There was a time not long ago when Call of Duty co-developer Treyarch was referred to, somewhat dismissively, as the B-team. For years it struggled to escape the shadow cast by Call of Duty 4: Modern Warfare, a game that fundamentally altered online multiplayer forever and cemented creator Infinity Ward’s status as the series’ rightful custodian. Treyarch’s immediate follow-up to Modern Warfare, World at War, was widely criticized for clinging to the franchise’s World War II roots despite Modern Warfare’s efforts to push Call of Duty in some exciting new directions.

But for Treyarch, World At War wasn’t about stagnation. It was about self-discovery. “In terms of where Treyarch got its identity, I think it starts at World At War,” says senior executive producer and longtime Treyarch developer Jason Blundell. “There’s a little bit of an edge to it. Previously Call of Duty had been very ‘rah rah America.’ Some could criticize it for being a little bit jingoistic at times. And Treyarch was searching for like, ‘What’s our take on it?’”

From the original Call of Duty: Black Ops

According to Blundell, Treyarch found its take by embracing its innate “anti-establishment” attitude, one that led to anti-heroes like Black Ops protagonist Alex Mason–a character convinced by a voice in his head to assassinate president John F. Kennedy. “At the time it was revolutionary speaking,” says Blundell. “Inside the halls at Activision it was like, ‘What are they doing to the franchise? They’re going to kill us all!’ That was exciting. I remember we used to go outside and drink coffee late at night, and I’d turn to some of the directors and say, ‘We might be killing Call of Duty at this point.’ I think that guided the studio’s future.”

In spite of these anxieties, Black Ops’ daring narratives did not kill the franchise. In fact, the Black Ops trilogy eventually helped Treyarch supplant Infinity Ward as the series’ premier development team, and in the process of turning its rebellious streak into an asset, Treyarch accidentally invented a signature. “World at War is where Zombies was first created,” recalls Blundell. “It was made out of those little bits at lunchtime. It’s literally made from the things from World at War. There was no speaking in the first one because there was no budget. There were no assets. That’s why there’s no VO. People always forget that.”

Inside the halls at Activision it was like, ‘What are they doing to the franchise? They’re going to kill us all!’ That was exciting.

Jason Blundell, Treyarch Senior Executive Producer

According to Blundell, there was no planning, no funding, and no support for its fledgling horde mode, and the final product was relatively bare bones as a result, with only a single room and a rudimentary weapon upgrade system to keep players interested. Still, there was something undeniably magnetic about the studio’s pet project–at least to its creators. “There was that kind of rock-and-roll aspect of it that we all identified with and gravitated to,” admits Blundell. “It’s got that kind of edge to it, and it speaks to us, and I think it speaks to the fans.”

The one party Zombies didn’t speak to? Call of Duty publisher Activision. “Even though Activision gives us wonderful latitude to explore different ideas, there are also certain guidelines that we have to keep within,” explains Blundell. “When it comes to Zombies, and if you look at the birth of it, it was done in the lunchtimes of World at War, which was already a very trying product in terms of schedule and so forth.”

In addition to scheduling and budgetary concerns, Activision may have been reluctant to include something as ludicrous as Nazi zombies in its otherwise weighty game about a real historical war. “World at War was a very serious game,” observes Blundell. “It was talking about the Pacific campaign and World War II, and the campaign finished with the atomic bomb. Not a light topic whatsoever.” Given Activision’s understandable concerns, the devs struck a deal: “Activision enjoyed it. We enjoyed it. People playing it all around the office enjoyed it. And so it got put in. The deal was: we wouldn’t promote it, we wouldn’t talk about it. It was purely an Easter egg.”

Originally, in order to unlock Nacht der Untoten–the very first Zombies map–players had to complete the single-player campaign. And prior to completing the campaign, there was absolutely no indication in the main menu or anywhere else that Treyarch’s brand new horde mode even existed. Of course, that all changed the instant the internet got involved. “The outcry was so massive,” marvels Blundell. “We didn’t have as much social media exposure as we do these days, but we still heard it loud, which means that it was really loud. So we patched the game so that everyone had access to it, and then it just took off. It took off in a way that was incredible.”

It was made out of those little bits at lunchtime. There was no speaking in the first one because there was no budget. There were no assets.

Jason Blundell, Treyarch Senior Executive Producer

What started as a playful afterthought lovingly assembled during the developers’ free time quickly grew in size, sophistication, and significance. Three more Zombies maps were added to World at War. The original Black Ops drastically expanded the scope of the experience by introducing new mechanics and a deeper story. Both subsequent Black Ops games layered even more complexity on top of that foundation, adding countless Easter eggs and continuing the rapidly snowballing narrative. Zombies eventually grew so big that it actually started to appear in Call of Duty games developed by other studios, including the upcoming Call of Duty: Infinite Warfare–a game developed by Infinity Ward.

So how did it happen? What allowed Zombies to flourish to such an unprecedented degree? “It [expanded] because of the fans,” says Blundell. “It sounds corny, but it’s the reality of what happened.” The ongoing zombies narrative is an excellent example. Even though Nacht der Untoten contained absolutely no story content whatsoever, fans and players “just started to make the story,” according to Blundell. “I always joke with our lead writer Craig Houston that he didn’t have to do any work because it was all written down at the beginning. They just went into it my sources. It was, ‘Who’s that guy? What’s that texture over there? Oh, look, I see the devil in the wall.’ Crazy stuff that we hadn’t even put in. We go, ‘Oh, let’s do something with that.’ Then I think over time what we’ve done is just put more and more resources and attention behind it.

“It’s always been about playing with the fans,” continues Blundell. “That was literally the only reason. The fans started writing it, and so we saw it as fan service to be able to reply back to them.” And with each reply came an even louder response from fans, excited to see their ideas reflected in the game. “It became this kind of prophecy to the point where they were able to write and make more stuff than we could keep up with because there’s just so many of them, and it’s grown and grown and grown,” says Blundell.

Fans also began to work together to uncover secrets, which meant Treyarch had to create deeper gameplay in order to challenge the community. “The need to put multilayer, multi-discipline complexity into the experience [emerged because players are] now working like a coordinated neural network,” affirms Blundell. “The whole community coordinates. It’s absolutely incredible. I can put an obscure thing like ‘You have to throw a grenade at that wall over there,’ and they will find that within 48 hours. You can look at it at the time when you’re making it and say, ‘There is no way anyone will ever think that.’ But just due to the sheer numbers, it will happen. They will find that Easter egg.”

Despite the community’s continued success in foiling his every dastardly plan, Blundell remains committed to delivering deeper, more sophisticated puzzles–and more of that trademark Treyarch spirit–with every new map. “Back when those original maps came out, it wasn’t anything like this. I celebrate the ingenuity of that and the passion behind that. It’s also then my job to make it harder for them,” laughs Blundell. “Even as we put more people on it and it becomes more of a serious endeavor to make a Zombies experience, one of my jobs as the director is to try and always keep that attitude alive. It’s hard work to keep something organic and edgy as you become more built up and structured, but hopefully that’s there and hopefully the enjoyment and the passion comes through in the DLCs we make.”

From Call of Duty: Advanced Warfare

Regardless of what the future holds for Call of Duty, Treyarch has already established a legacy thanks in part, if not entirely, to its humble undead friends. For that, Blundell seems genuinely grateful. “When we started out, no one thought that it could keep going and keep going,” confesses Blundell. “My first map I got involved in was a map called Der Riese, which was on World At War. That’s where we introduced a thing called Pack-a-Punch, which is the way to upgrade your guns. At the time, people were shouting heresy at me. The idea of upgrading. Then the teleporter. I was just crapping myself because my thought was, ‘I better not destroy this thing that people are really enjoying.’ It worked out well in the end.”