First published in 2010, it is the third role-playing game by Fantasy Flight Studios based on the tabletop wargame Warhammer 40,000. The players take the role of memebers of the eponymous Deathwatch, a military organisation created to train its members in fighting the various alien horrors trying to bring forth the downfall of the Imperium of Man, the human faction within the setting.
The players take the roles of Space Marines (more formally, the Adeptus Astartes), a group of superhuman soldiers who through extensive training, conditioning and augmentation are amongst the most powerful soldiers within the Imperium. Normally operating in independent armies (or "Chapters") ranging between 1000 to 100,000 Space Marines, they take it upon themselves to attack the most important parts of enemy armies (leadership, logistics, facilities) to cripple said parts so that the conventional armies of the Imperium of Man can more easily defeat their foes.
Space Marines can be liased to the Deathwatch. How they are chosen differs per Chapter: some are chosen based on their bravery and skill, their experience with alien foes, their tactical genius, as punishment or to further the agenda of the Chapter. If a Marine is chosen (refusal is unheard of) they travel to a base of the Deathwatch, a Watch Fortress. Here they take their pledge of service (the Apocryphon Oath) and serve for an undetermined amount of time as a member of the Deathwatch (a Long Watch).
Given that all Space Marines are all 7' human men who dress in black and silver armor the race system works a bit different: a player chooses the Chapter that their Space Marine originates from, which gives different bonuses, psychic powers, relics and types of personalities. These Chapters are sometimes based on historical forces, such as the Mongolian White Scars, the Native American Raven Guard, the classic Greek Minotaurs, the Viking Space Wolves, the Roman Ultramarines and so on.
Character classes are in place, giving options like a tactical expert, a heavy weapons man, a close combat expert, a psychic (psyker) and a technical expert (a techmarine), with more options available.
Gameplay is based on a roll-under 1d100 system, and characters advance by spending the EXP they earn on mission. As such, there is no traditional leveling system in place.
Players determine their inventory by requisitioning items: based on the difficulty of the mission and the skill of the Space Marine they get an amount of requisition points: more powerful gear costs more points.
Combat is highly lethal in Deathwatch: the superhuman Space Marines and their highly powered foes make for a game that is far higher in power than most other Warhammer 40,000 games that Fantasy Flgiht Studios makes. The games can be combined, but a Space Marine of a starting level will be significantly more powerful than a character from Dark Heresy.
There are splatbooks for both players and DMs: Players get more classes, Chapters and items to use, while DMs get more enemies to put the players against and settings to send them to.