Left 4 Dead 2: Swamp Fever and Dark Carnival

Unlike the movie and music industries, which leave you in no doubt you're watching a McG picture or listening to a Lady GaGa record (and in my case leave me wondering why), videogames haven't always been great about acknowledging the people who deserve the most credit.

So it's nice to see Valve paying tribute to the AI director. He's the king of Left 4 Dead, the unseen hand on the panel of instruments that determine the flow of infected zombies into your four-man team's path, and in Left 4 Dead 2 he can punish you in tons of new ways - he has new special infected to divide you up and drive you into peril, he can smash you with storms and force you down new paths - all the while he continues to shape and reshape the pace and tone of the adventure with an intelligence that belies the fact he's just a bundle of scripts and programming routines designed to mess you about.

A lot of developers would keep quiet about him, and leave you to draw your own conclusions, but Valve's proud of him. Left 4 Dead 2's their product, but the AI director's their baby.

We can talk all day about the clever things he does, like understanding not to back off completely just because you're down to one health point, or hitting you again and again through one section of a level only to leave it virtually empty the next time you play it (and this being Eurogamer, when we can talk all day about something, we do - check out our interview with Valve's Chet Faliszek about exactly this sort of thing). But playing through the latest campaign, Dark Carnival, in Valve's Bellevue headquarters last week, it's easier to stop hunting for the invisible, hugely brilliant hand of the director, because there are much more immediate delights to consider.

The clown's pretty easy to take down - his uncommon skill is his ability to attract the horde, which forces you to prioritise him.

Dark Carnival begins on a highway, amidst abandoned cars pointing towards a Ferris wheel and the red and white funfair stripes of a marquee on the distant twilight. Initially it's functional, although typically fraught, as regular infected streak between and over the lifeless husks of cars and buses from the roadside to get at you. Following a slippery hillside descent, it progresses into a two-storey motel complex horseshoed around a car park, with zombies bashing holes in seemingly every door you pass as you ascend the naked stairways in search of a route through.

The Whispering Oaks carnival itself, past concession stalls and rides, is a manmade maze designed to drive people round and about to commerce, but you still find your way into the back-rooms and over rooftops as usual, and here you sometimes encounter the campaign's uncommon common infected, the clown. His squeaky shoes attract the horde, and sometimes during a break in the action all you hear is the squeaking. "You're going to want to kill the clown first," says Faliszek. He joins the hazmat-wearing CEDA infected and the riot cop, who can only be perforated from the rear, on the six-strong uncommon common roster.

The carousel crescendo moment at the peak of the two-chapter Dark Carnival reveal. Much easier without a Witch onboard, thanks.

The second of the two chapters being shown this month (each campaign consists of four chapters and a finale) culminates in one of the game's crescendo moments, familiar to fans of the original (the elevator button at Mercy Hospital, for instance), where you have to fire up a carousel and then race around the other side to switch it off again, with an unending horde of zombies pelting themselves at, up and over the wire mesh surround as you do so. The director throws a Witch in for good measure - she's still a class apart from the other specials, and in her new Wandering Witch guise she's a paralysing presence in even an empty street, stumbling around at random as you try to get the hell out of her path.

It's crowd-pleasing all the way. We're warming to the new survivors already, for instance. "Whispering Oaks. I used to go there as a kid," says the righteously indignant Coach during a moment of burly reflection. "Great," says Nick the gambler. "Now we can die there as adults." There's the wooden cutouts of Mister Peanut and his arch enemy Mustachio (they vie for the affections of Nutasha). There's the gibbing and the limb loss. Oh the limb loss. The pipe bomb was always fun, but now it's worth hanging around for. Nothing but the rain!

Valve says that the new weapon progression introduces greater diversity, and this is seemingly evident on the full-auto route, as you move up from silenced Mac-10 (or perhaps TMP?) past AK-47 to the biggest assault rifle. The most obvious and striking departure on the weapon front, however, is the grenade launcher - a one-shot-at-a-time friendly-fire nightmare that makes the player a lot more dangerous to both sides. You can destroy a horde of zombies in one blow, but you have to switch to a side arm or think twice about trying to dislodge a Hunter from an ally, unless you want to loose them both the mortal coil. It's such a game-changer that it effectively represents a profound change of player class in a game that only does ad hoc classing.

It's enormously useful when I uncover it in the second campaign we try out, Swamp Fever, which is also the most exotic of the three shown so far. Almost everything about the two Swamp Fever chapters we play is brilliant - the mud men uncommon common biting at your knees and splattering mud in your eyes to announce their presence in the waist-deep swamp water; the plank pathways in the mud and raised walkways; the downed aeroplane with one wing sunk into the water and a massive rip through the fuselage that you have to scramble through; and the best crescendo moment so far, on the banks of a river waiting for a rusty old ferry platform to be hauled across to your side of the water.

The level configuration is cunning and, despite the setting, fresh, driving you into new patterns of play. Infected rise out of the water, the cloying dankness of which slows you down terribly whenever you need to wade into it, and there are some lovely touches here, closer to the marginal flourishes in Portal or Episode Two, like an assault rifle upgrade to be pulled from the cold dead hands of a CEDA man dangling by a parachute in the trees, and all sorts of junk you wouldn't expect to see in a swamp. Faliszek explains that this sort of stuff really is there in real life - a veteran of New Orleans, he even sent unbelieving researchers down to the bayou to see for themselves.

The Parish, previewed elsewhere on the site, introduces the excellent cricket bat, and takes place during the day. Dark Carnival progresses from dusk to nightfall, altering infected behaviour in the process.

There's much more to talk about - so much more, in fact, that I brought Christian Donlan along with me, a man as horribly polite as the new Jockey infected is horribly effective. You should check out his alternative hands-on to find out about the special infected updates, melee weapons, and the manner in which Left 4 Dead's visual identity is beginning to take shape.

The three main things we don't get to play around with this time, apart from the remaining two campaigns of course, are new finales, the special infected side of Versus mode, and the promised new game mode, which has yet to be announced. Faliszek says the upcoming Crash Course DLC for Left 4 Dead 1 will introduce a few of the key special infected interface changes, like allowing you to see recharge times for your team-mates' attacks, and the one finale we have played - the gauntlet conclusion to The Parish - at least bodes well. Fantastically well, actually. All questions about the new game mode are beaten back with a frying pan.

Disco!

But that's okay, because the last word should go to the AI director. He's still in training, no doubt practicing his sudden, visibility-slashing rainstorms and working out where to drop the dynamic crypts in one of the graveyard sequences. But he's everywhere throughout our two hours, dumping a Witch on the wing of the swamp-sunk plane-wreck, lamping me with a Charger when I'm the grenade-launching survivor of a group with three men down and what I really need is room to manoeuvre, and laughing us out of the building with a Tank at the last of gauntlet Parish run, just as we haul ourselves off the bridge and make for safety.

He's older, he's bolder, and he still deserves his star billing. Unless he goes a bit Lindsay Lohan, it's hard to imagine Left 4 Dead 2 being anything other than magnificent.