Space Marine chapters had been one of the main obstacles — alongside Games Workshop’s habitual fickleness — to the creation of a cohesive 40k themed role playing game. The Adeptus Astartes had been the de facto mascots of the franchise, with their original beaky-faced plastic figure kit preceding the original set of tabletop rules. The Astartes were too significant to omit as a playable “race,” but their superhuman powers and regimented militant-monastic chapter organization made it difficult to incorporate them into heterogeneous adventuring parties without breaking game balance or the franchise’s established lore.
Dark Heresy and Rogue Trader sidestepped the issue by focusing on aspects of the in-game universe where the marine chapters were the awe-inspiring stuff of NPC encounters. (A later Dark Heresy supplement did add rules for playing Astartes characters from the daemon-slaying Grey Knights, but these were buried under scores of caveats which only highlighted the headaches such characters could cause in a campaign.) The modular nature of the 40k RPG system, however, did make it possible to center an entire component system around player character Space Marines through the Deathwatch ruleset.
In 40k lore, the Deathwatch functions as the chamber militant of the Imperial Inquisition’s alien-fighting Ordo Xenos. It is an Astartes chapter drawn from existing chapters, which loan out battle brothers to the organization to honor ancient pacts and serve as a form of professional development. As each standing Astartes chapter possess its own culture, traditions, and deviations from the “Codex” norm, the ad hoc structure of the Deathwatch allowed players to create characters from whichever canonical or custom-generated chapter they wished while providing a lore-correct throughline and the seeds of internal “Kill Team” drama. (Think of it as “the slobs versus the snobs” but the snobs are puritanical zealots and the slobs ritually eat the corpses of their slain foes. A LAFF RIOT!)
The game’s setting ties back to the previous 40k RPGs through an ancient warp gate discovered in the conduit between Dark Heresy‘s troubled Calixis Sector and Rogue Trader‘s wild Koronus Expanse. The gate leads to the Jericho Reach, a long isolated stretch of space on the opposite side of the galaxy. Formerly an Imperial sector, centuries of separation have seen humanity’s domains fall to the expansionist anime-fans of the Tau Empire, Chaos-worshiping reaver legions, and a splinter fleet of the world-munching Tyranid hordes. Unable to win a decisive victory, yet unwilling to retreat, the Imperium of Man embarks on a multi-front war of attrition to keep the gate from falling into enemy hands — and the Deathwatch are just kind of hanging around helping out as they see fit and pursuing some mysterious agenda. The point is the Reach provides a rich source of adventure seeds against various factions and challenges, as any decent default setting should.
The character creation and development mechanics were yet another evolutionary step from the original Dark Heresy rules, only up-scaled to reflect the greatly increased power levels of the individual characters. Creation and progression moved away from levels in favor of a multifaceted focus on (rigid) home chapter and (fluid) specialization pools of skills, talents, and characteristic advancements.
Wargear is requisitioned on a per mission basis, persistent buffs can be granted alongside experience in the form of “battle honors,” and combat against massed minions is handled through a “horde” mechanic which bundles the opponents into a single unit in terms of giving or receiving damage. Depending on the composition and “cohesion” (distance from each other), members can evoke “orders” which proc situational advantages of limited duration.
The system is extremely weighted toward blasting and blowing shit to pieces, which can feel a bit narrow but is in keeping with the spirit of the source material. If that kind of action isn’t your deal, then you’re probably not the type of person who’d play Deathwatch in the first place. The game also has some dodgy gender politics going on, thanks to the established lore stating Space Marines are an exclusively male organization because of made-up “future science.”
Yes, the rules do some performative hand-wringing about it and put forth high-level Inquisitors or members of the Adeptus Sororitas (i.e. “battle-armored space nuns”) from Dark Heresy as suitable options for those seeking a distaff option for character creation, or simply saying “fuck the canon” and opening up the Astartes ranks to any gender, but that kind of end run really should be necessary in this day and age.
Times have changed quite a bit since 1987. Warhammer used to be the punk-metal upstart against the stodginess of D&D. Now the latter system has been undergoing a diversity driven renaissance while the Warhammer franchises have become synonymous with developmentally arrested neckbeards and edgelords. Games Workshop hasn’t had any problem with radical lore revisions when it comes to adding new lines of product to sell (or drop). I’m not sure why that they can’t (won’t?) apply that to the more retrograde remnants of their franchise lore.