A strange accident has stranded three mighty warriors in Equestria: Horus Lupercal, Sanguinius, and the Emperor of Mankind. Trapped in the bodies of ponies, and fresh from the battle on the Vengeful Spirit they must navigate this strange new world. But their very arrival has altered the course of Equestrian history: Nightmare Moon has escaped her end, and still haunts Equestria. With Horus at large with her, and the Emperor crippled by his injuries, Sanguinius must work with Twilight Sparkle to find a way to save the world, while maybe learning a few valuable (and grudging) lessons about friendship.

Warning: Silliness inbound.

By order of the high lords of Terra themselves, a lone soldier from the Death Korps of Krieg has been sent to the planet Epona to wipe out its Xeno population. Starting with Equestria, the Korpsman has sworn not to leave the planet until he falls or completes his mission.

However, no Korpsman should fight without setting up a proper trench first, and so the Korpsman must dig...and dig...and dig.

Destroying Equestria is going to take a while.

Princess Celestia and Princess Luna are leading together a combined Army of several different equestrian Races against an supposedly terrifying and unfeeling Enemy.

What kind of Threat are our beloved Princesses are really going to face?

A Warhammer 40K/MLP:FIM Crossover.

Special Thanks goes to ARchy4288, Tyrannosaurus_Tux and AlicornPriest for Editing, Proofreading and otherwise helping.
I can only recommend those guys.

This story is a sequel to A New Friend

Ten thousand years have passed since the golden age of ponykind collapsed and the horror of Old Night fell across the galaxy. Equus itself is now but a shadow of the shining beacon it once was.

The rulers of that golden age are long vanished, and few believe them to be anything more than myth and legend if they existed at all.

But one mare means to change that. None know her name, but none can deny her prowess or ability to lead. All will one day call her by a simple title. The Empress.

In her heart though, the Empress, the mare once known as Twilight Sparkle, will carry that title even though she feels she does not deserve it.

The Imperium's Western Fringe Crusade discovers a remote planet and attempts to claim it for the Emperor. However, they encounter unusual local inhabitants, and they soon find that they are not the only ones who lay claim to this remote and strange domain....
MLP x Warhammer 40k Crossover

The mirror sent Sunset into the Warhammer 40K universe to meet her father.

This story is a sequel to Celestia Uses An Online Dating Website

The God Emperor of Mankind is in Equestria, and he's looking for love. Good thing for him that Celestia is looking for the same thing. The problem is that he's a warmongering immortal monarch, and she's a benevolent alicorn princess, so clearly they'll get along swimmingly. Here's hoping Equestria survives their relationship!

Equestria is dead. The sun and moon have stopped, and the planet is in ruins. The xenos that built the civilization are gone, their works in ruins. Investigating the planet, Interrogator Kylara makes a surprising discovery, one that might hold the key to what happened to this once-fair land - and what might be done to stop it. A Warhammer 40k crossover.

Now featured on Canterlot's Finest and Equestria Daily

Also now featured on TV Tropes.

This story is a sequel to The God Empress of Ponykind

A message rings through the Warp: "The Game is broken, only Destruction remains". Celestia hears it, fearing for the lives of her former subjects and those of her beloved ponies. Unwilling to sit idle, she has chosen a dangerous path: return to the Imperium, take up the mantle of Emperor of Mankind, defeat the coming storm, and save Mankind once and for all.

With Twilight Sparkle to aid her, Celestia faces new challenges and new enemies. Her forces are scattered, her people assaulted, and the enemies of Man grow stronger with each passing day.

The Time of Heroes is gone. Now comes the Time of Ending.

Sequel to God Empress of Ponykind. Reading that first is strongly recommended.
Rated T for 40K related violence, but that might change at some point.
Comments contain spoilers. You have been warned.

Princess Celestia: Goddess of the Sun, ruler of Equestria, beloved Princess of ponies. She is loved by all, and loves all in return. A calm and wise ruler, she sits on the throne in Canterlot, maintaining peace and harmony throughout the land.

What most don't know is that her past is much darker: She was once a commander of legions, a warrior and scientist of unmatched skill. Thousands of monsters and abominations fell to her forces, slain in the name of an old leader, one entombed within his own throne. Free of her old form, free of the powers of Chaos, she was cast into a new realm, in a new form, with a new lease on life.

She is the God Emperor, Eternal Ruler of Equestria and Lord of the Imperium of Man.

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Comments contain spoilers. YOU HAVE BEEN WARNED!!!