The 'Iron Hearts' trilogy focuses around the Iron Warriors chapter of the Chaos Space Marines, namely the 38th Company (a fictional branch made up of the 'losers' of the chapter; they've only got a few Terminators), and the Lamman Sept (probably another fictional faction) of the Tau Empire. They both wind up in orbit over Equestria, which is called 'Centaur III' by the Iron Warriors. Both the Tau and Iron Warriors engage in glorious battle both on and off world, with the humans coming to have a tentative alliance of sorts with the ponies. It starts with the 38th Company's flagship, the
Harvest of Steel, getting ready to build a manufactorum facility on-planet. Several tens of thousands of words later, it is currently at the point where the Tau capture and fortify Canterlot as a staging ground for their operations -- which doesn't really go so well.
I was BLOWN AWAY by how good this series was, honestly. The dark and gritty feel of WH40k has been
masterfully spliced with the lighthearted atmosphere and comedy of MLP, and even though there's plenty of violence and gore where it counts, it never stays in the 'Dark' area for more than flickers at a time. The characterization and flow are incredible; I'm planning on using this as an example for anything I write and helping others figure out how things should be written, in fact. The series is regularly updated on FiMFiction and generally consists of chapters 15,000-25,000 words long. It doesn't get
nearly enough views as it should for some reason, though, so I thought I'd make a post here to advertise it. If you guys like MLP and WH40k, then you will
love this. Check out the links below the notes (start with Book 1, of course)!
(Note: though there's plenty of bullets flying, no pony characters or even any human or Tau major characters have died yet; I think this fic is staying firmly away from Grimdark like that. At least for now.)
(Note: also, if the Tau are your favorite faction and you don't like seeing them get curb-stomped (like Applejack beating a Shas'vre in hoof-to-hoof combat), this might slightly annoy you.)
http://www.fimfiction.net/story/147183/iron-hearts-book-1-planetfallhttp://www.fimfiction.net/story/150384/iron-hearts-book-2-ferrous-dominushttp://www.fimfiction.net/story/176991/iron-hearts-book-3-the-sept-lamman