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.

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 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!