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There have been a number of official Warhammer 40,000 RPGs over the years:
- The original Rogue Trader was halfway between a skirmish game and an RPG.
- Inquisitor was a skirmish game that zoomed in on the action. It required only a handful of models, and the combat mechanics were way more detailed than other wargames (and much closer to an RPG).
- Black Industries and later Fantasy Flight Games released a number of d100-based games with similar mechanics, each focussing on a different aspect of the setting:
- Dark Heresy and Dark Heresy 2nd edition (inquisitorial acolytes)
- Rogue Trader (rogue traders and their retinue)
- Deathwatch (space marines)
- Black Crusade (followers of Chaos)
- Only War (imperial guardsmen)
- Cubicle 7 publishes Wrath & Glory (originally by Ulisses Spiele), a d6 dice pool system that can handle various power levels, from lowly hive scum to mighty space marines.
- Imperium Maledictum is the latest offering by Cubicle 7. It's similar to Dark Heresy in the sense that characters are Imperial agents for a patron (although it doesn't have to be an Inquisitor). Mechanically, it uses a streamlined d100 system.
However, what if you don't want to use any of the official games? What if you're looking for pastiche?
The Kaos Rules adapt WFRP 1E to the 40k universe. It's entirely unofficial, of course, and you need to have access to the 1st edition rulebook. It comes with lots of detail regarding the sprawling 40k universe (and a bunch of useful stat blocks).
I feel like mechanically the BRP system is closest to the Warhammer RPG system, not just because of the d100-based resolution system, but also because of its grittiness. It's no wonder you can find an unofficial BRP conversion of 40k. You can find the PDF over at BRP Central (along with some extra creature stat blocks), or you can read the whole text on the author's blog.
There's also another game in development that began as a Mythras-based 40k adaptation (called Mythammer 40,000), but it's now become its own thing known as O MYTHODES AKMON. You can still find it on the Mythras Discord server.
There are two further d100-based free games I know of that are 40k-adjacent, both by the same designer, David Johansen. The first is Blood Red Future, which is XX. The second is Darker and Grimmer, which describes itself as a futuristic medieval fantasy game.
Outlaw Merchant is an add-on to Electric Bastionland to play in the 40k universe. It's written by Milton from The Last Redoubt, one of my favourites of the lesser known blogs. It uses a similar framework to Dark Heresy: the characters are acolytes of an inquisitor and conduct investigations into heresy, going for the more personal and human-scale aspects of 40k instead of big space marines.
RoosterEma shared a hack/mod of Blades in the Dark back in the G+ days called Blades of the Inquisition. It's not a complete game in and of itself but just a reworking of Dark Heresy's career types into the BitD framework.
Warp Cult, in contrast, allows you play cultists: heretics who fight against the Empire's oppression and injustice. It uses a detailed d6 dice pool system. It assumes (or the very least encourages) the use of miniatures unlike most other games on this list.
Warpstar! is a d20-based faux-40k game by Greg Saunders of Fire Ruby Designs. It's a sister game to Warlock! (which I reviewed here) and uses much of the same mechanics, but whereas Warlock! takes its inspiration from early WFRP, Warpstar! tries to reimagine the early days of 40k as an adventure game. It's even got a few supplements (although not as many as Warlock!).
3:16 Carnage Among the Stars is generally more like Starship Troopers, but it wouldn't be too hard to reimagine the player characters as members of the Imperial Guard and the enemies as the various alien threats to the Empire.
Necropolis 2350 (for Savage Worlds) is another game that hews a bit close to 40k; this time it's the religious crusade against demons in a dystopic world (although it lacks the interstellar grandiosity of "proper" 40k).
Of course, no list would be complete without Mutant Chronicles, which is another gothic dark space fantasy game (this time limited to our solar system). Sadly, neither the amazing 2nd edition nor the 2d20-powered 3rd edition are in print.
Last but not least, there's Renegade Scout, which is kind of a retro-clone of Rogue Trader by Ivan Sørensen. The game's implied setting is the same Unified Space as seen in some of Ivan's games (such as Five Parsecs from Home and Unity Field Agent), but it's very easy to substitute it with 40k as you can essentially take any RT stat block and match the values with those of RS one-to-one (except for point values).