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!)

Princess Celestia tires of constantly being surrounded by decorum, deference, and formality, and decides to take a day off from being Princess. But visiting Ponyville incognito is harder than she expects. Will she be able to fit in and make friends without blowing her cover?

Rainbow Dash struggles to figure out why Pinkie Pie keeps acting weird, and seems so happy about spending Hearts and Hooves with hanging out with her, while also puzzling over the fact that Cloud Kicker and Blossomforth seem to be going out of their way to help the two of them enjoy the day.

Cloud Kicker and Blossomforth need to have their own tags, because they're awesome characters who deserve to have more stories written about them.

Has a semi-sequel-follow-up-side-story:"The Life and Times of a Winning Pony."

Suddenly finding herself in hell, Twilight is presented with one fact: for the rest of eternity, she's stuck in a library holding every book possible. She's expected to organize them and is allowed to read them.

What makes this hell, again?

Now has two Spanish translations, one by SPANIARD KIWI and another by Pugg-Senpai.

Trixie's line of work is dangerous. But she's a careful showpony, and that's why she's only died a few dozen times in her career.

She thinks this is normal.

Luna suspected something was troubling Twilight, but she didn't realize just how deep the hurt went...or how close she was to losing her.

Art by FidzFox

Featured from 4/5/2021 to 4/8/2021, peaking at #1. Thanks, everypony!

An audio reading of this story by Rainbow Infinity is available at this link.

Celestia, the Lightbringer.
Luna, Mistress of the Night.
Twilight Sparkle, Master of Magic.
Cadance, Princess of Love.

Of the Four Princesses, Cadance is the one that ponies think they understand the most. After all, what could be so hard about being Princess of Love?

A lot of things, actually.

The first story in the Four Princesses, Four Demons series. Sequels are Celestia the Indomitable, Twilight the Triumphant, and Luna the Faithful.

Featured from 1-26-21 to 1-29-21, all thanks to the kind folks of this site, who even pushed it to the #2 spot.

This story is a sequel to Melt

This is a sequel to Melt, but can be read independently.

The common cold is, for Celestia, quite rare. Whenever she does catch one it's a time for privacy, reflection, and no shortage of small miseries.

That said, she's never had Twilight Sparkle on her side before. Maybe, just maybe, this cold won't know what hit it.

My thanks go to CosmicAfro for prereading

Now with even more perfect cover art!

Twilight's caught a special type of flu, and it's up to none other than Princess Celestia to nurse her through it. A special, tender little moment follows in this most unexpected of circumstances.

A machine made for war awakes in a new world. Stuck in surroundings which puzzle it; can it, or will it be able to become something more than what it is?

Crossover with Keith Laumers Bolo-verse
Other tags include : Slice of Life, Possible Romance believe it or not! And a bit of humor.
Please beware of any spoilers in comments as well