An AU Equestria Girls fanfic.

In an alternate world of steam and gunpowder, the mysterious ruler known as Celestia is dead and the dark god Nightmare Moon has already sentenced half of Greater Everfree to the apocalypse. The future now depends on six strangers uniting before the world either falls into darkness or self-destructs.

Yet there are far greater forces at work than anyone realizes, and secrets far more terrible and destructive than any monster lie deep in the realms of eternal night. As the same power that brought the world to its knees becomes the means to fulfill the dreams of human ambition, what sort of future these six strangers will bring is called into question: a harmony of peace or a far greater cataclysm than the one currently facing the world.

Brought to you by Loyal2Luna, this new Downloadable Content for Mass Effect 2 brings the fight to Equestria.

Just about ready to embark on their mission to stop the Collectors from further abducting human colonies, Commander Shepard and the crew of the Normandy SR-2 are given a new priority, when the Illusive Man tasks them with investigating the mysterious disappearance of a Cerberus science team.

Soon, what begins as a simple search-and-rescue unveils a mystery millions of years in the making, and the Commander finds himself thrust into a situation he couldn't possibly have prepared himself for.

Join the Hero of the Citadel as he uncovers the secrets, combats the demons, and ultimately decides the fate of the impossible world known as "Equestria."

After the Changeling invasion, Celestia and Luna make the decision to have Twilight investigate the Elements of Harmony and solve the mystery of their power.

A thousand years later, Rainbow Dash awakens in a world she does not recognize. She has no idea how or why she came to be there. She does not know the language, the geography or any of the rules of this new time. She soon finds allies in Star Fall, a scholarly pegasus whose Talent is Magic, and Astrid, Star Fall's Griffin guardian. Together they set out to discover why Dash has been sent to the future, and how she might return to when she belongs.

War threatens the nations, a Nightmare stalks the shadows, and Rainbow Dash's arrival has turned her into a wild card in a deadly game that pits the law on both sides of a broken Equestria against a mysterious criminal figure, Max Cash. Yet the stakes are higher than anyone knows, for in her research Twilight Sparkle discovered a dark secret about the Elements, something that will shake the foundations of the world.

Cover image by SierraEx
Has a TvTropes page

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!)