There are a few official Warhammer 40k RPGs that let you play as Space Marines: Deathwatch, Wrath and Glory; or if you want to play as Grey Knights or Chaos Space Marines, the Daemon Hunters supplement for Dark Heresy, or Black Crusade. But that didn't stop indie RPG designer Erika Chappell when she was struck with inspiration this October about how to make the "Perfect Space Marine RPG" - in just under a week, she hammered out 34,000 words of rules for her original game 'Oath of Moment', and uploaded them for free to the internet.
This is very much not an official Warhammer 40k RPG, and Chappell and her editor Lexie (surname not given) have inserted narrative crowbars into the cracks in the Space Marine concept to pry it open and get at the dramatic good stuff inside. "Oath of Moment tells stories about who [marines] are supposed to be, who they are, and the vast gulf between", Erika says in the introduction - but adds that the game "also tells stories of killing a lot of things with incredibly cool weapons, because that's a big part of who Space Marines are, and also, it's awesome".
You can grab Oath of Moment for free via this link. The game is utterly packed with content: there are rules for creating Marines from just about every Space Marine chapter, including Marines with uncertain gene lineages that may be derived from the traitor Primarchs; seven different combat Specializations, such as Devastator, Techmarine, or Librarian; a massive arsenal of weapons and ammunition; and a packed bestiary of foes to use them on.
However you build your marine, they're going to be faced with conflicting Demands that will tug them in multiple directions and often distract them from the mission. A Marine's combat Specialization gives them a Duty - a Tactical marine's Duty demands they draw fire and shield their fallen allies with their own body, while an Assault marine must pursue fleeing enemies and charge into enemy gun emplacements.
The culture of a chapter (which could be Refined or Dour or Hateful or something else) places Demands of Honor on a marine, while their Personality will weigh them down with the Demands of their Conscience. Every marine inherits unique Demands of the Blood from their Primarch - and you'll swear a special Oath of Moment each time you're assigned a mission.
These Demands provide the stats you roll against in situations that aren't purely about annihilating the enemy. You'll make Duty tests to take non-combat actions during a fight, Honor tests to interact with other Space Marines, Conscience tests to interact with non-Space Marines, and Blood tests under unusual, Primarch-specific circumstances. You can increase the stats by achieving the Goals of each Duty and succeeding at your missions - but you're likely to be faced with a downward spiral.
Sometimes, meeting a Demand will be the dumbest thing you could possibly do; other times, meeting one Demand will mean failing a different Demand. Any time you fail to meet a Demand, you lose a point from it (or two points if you break your Oath of Moment). There are very bad consequences when your stats hit zero - an Honorless marine is marked a pariah by their Chapter - a marine who failed in their Duty is sentenced to death, likely in a penitent suicide mission - and the penalty for betraying your Blood is unique to each geneline.
Your own Conscience will fight you. Some actions you might attempt will force a Trial of Conscience: a Thoughtful marine cannot easily accept taking an order without question, while a Quiet marine's conscience is tested when they must bring war to a place not yet touched by it. The marine must make a Conscience test, and can only proceed with the action if they fail the roll.
The commentary is baked into the system. You're very, very good at blowing stuff up, and you can spend XP to gain extra combat Specialization advancements - but just trying to keep the conflicting Demands on your character squared will eat away at every other part of them. If a character ever hits zero Conscience, they're no longer playable - unable to question orders and exert their own personality they're now a perfect Space Marine, incapable of thinking for themselves.
This is another 40k-themed indie RPG that I haven't tested but deeply want to. The focus on simple ultraviolent combat with narrative complications that show up in the surrounding campaign structure really reminds me of the indie TTRPG 3:16 Carnage Amongst the Stars, though Oath of Moment is considerably more fleshed out and granular. If you want an even more darkly comedic take on Space Marines, I wrote about Chapter Serf earlier this month, which sees you playing as their fragile underlings, and looks equally cool.
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