On October 5th, Valve will release a new campaign for both Left 4 Dead 1 and Left 4 Dead 2. It's The Sacrifice, a story which bridges the gap between the two games, and explains the mysterious events of previous downloadable campaign The Passing.

As a special treat for Left 4 Dead 2 owners, they've also carried the original game's No Mercy campaign across to Left 4 Dead 2. And everyone will get a huge 190 page comic beforehand, leading up to the events of The Sacrifice. I spoke to producer Chet Faliszek about, like, what the hell.

The original plan was to make The Sacrifice for Left 4 Dead only. L4D2 had The Passing, and its counterpart was meant as an exclusive for the original game. Chet explains. "It's a really fun campaign, it's got lots of high Hunter leap points and stuff. And we said 'You know, this would be really cool in Left 4 Dead 2. Some of these locations would be cool for the Spitter and the Charger.' So we decided we'd release The Sacrifice not just for Left 4 Dead 1, but for Left 4 Dead 2."

He says this DLC links the two stories. "Do you really want to make the people from Left 4 Dead 2 buy Left 4 Dead 1 if they don't own it?" Yes, I suggest, from a corporate perspective. "That's kind of weird, right?" Apparently, to Valve, it is.

The Sacrifice will be a three-map campaign, but a longer one than Crash Course or The Passing due to the size of the maps. "The finale's a new kind of finale," says Chet, "there's three objectives you have to go and do."

The decision to add No Mercy to Left 4 Dead 2 was simply that it was very popular with players, and Valve like it a lot. "When we first changed them over," Chet says, "they were brutally hard. Your strategy of holing up in corners just disappears, because the Spitter is going to wreak havoc. The Rooftop of No Mercy now, with Chargers and Tanks, is... you'd better not stand by a ledge."

They addressed that by giving players the best of Left 4 Dead 2's arsenal, including several of the mighty M60 machineguns added by The Passing. "You're playing as the Left 4 Dead 1 characters in Left 4 Dead 1 maps, but you get melee weapons, the new weapons, the defib - all the stuff from Left 4 Dead 2. You get to see Zoe with a chainsaw, Zoe with an axe," Chet says with ill-hidden lust.

The comic is part of a conscious effort by Valve to expand their game worlds into other mediums. The short films they've made for Team Fortress 2, and that game's own comics, have been incredibly successful - both in pleasing players, and attracting sales. It'll be released in episodes starting in early September, and the final chunk will cover the same ground as the DLC itself. "The final part that explains what happens to Bill takes place the same time as the game," says Chet, "so you can either play it or read it - or both. You'll know exactly how and why Bill died."

The final twist is that the launch of this DLC will coincide with the launch of the Mac version of Left 4 Dead 2. The DLC will be available for all three versions - free on Mac and PC, the usual 560 points (£4.80/$7) on Xbox 360.

Everything in this announcement is positive: L4D1 players are still getting what they were promised, and L4D2 players are getting extra. But that doesn't always mean it'll go down well. Those who only own Left 4 Dead 1 could be put out that L4D2 players are getting a better version of their DLC, plus a whole campaign from their game.

Probably not, though. Most of the fuss over Left 4 Dead 2 was down to a fear that the first game would be abandoned. That clearly hasn't happened - this will be its second DLC campaign, on top of the Survival mode they added in April last year. With this update, they'll even be adding in some improvements from Left 4 Dead 2 into the first game. "We're going to update the Sniper Rifle to be more like the one in Left 4 Dead 2," says Chet, "because people seem to like that one better."

In a separate interview, Valve MD Gabe Newell told me, "the thing that makes me happy is when you look at the customers who self-identified as boycott members, they actually bought Left 4 Dead 2 at a higher rate than any other group of Left 4 Dead 1 owners." In other words, the backlash was quelled by some diligent PR by Valve - including flying boycotters to their offices to let them play the game ahead of release.

I have to ask Chet, and marketing director Doug Lombardi, if there's a cut off point for making Left 4 Dead 1 DLC. "At this point, who knows?" says Chet. "We haven't quite arm-wrestled that one out." Doug adds. "If people tell us by the amount of play time and purchases on the Xbox side of this that they want more, then we'll probably make more."