An MLP/Warhammer 40K Crossover

The Iron Warriors 38th Company has landed in Equestria, entrenching themselves like a barbed hook. They hunt the aliens of the Tau Empire, intent on destruction and pillage.
A small squad is dispatched on a reconnaissance mission, only to find that there is intelligent life on this world besides the feuding space-farers.
Will these improbably adorable and friendly equine aliens turn the damned warriors of Chaos from their blood-soaked path, cleansing them of their dark legacy with the magic of friendship and harmony?

No.
No, that isn't happening.
EXPLOSIONS!!

(Dark humor warning; limited grimdark themes)
(Cover art by Nicholas Kay)
(This series now has its own TVtropes page!)
(This story is a contestant in the Tournament of Canterlot, a charity event to raise money for Syrian refugees! Go here for details and donations!)
(Featured on Equestria Daily 8/20/15 )

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!)

Ancient Greece: A land of gods, heroes, monsters... and very few roles for women beyond wife and housekeeper.

In other words, it's a pretty terrible place and time for a renegade student of Princess Celestia to find herself. But Sunset Shimmer is nothing if not determined and resourceful. She will bend this world to her will.

That's the plan, anyway. Given how the heliocentric model is still a few millennia away from wide acceptance, maybe she can settle for finding happiness.

Winner of the Sunset x Villain Period Piece contest.

This story is a sequel to Crystal's Wishes

Love had finally happened to her. Crystal Wishes married the stallion of her dreams and couldn't be happier with her life... until she discovers why ponies treat military spouses with so much respect. Life after the happily ever after isn't as easy as she hoped it would be when married to a royal guard.

This story is a sequel to Memoirs of a Royal Guard

From the outside looking in, life has never been better for Silent Knight. He is the first pony in a long line of Royal Guard ancestry to earn a commission, he has a loving mare by his side, and he bears the kingdom’s highest honor for bravely risking it all to protect Princess Luna. The future shines bright with potential, but the shadows of the past lurk at every twist and turn that lies ahead.

Guilt, anger, and regret are constantly at war in the depths of Silent Knight’s mind. Despite his best efforts to enjoy the life he has, he finds himself trapped in the past, reliving the moment when his unit was ambushed and his ponies were killed. He refuses to show any weakness, instead hiding everything behind a tough face to protect himself and those he loves. But his true feelings—and what he plans to do with them—shackle everything he holds dear with chains forged in secret.

Secrets of a Royal Guard is a comedy series with romantic, dramatic, and adventure elements. It features an original character cast and their interactions with the princesses they protect. It is recommended that readers be familiar with Memoirs of a Royal Guard.

Quill & Blade Universe

Everypony has a story. Some tell of heroic feats and grand adventure. Others tell of villainy against the forces of Harmony. The story of an average guard, however, is one of obscurity. They live in the background, unnoticed but ever-present.

Silent Knight assumed his tale would be like that of his father's: join the Royal Guard and serve Equestria. He would be just another helmet and spear, part of the rank and file. And that was all he ever wanted.

However, when Captain Shining Armor asked for volunteers to form a House Guard for the recently returned Princess Luna, it was Silent's hooves that stepped forwards to set his new life in motion.

Velvet Step remembers her grandfather.

But her grandfather seems to be having trouble remembering her.

This story is a sequel to Secrets of a Royal Guard

Duty, honor, and loyalty above all else. That was what Silent Knight had been told ever since he was a colt. They had guided him all his life and he was more than ready to follow in his father’s hoofsteps by dedicating himself to the Royal Guard. Then he discovered something else he had never expected: friendship and love. He learned that he owed these values not just to his job, but to his wife, his friends, and his family. Unfortunately, he learned that too late.

War has broken out between the gryphon kingdoms and Equestria is stuck in the thick of it trying to protect their ally, Nordanver. As Silent marches off to war, he must contend with the guilt that battles within him. He played right into the claws of Sudramoar's sovereign, and now he must leave his newlywed wife behind so that he can pay the price for his foolish actions.

Knowledge of the Quill & Blade AU is recommended to understand this story. It is a direct sequel to Secrets of a Royal Guard.

This story is a sequel to Luna's Daughters

(Can be read without first reading the prequel.)

Centuries ago, in the years after Nightmare Moon's banishment, Luna and Celestia's descendants helped Celestia build the new nation.

Luna's great-granddaughter, Gale Glider, forged the Royal Equestrian Coast Guard, imbuing it with traditions of valor, sacrifice, and bravery. Her final words remain their sacred oath:

Preread by Sledge115 and RDT, thank you!

Cover art found on Google without any attribution, alas.

Author's notes blog, containing spoilers.

A head full of romance is what many Canterlot ponies would call a frivolous waste of time. Everypony was meant to play by the rules of high society: keep your nose in the air, laugh at important ponies' jokes, and attend all of the right parties. Upper Crust and Jet Set had been diligent in raising their daughter Crystal Wishes to be a pony everypony would know, but she wanted more out of life than that. She hoped she would someday meet a handsome prince and fall in love, just like in the stories she wrote.

Crystal knew life wasn't as easy as a foal's tale, but that didn't stop her from dreaming. All of the wishes, hopes, and dreams inside her heart escaped into her precious notebook as she wrote about princesses in power and dashing knights. So long as nopony found out her secret, she could continue to play the part of a dutiful daughter while waiting for her own love story to begin.