I'll let FFG describe it, after the jump.
Welcome to the front lines, Battle-Brother!So who's going to get the game? For me, this is right up there with the Horus Heresy as something I want, but can't really justify with so much other actual 40K stuff I still need to get.
Deathwatch is a roleplaying game in which you take on the role of a member of the Adeptus Astartes - the devout, bio-engineered super-soldiers also known as Space Marines. After many years of exemplary service within your own chapter, you have been singled out and selected to join one of the most unique and specialized collectives of Space Marines - the Deathwatch.Only the most exceptional candidates from the fighting forces of the Adeptus Astartes are invited to join the mysterious Deathwatch; to take on a new oath to safeguard the Imperium from the darkest of threats. You will now be called into service to hunt down and destroy hostile xenos forces, to seek out and crush the root of heresy and sedition, and to continually fight against the foul daemon menace that crawls forth hungrily from beyond the Warp. You are at the fore of Mankind's war for survival against an extremely hostile universe.Along with the other players, you will unite together as a Kill-team of Deathwatch Space Marines, but the challenges you will face are not all external. Since you have each been recruited from a separate Space Marine Chapter, you will be met with wildly differing traditions, beliefs, experiences and backgrounds. You must learn to work together with Space Marines who are strangers or even rivals.As a Deathwatch Space Marine you have been selected as an exceptional candidate from amongst the warriors of your primary chapter, to bring your unique talents and abilities to the fight at the furthest fringes of Imperial space. United in this newly forged brotherhood, all Deathwatch Space Marines must learn to put aside their differences and work together to succeed in the most extraordinary of missions — or face the threat of total annihilation when confronted by implacable alien foes.The Deathwatch RPG drops you and your group of fellow Space Marines squarely in the thick of the action, stationed at the heart of the Jericho Reach – a region of the Imperium long ago devastated by war, and now on the brink of destruction.For centuries, the Deathwatch have maintained their silent vigil over the ancient ruins of the Reach, but now you find yourselves on the front lines of a grand Imperial Crusade that has been launched to reclaim this vast region of space from darkness and heresy.The fate of billions rests in your hands!