Do not ask "Why Kill the Alien?", rather ask, "Why not?"
Deathwatch is the third in the Warhammer 40,000 roleplaying game series. While Dark Heresy focuses on agents of the Inquisition, and Rogue Trader focuses on rogue traders and their crew, Deathwatch casts the players as Space Marines of the Ordo Xenos Deathwatch, a sort of special operations group tasked primarily with defending humanity from alien horrors. Even among the Space Marines, the Deathwatch are considered to be an elite unit—and when the typical Space Marine is a seven-foot tall Super Soldier with a standard-issue fully-automatic grenade launcher and armor that can stop a tank shell, that's pretty impressive.
Now think about what you're going to be up against...
Six Chapters are included in the core rulebook, five of which already exist in the Warhammer 40,000 canon: the Space Wolves, Ultramarines, Dark Angels, Black Templars, and Blood Angels. According to the blog, these particular Chapters were chosen to create stark contrast in tactics and personalities within the party between members of different Chapters (though the fact that they're also 40k's flagship Chapters, each with its own codex, probably doesn't hurt). The sixth Chapter, the Storm Wardens, are an original creation of Fantasy Flight Games; the lessons the dev team learned in creating this Chapter are expounded upon in the Rites of Battle supplement, so you can have a go at creating Chapters of your own, you special snowflake, you.
See also Dark Heresy, Rogue Trader, Black Crusade, and of course Warhammer 40,000.
- The Ace: Deathwatch really hammers home that this is the Ultramarines' hat. Not only do the Ultramarines get a bonus to interactions with other Imperial soldiers because of how well-respected they are, but their Chapter-specific Squad Mode abilities are both based on the Ultramarine's Fellowship—meaning that a Kill-team that includes an Ultramarine will actually fight better because he has a Ph.D. in being handsome and wonderful. What a guy!
- This is taken so far that the Ultramarine's Primarch's Curse (caused by getting too many Insanity points) actually causes the Ultramarine in question to lower squad cohesion if he ISN'T put in charge.
- Ascended Meme: Mark of the Xenos gives stats for greater daemons, and each entry begins with a relevant quote. The quote for the Lord of Change entry?
"Yesss! Just as planned!" -- Spoken by Xi’aquan, Lord of Change, in its death throes
- Badass Crew: Deathwatch characters get squad-based abilities that improve their effectiveness while they're working together as a cohesive unit.
- The Berserker: Blood Angels characters tend towards this, unsurprisingly. Their Chapter-specific abilities revolve around hitting better and harder in melee, and taking fewer Wounds by virtue of simply not feeling any pain.
- The Flesh Tearers (a Blood Angels successor Chapter given rules in Rites of Battle) are even worse; with the right combination of talents and wargear, they can bring down boss-level monsters in one or two rounds of combat, simply due to the insane bonuses they get from charging and the raw power their many MANY attacks provide.
- BFG (weapon): Oh Lord, yes. A Space Marine's bolt pistol sidearm actually deals more damage per shot than the heavy bolters available to Dark Heresy and Rogue Trader characters, and it goes up from there. (This discrepancy is justified in-universe by saying that these guns were originally designed for the Space Marines in the first place, and that everyone else in the Imperium is using knock-offs scaled down for ordinary human use.) Errata 1.1 introduces alternate weapon rules that scale the bolters back a bit—and make several of the Marines' other guns even nastier.
- Black Knight: Rites of Battle details the "black shields", Astartes who forsake their Chapters of origin and dedicate themselves to the Deathwatch.
- Blade of Fearsome Size: The Sacris Claymore, signature weapon of the Storm Wardens, is a sword big enough that a Space Marine, a seven-foot tall killing machine that can bench press a truck, needs two hands to wield it effectively. The Power Claymore described in Rites of Battle is basically the same thing with a power field wrapped around it, and the Relic Blade is an even bigger and nastier version of that. It's also worth noting that every Space Marine's standard-issue combat knife deals more damage than the basic swords available to most other folk (or at the very least the same amount of damage, if using the optional weapon rules from Errata 1.1).
- Bling of War: Chapter Trappings of various sorts, which have in-game rules effects, and Honours, which are primarily a roleplaying device.
- Bug War: Deathwatch characters are likely to find themselves in the middle of one of these, courtesy of the Tyranids. Examples include the free quickstart adventure, Final Sanction, and the introductory adventure given in the core rulebook, Extraction.
- Cool Gate: One of the main objectives of the crusade into the Jericho Reach (the main setting of the game) is to secure a warp gate that leads to the other side of the galaxy. Conveniently, this also allows Deathwatch to cross over with the default campaign settings for Dark Heresy and Rogue Trader, despite the literally astronomical distances between them.
- Crowning Moment of Awesome: Invoked, by name and link, as a goal of the Demeanour game mechanics.
- Death of a Thousand Cuts: The Horde rules allow even the puniest of enemies a chance to pull down Space Marines through sheer, overwhelming numbers.
- Dying Moment of Awesome: The game provides rules for this; see Heroic Sacrifice, below.
- Enemy Mine: The Achilus Crusade was originally intended to drive the Tau from the worlds of the Jericho Reach. Following the arrival of Hive Fleet Dagon in the Reach, certain Imperial commanders and Tau leaders have suggested temporary alliances in order to stand against the Tyranid threat. The Tau Commander Flamewing exemplifies this mindset.
- Foe-Tossing Charge: An Assault Marine with the right Talents (Thunder Charge, Whirlwind Attack, Preternatural Speed, Lightning Attack, Wrathful Descent) can easily perform one, annihilating an entire Horde within a five-second turn of combat.
- The Hero: While all Space Marines are heroic (in the classical sense, if nothing else), Tactical Marines fit the trope best. Their abilities tend to skew towards leadership and command, with a secondary role as the Jack of All Stats.
- Heroic Sacrifice: There's even a mechanic for it. A Marine can forfeit his last 'get out of death free' ticket (Fate Point) to receive about a minute's worth of near-invincibility after which he's irrevocably dead.
- Hunting the Most Dangerous Game: The heretek Magos Phayzarus from Mark of the Xenos takes this a step beyond the norm, hunting Space Marines in order to consume their precious gene-seed. This is a major Berserk Button for any Space Marine worth the title.
- It Got Worse: Very likely to happen - the Deathwatch doesn't get sent in until things get a lot worse than usual in the Crapsack World of 40K.
- It's All About Me: Ultramarines who become insane automatically cop this attitude.
- Jack of All Stats: The Ultramarines, as usual. While Marines of other Chapters get to add a bonus to two specific abilities, Ultramarine characters get to pick which two abilities to improve.
- Leave Him to Me: The Storm Wardens' Thunder's Call ability allows them to invoke this against a single enemy per combat. The rest of the Kill-team is generally expected to kill everyone else in the room while the boss is occupied.
- Lightning Bruiser: The Assault Marine—strong and tough as any other Space Marine, able to make two melee attacks straight off the bat, and thanks to his Jump Pack, very, very fast.
- The right character build can take this to Beyond the Impossible territory with a two-ton, Powered Armored Space Marine who can sprint at a theoretical rate of nearly three-hundred miles per hour. That is going to do some serious damage to whomever he crashes into...
- The Medic: The Apothecary, whose abilities mainly center around healing and boosting the performance of his Kill-team. Can also learn to create toxins. Kind of subverted in that Apothecaries are no more squishy than other Space Marines.
- Military Science Fiction: More so than the other two 40k roleplaying games, since characters in Deathwatch are specifically intended to engage in military operations and deal with military-grade threats.
- More Dakka: Devastator Marines. Much like a well-built Assault Marine, a well-built and well-equipped Devastator Marine can destroy an entire Horde of enemies in a single turn (though the latest round of errata has scaled this back significantly).
- One of Us: The FFG newspost on Demeanours includes a link to the Crowning Moment of Awesome page.
- One-Man Army: Deathwatch Kill-Marines, introduced in the Rites of Battle supplement, are specialists trained to perform solo missions without the support of their battle-brothers.
- The Perfectionist: The Iron Hands' Primarch's Curse turns them into this; mixed with Jerkass and Insufferable Genius.
- Scarily Competent Tracker: Space Wolves get bonuses that lend themselves well to tracking and infiltration, filling an advanced scout role.
- Shout-Out: The Expanded Wargear chapter in Rites of Battle has an opening quote from one Commissar Holt.
- Space Marine: Of the Warhammer 40,000 variety, of course, with everything else that implies.
- Stone Wall: Dark Angels characters tend towards this, with abilities that trade movement ability for heightened defenses, temporary Wounds and improved firing capabilities.
- Their Librarians even get a psychic power that lets them essentially become an actual stone wall, with massive amounts of armor as long as they don't move.
- Walking Tank: The Rites of Battle supplement allows for player characters to be interred in Dreadnoughts. However they have to be near death and of high level and renown.