It is a problem that has befuddled RTS developers for years: how do you create a compelling single-player campaign that's more than just a glorified multiplayer training manual? So many games, even now, don't even bother to try. Relic Entertainment, with standalone Dawn of War II expansion Chaos Rising, has dared to, and the result is perhaps the best effort yet.

When it comes to daring to try, Relic has form. Before Dawn of War II was released, World War II RTS Company of Heroes wowed PC gamers with its incredible graphics, fast-paced action, and... shock horror... stripped down gameplay. Dawn of War II took the streamlining of the genre one step further, dropping base building and resource gathering almost entirely (it remained in multiplayer, but you got the impression it did so through gritted teeth). Some lambasted Dawn of War II for hardly being an RTS at all, labelling it a slightly more complex Diablo-style dungeon crawler. Others praised Relic for moving things forward. I sort of sat on the fence.

My main beef with Dawn of War II wasn't this shift in focus, but the distinct lack in variety in the campaign missions. Most of them followed one of two repeating patterns: last stand against Tyranids/Orks/Eldar; or make your way around a map and fight a boss at the end. Truth be told, it got boring after a while.

This is where Chaos Rising excels. It not only improves mission variety (to my mind, I can't remember any two missions the same), but adds layers of strategy and excitement, so much so, that you start to forget you're playing an RTS, and instead believe you're embroiled in some kind of Warhammer 40k-fuelled Modern Warfare.

Relic has surely played Infinity Ward's influential shoot-em-up; Chaos Rising's campaign is packed full of set-pieces, scripted moments, and hurtles along at such a relentless pace you worry it may reach its exciting end without you. Put simply, there's a lot more going on.

Take, for example, one early mission set on the frozen planet of Aurelia, which has mysteriously reappeared after thousands of years lost in the Warp. The Blood Ravens, once again guided by the disembodied voice of Gabriel Angelos, are fighting Chaos, as they have a want of doing.

Dreadnought Davian Thule - still one of the coolest units in RTS gaming - is holding the line at the bottom of the map, but he can't hold it forever. You rush to save him, with the powerful librarian Jonah Orion - the Space Marine's new elite unit - in tow. In Dawn of War II, you might have moved down the map in a breezy fashion, saved Thule, and instantly forgot the act. But in Chaos Rising, all sorts of hell breaks loose as you frantically scramble to Thule's position. Ice cliffs crumble and fall as you pass through valleys. Glaciers shatter and slide down into the water below, preventing any retreat. It feels as if the world is falling apart right behind you.

Another example: one of the game's best missions sees the Blood Ravens explore the dark and dank corridors of a Space Hulk (remember that board game?). There's little room to manoeuvre, and it's quiet - too quiet. Then, of course, the terrifying Tyranids, bolstered by Genestealers (yay!), literally burst out of the walls and head straight for your armoured flesh in a flurry of claws and razor-sharp teeth. Carnifexes blow out of pipes, leaving twisted metal and steam in their wake. You can see little monstrosities crawling about all over the place before they even get to you. It's brilliant - atmospheric, scary, and totally, totally, Warhammer 40k.

Then there's Corruption - a neat new feature that adds a degree of morality to the bloodshed. At its most basic, each of your units can be corrupted by player decisions: do I battle back a Chaos uprising by destroying the great gate that defends Angel Forge, or use Jump Packs to take the long way round? Do I take care not to destroy a Blood Raven monument as I assault a group of Chaos, or rain down orbital bombardment and missile launcher fire from a safe distance with nary a care for relics? Do I care about entirely optional, pain in the arse redeeming missions, or get on with the good stuff post haste?

The game always makes clear which decision will result in corruption, so don't expect any philosophical head-scratchers, but it's cool all the same. More interesting is the use of special corrupted wargear and the pursuit of cool traits unlocked by climbing up individual unit corruption skill trees. If some wargear is used in missions, it'll add a couple of points of corruption to the user. Now, the question you have to ask yourself is, do I want corruption? Some of the corruption abilities look the business and are incredibly useful on the battlefield. But climbing up the corruption ladder prevents you from using other wargear designed for the pure of heart and mind, and takes away already unlocked traits. So, as you're fussing over equipping Terminator armour to your Force Commander, and working out which heavy weapon to give to Avitus, you've always got corruption clawing at the back of your mind. You think, I really want Jonas to walk around the battlefield covered in flames, but if I head for that trait, I'll lose his incredibly useful healing trait. Really, that's what the corruption mechanic boils down to: loot and traits. This isn't a criticism - you genuinely care about your men and their tools of destruction, itself an impressive achievement. Just don't go expecting a Fallout 3-style karma system.

All the while, though, you wonder, is this going to impact on the storyline in any way? Are my decisions going to meaningfully change the tale Chaos Rising is telling? I won't spoil it for you - even though I really, really want to - but I will say that the answer is yes. Corruption adds replayablity to the game beyond just maxing out the level 30 cap and getting the best wargear. Chaos Rising has multiple endings, and with good reason. I'll let you discover them for yourself.

It all adds up to a campaign experience that's more varied, exciting, and dynamic than the one we enjoyed but quickly tired of in Dawn of War II. The environments are eye-catching, the plot - a mildly compelling detective story packed with political intrigue, traitors, and, of course, the kind of po-faced heroic sci-fi fantasy we know and love from the 40k universe - is as good as we could have expected. And the gameplay... well, we know from Dawn of War II that it's heart-pounding, accessible, and heavy on micro-management. If you hated Dawn of War II, Chaos Rising probably won't change your mind. But you can't escape the feeling that Relic's RTS/RPG hybrid is perfectly suited to the kind of campaign we're getting here. It's hard to imagine how base building and resource gathering would fit in.

You may not care about all this namby pamby story stuff, of course. For you, Dawn of War II was, is, and always will be, all about the multiplayer. That's fine. Chaos Rising, with the addition of the Chaos Space Marines; new elite units for the existing races (Genestealers - yay!); new upgrades, such as multi meltas for the Tech Marine and twin las cannons for the Predator; and nine new maps, freshens up the multiplayer experience. But it doesn't revolutionise it. Multiplayer is, essentially, as it was: very different to the single-player, but still very good.

The headline addition is the new race, the Chaos Space Marines. They work similarly to the Space Marines - perhaps too similarly - but they're a hell of a lot cooler, and loads more evil. They've got spikes and skulls and other bits you don't ask questions about adorning their shoulder pads and tanks. They've got daemons - horrible ones too - that spew out of the Warp and bring terror to anything that gets in their way. Chaos are an interesting alternative to the Space Marines; extremely powerful in melee, and tough nuts to crack. They're also versatile - by pledging allegiance to one of the Chaos gods, units can specialise in certain areas, making reacting to your enemy's strategy as easy as pie. But their most powerful unit, the Great Unclean One, doesn't need to worry about pledging allegiance to anything. Essentially a massive ball of flab, it cackles as it tramples all over its enemies and spews vomit. Still, I reckon Davian Thule could have him. Maybe.

Problems? Complaints? While you care about your units - more so than in any other RTS - the voice acting and plot is often embarrassingly hammy. I'd hoped we'd get more than just one new race; one day my beloved Dark Eldar will make an appearance, and that day will mark the end of my social life. There's no new opening CGI movie - disappointing because I adore Relic's work in this regard. What else? Oh yeah. The Eldar are still crap. And by crap I mean I just don't get them in the slightest.

Bar these mere niggles, there's little wrong with Chaos Rising. In almost all respects, it's a better game than Chaos Rising, and from a campaign perspective, the best since Warcraft III. Heart-pounding, cool, and relentless, Chaos Rising is a must-buy. If you're not already immersed in Relic's Dawn of War universe, there's no better time to orbital drop in.