Ciaphas Cain, Warmaster of Chaos and Age of Iron Crossover. These are various fics that I've crossposted on the CC:WC thread on Spacebattles.com, posted here for your entertainment and amusement.

This story is a sequel to Iron Hearts: Book 3 - The Sept Lamman

An MLP/Warhammer 40K crossover, part 4.

The Tau threat has passed. The fleet that once thought to sweep aside the tainted pirates of the 38th Company has fled the system. The Lamman Sept is broken, demoralized, leaderless...
... And victorious.

With Lamman's plan put into motion, an alien horde hurtles through the Warp toward the Centaur system, its soldiers without number and thirsting for violence. Equestria's heroes must find a way to stop them, or the entire planet will be overrun with ease and its inhabitants butchered. While they could afford some pretense of neutrality and disinterest in the fight between the Iron Warriors and the Tau, now it is their world that lay in the crosshairs of the foe. Ponykind will need help to survive, and they look to the forces of Chaos for aid. But is there any hope to be found in the darkness?

EXPLOSIONS ARE IMMINENT.

(Dark humor warning; mild grimdark)
(Cover Art by Ahrimatt)
(This series now has its own TVtropes page!)
(This series is a contestant in the Tournament of Canterlot, a charity event to raise money for Syrian refugees! Go here for details and donations!)

This story is a sequel to The Sun, the Infinite, and the Divine.

Trazyn the Infinite and Orikan the Diviner are back in Equestria. That time, however, Orikan is frustrated. The poor thing is lost in the comprehension of what has happened to the time thingy after Starlight's bizarre adventure with Twilight... and Spike. Yeah... him... Forgot about our little fella.

Anyways, in brief: Orikan is confused about the wibbly-wobbly, timey-wimey stuff caused by magic, and everyone of importance is told about our bombastic duo.

This story is a sequel to Friendship For Gearheads

Barron Secht, Archmagos Dominus, has done things which less flexible Imperial factions would call Heretical. He has adapted designs from Xeno tech-heresy, offered to negotiate with the unclean, and he's even gone so far as to cancel an Exterminatus--on the grounds that the very Machine Spirits themselves willed the ship's thrusters to quietly die.

He has spent the past two weeks attempting to understand a new kind of xeno--a race of quadrupedal sapients who have no knowledge of the Warp, or of War--but they have shown a capacity for learning and understanding beyond what the Imperium has ever reckoned with. The one called Twilight Sparkle recreated a lasgun design long thought lost. What other knowledge do they hold? Secht cannot afford to lose such valuable tech-lore.

Suffice to say, when he learned about the chaos god living on the edge of Ponyville, he was not thrilled.

Contains my best guesses at what Warhammer 40K canon is. Special thanks to Opponent, who wrote the comment that inspired this sequel, and who graciously loaned me his OC to play with. Also, shoutout to the WH40K wiki, and my buddy Tupo over in Estee's discord server, because dear Emperor this game's lore is dense.

This story is a sequel to Entrenchment

The 38th Company brought many strange and novel things to Equestria.
Technology beyond Canterlot's wildest fantasies. Horrors beyond its bloodiest nightmares.
War, strife, blight, and the dominion of monstrous gods long hidden from the eyes of Equestria's innocent populace.

But the planet known as Centaur III knew the touch of dark magics and evil cults long before the Iron Warriors took their first step upon its soil and constructed their blasphemous idols.
The plot to plunge the world into darkness left many wounds and ancient grudges to fester.
The story of the thestrals, the bat-winged soldiers of the Lunar Guard, is one of misery, bitterness, and betrayal. Their history is little more than desperate quests for power and anticlimactic failures, all mere footnotes now as the survivors quietly return to service under their humbled, purified Princess.

Alas, some scars do not heal.
You reap what you sow.

(Dark humor warning; limited grimdark themes)
(Cover art by MagicStarFriends)

It began like any other day in Equestria, ponies peacefully living their lives, knowing that if any danger appeared, Princess Twilight Sparkle and her friends would keep them safe, never even entertaining the idea that something was out there that not even the Elements of Harmony could defeat...and yet there was.

As everypony lived their lives in blissful ignorance, fire rained from the stars, destroying cities and burning forests.

By either luck or fate, Ponyville survived, but as the Element Bearers rushed to gather the Elements of Harmony, they found them powerless, for the Tree of Harmony had been destroyed.

When the fire stopped falling, metal pods fell in their place, and from these pods came horrible monsters the size of minotaurs covered in blood red armor, bearing a mark on their shoulders of a black bird and a crimson tear.

The monsters were without mercy or weakness, but as all hope was lost, the Element Bearers heard voices, promising them deliverence in exchange for servitude, caught between annihilation and slavery, Twilight and her friends made a choice...a choice that changed everything.

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!!!

It is generally not intended for the Mechanicus to be the ones making first contact with a new race of Xenos. Brother Gentoo has the dubious honor of being stationed on Equus, for reasons that nobody can really explain, because the explanation was taken out back and shot by someone three or four links up the chain of command. Frankly, he's done this song and dance before. The Mechanicus are used to bending the rules a little to ensure that the Imperium's gears continue to turn.

This is how Brother Gentoo ended up being the one to make first contact with the inhabitants of Equus--a group of oddly-colored, unusually intelligent equines.

And then one of them asked him how lasguns work.

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

An Ark Mechanicus commanded by Arch Magos Cassius discover a world rich in resources and potential. The only issue is the inhabitants.

Cover art is from the game Warhammer 40,000 Mechanicus.

Warhammer 40,000 is property of Games Workshop

MLP is property of Hasbro

EDIT: Inspiration
The original idea for this is actually from an unfinished comic on deviantart by someone called Telgin I found a few years ago. Basically I just kept thinking about what happened next and this is the result. If you read the comic then the first chapter you can probably see the similarities.

Also many characters have their parallels in the game Mechanicus as well.