mare from Ponyville and a time-looping Prince from Canterlot. This most unlikely union now stands at a crossroads: a ruthless changeling invasion; a conspiracy within the highest circles of the Equestrian aristocracy; a half dozen power hungry noblemares looking to become a Princess in all but name. With the prize of Canterlot and its Platinum Crown at stake, it falls to six Elements of Harmony to once again save not just Ponyville, but all of Equestria.

---
---
TVtropes page is here: link

The prequel to TPC, "The Best Night Ever" is here. link
The side-story crossover with "Sweetie Belle Chronicles" is here. link

Someone out there made a playlist for Platinum Crown: Rarity, Blueblood and Chrysalis. Pretty sweet!
Wish I knew who, or even when they did it, though. I especially like the Chrysy playlist.
https://8tracks.com/explore/this_platinum_crown/hot/1

I've gotten some cool TPC related art done, too.
So Sweet and Tasty's TPC Cover Art
link
Thanks to Madmax for these two:
Rarity, Blueblood and Antimony
link
Dash and Ritterkreuz
link
The Terre Rare Founder - Lady Arsenic
link
Twilight and Alpha Brass Battle Duet
link