Chaos Marines. Emo Space Marines, basically. They're an unhappy bunch - something to do with a "disagreement" with the Emperor. In Chaos Rising, the first Dawn of War II expansion, however, they're more than unhappy. They're positively vexed.

Why? It's all down to those pesky Blood Ravens. They've only gone and returned to sub sector Aurelia where a long lost frozen ice planet has reappeared from the Warp. This simply will not do.

If that wasn't enough, the CSMs have to contend with the Space Marines, Orks, Eldar, and Tyranids all bolstered by new units. Of these, the showstopper is clearly the Tyranid Genestealer - the fan favourite, the jewel in Warhammer 40k's blood-soaked eye, the... horrible, nightmarish blur of claws and teeth that keeps all budding Space Marine commanders up at night. As any 40k fan worth his or her salt knows, the Genestealers are the tabletop equivalent of the xenomorphs from the Alien films, and the source of terror in cult Games Workshop boardgame Space Hulk.

If there's a unit we're most pumped for during our hands-on with Chaos Rising, it's the Genestealer. But, since the CSMs are the new race, it's only right to start with them. They work, predictably, like the SMs, but with an interesting hook: units can specialise by committing themselves to one of the three gods of Chaos. Licking Khorne's arse will lend you a melee focus, bowing down to Tzeench will improve your ranged abilities, and, finally, giving disgusting fat slob Nurgle a great big hug will improve your support abilities (as well as give you a horrible disease. Probably).

The three CSM commander units, however, have already pledged their allegiance to one of the Chaos gods, and so fulfil differing roles in a similar fashion to the other races' commander units. Of these, the Tzeench worshipping Chaos Sorcerer is most interesting. It's a powerful caster with useful summoning abilities, and it can take control of enemy units and turn them against their comrades. Take that, SMs.

Chaos Rising brings a new single-player campaign, which we're not allowed to talk too much about... yet!

The CSM base unit is the Heretic - melee fodder, basically. They're cheap to produce, making them useful control point and power node capturers during a match's early moments. And they have a useful trick up their sleeve: Worship. Activate this ability and the Heretics will gather in a circle around a sacrificed unit, supporting nearby friendlies with heals or damage buffs.

Havok Marines are the CSM version of the SM Devastator Squad, and specialise in heavy weapons. They can be upgraded with rocket launchers, making them great tank busters. At tier two, Bloodletters become available. These red-skinned daemons are great melee units, and perfect for strike assaults - teleportation makes them quick movers, and they're hard to kill, too, because of their phase shift ability. Then, of course, you have the CSM Dreadnought - a terrifying prospect, particularly when upgraded with a missile launcher which enables the rocket barrage ability. For the Bloodcrusher, picture a daemonic rhino charging into enemy infantry. Ouch. And then there are the durable Plague Marines, who are immune to suppression. That's right, kids, they're immune to suppression.

DoW II's quick, action-oriented pace means competitive multiplayer matches rarely reach beyond tier two, but our play test afforded us the luxury of taking our sweet time with the CSMs - enough time to pump out some devastating tier three units. The CM Predator is, like the SM Predator, one of the best vehicles in the game. Is it better than the SMP? Hard to tell at this stage, but it does have bloody spikes on the front, so it's definitely cooler.

You're only here for the Genestealers, aren't you? Sniff.

There is no doubting the cool factor of the CSM's most powerful unit: the Great Unclean One. This huge, disgusting ball of flabby flesh has loads of cool abilities. It can projectile vomit in a way Left 4 Dead's Boomer can only dream of, and poison anything that dares to invade its personal space. It's also got a huge sword, which pretty much cleaves the crap out of anything that moves. And, like any mega elite unit in the game, it's got a ton of hit points, making it a bitch to take down. But doing so is worth it: when the Great Unclean One finally bites the dust, it laughs its head off before exploding in a damaging shower of blood, guts, and god knows what else. Fun, fun, fun.

How do the CSMs feel to play? Like the SMs on evil juice. They're not particularly complicated, which is good for newcomers. The Chaos god specialisation options afford an impressive degree of control over your offensive and reactive strategies, but they're a familiar force. The basic CSM units are great, and the unique units, like the Bloodletter and the Bloodcrusher, are great fun to use. Put simply: they're a welcome - and overdue - addition.

Now the CSMs have had their 15 minutes of fame, it's time to talk about the new units that have been added to the existing races. The SMs get the Librarian, a nutball psycher who doesn't ever wear glasses or tell people to "shush". His main role is to support - healing and protecting allies with force fields - but he's handy in a scrap, too. The Orks get the Weirdboy, who may or may not be a distant relative of red-skinned comic book daemon Hellboy. The Weirdboy is a caster who can teleport friendly units across the battlefield. When he dies, he overloads, doing damage to anything stupid enough to stick around to watch the show.

The Eldar get the Wraithguard, a powerful walker unit slightly smaller in size than the elite Wraithlord. We can confirm that they're tough nuts to crack - Eldar players, we imagine, will quickly warm to their devastating laser fire and work towards pumping out as many of the lanky gits as possible. Interestingly, Wraithguards are buffed by Eldar Warlocks. So, taking a few Wraithguards supported by some Warlocks out for a stroll will no doubt prove a sound tactic. If your supporting Warlocks are killed, though, your Wraithguard will be stunned for a few seconds, leaving it prone to being blown to bits.

And finally, the Tyranids have the Genestealers. These elite melee units hack and tear with all the ferocity you'd expect from one of the most feared beings in the galaxy. They're fast, but they can get faster - their Adrenal Rush ability gives them a speed boost, as well as increased damage. But they're great scouts, too. If they stand still, they infiltrate (turn invisible). Ladies and gentlemen, the Genestealers have arrived.

We're trying to think of reasons why Chaos Rising won't be awesome. We're struggling.

As you'd expect, Chaos Rising will ship with new maps, but they'll also be given to owners of Dawn of War II via a free to download patch. Somewhat surprisingly, the new units and wargear will be free in a patch, too. But you'll be limited to the races included in the game(s) you own. So, for example, if you only own Chaos Rising, you'll only have access to the CSMs in multiplayer and the Blood Ravens in the new 15 mission single-player campaign (more on that in a future preview), but you'll be able to play against any race online. This is Relic's attempt to keep the community from splitting in two (and to balance the old races with the CSMs). We can't see any reason why it won't work.

In any case, THQ plans to release a Gold Edition of the game, which includes the original Dawn of War II and Chaos Rising, and all of the patches, and the Last Stand downloadable game mode, for ten quid more than Chaos Rising, which, when you factor in online retailer discounts, will probably equate to five quid more. Really, if you haven't already played Dawn of War II, there's no better time to get stuck in.

There loads more to discover, of course. We haven't talked about the new single-player campaign, which has been designed to address many of the complaints of the original game, as well as provide a more set piece driven experience. Excitingly, our THQ rep teased "big twists" in the story; twists that he guaranteed will shock and impress Dawn of War fans in equal measure. But we have seen enough of the multiplayer to know that Chaos Rising's already looking like an essential purchase for all Dawn of War fans. If you're sitting on the fence, consider this: Chaos Space Marines and Genestealers. 'Nuff said.

Chaos Rising will be released in Europe on March 12 2010.