The delicious brains behind Back 4 Blood and World War Z’s zombies

By Julian Benson, Contributor
There's a horde of zombie titles on the Epic Games Store—Back 4 Blood, World War Z, Days Gone, and Dead Island 2, to name a few—and they’re all looking to get into your game library. It would be easy to assume the shambling corpses of one game are much the same as the others, but if you delve into each of these titles, you will find that not all zombies are created equal.

There’s a surprising amount of thinking that goes into these reanimated brain-munchers. We spoke to the developers behind several of these hit games to discover just how much effort goes into differentiating one type of stereotypical "zombie" from another. The shambling undead have to be molded to fit the specific needs of different games—even when they’re in the same subgenre, such as Back 4 Blood and World War Z.

Not breaking the formula

One key challenge of making a zombie game in the 2020s is the expectations set by everything which has come before. For decades, zombies have appeared in films, TV, comics, books, and—yes—games, and each new addition to the canon solidifies certain looks and behaviors. Zombies bite, they groan, and they want to eat brains. That framework of expectations can be a powerful tool to shock an audience—such as in the early 2000s, when cinematic "fast zombies" began to run after decades of only shuffling—but also a weight to contend with.

“If you change too much, there can be this misalignment in how players feel about your game,” says Brandon Yanez, Turtle Rock Studios' Gameplay Director on Back 4 Blood. That misalignment can even happen if the “gameplay is really similar” to other games in the genre.

Back 4 Blood is a first-person shooter where you and three friends can face off against hordes of AI-controlled zombies. The game would be compared to Left 4 Dead even if it weren’t for the fact that developer Turtle Rock itself created L4D before being acquired by Valve and finishing the game as a studio within that company. With that no doubt in mind, the team was very careful with its deviations.
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In Back 4 Blood, the basic zombies function much as they do in other games in the co-op shooter genre. They’re “almost like a tar pit,” Yanez says. “They're there to get in your way and slow you down and distract you, but they're also there for you to feel powerful.” You can shred the undead with just a couple of shots from a rifle, and their bodies come apart in satisfying chunks of gore.

“They’re part power fantasy and part pressing threat,” Yanez says, describing them as “poster children” of the genre. You also face special zombies, each designed to encourage specific player behavior.

“We try to build them, like 'this is the zombie that comes in and grabs you for your friends to try and break you out,' 'this is the one that sits behind the Horde and fires at you, [forcing] you to choose between focusing on what's right in front of you and what’s in the distance,'” Yanez says.

With that familiar foundation reassuring players, Turtle Rock had more latitude to build something unexpected on top. “Back 4 Blood was inspired by the roguelikes we were really into at the time,” Yanez says. The team created over twenty special infected variants, three times more than appear in Left 4 Dead.

At the start of a run, the "AI Director" drafts a roster of special infected from the pool of variants and deploys waves of them against you and your squad. You might face a wave made up of multiple close-ranged specials, such as club-wielding Tallboys and acid-vomiting Retches, that race into the center of your group and scatter you to be picked apart by the horde; or a mixed wave of Hockers, who throw harpoons that can pin you in place, and Bruisers, who are slow-moving, high-damage melee specials that can do a lot of damage to a player who can’t run away.

“One thing we found really early is if you throw everything at the player all the time, then it feels more generic than if you try to brand waves,” Yanez says. “[With that variety] if you were to play a campaign again and again, you would get different pairings of enemy types every time you replayed the same mission.”

Aiming for the expected

Whereas Turtle Rock leaned into expectations to make space to create the unexpected, Saber Interactive—developer of third-person co-op shooter World War Z—knew many in its audience were coming to their game specifically to play an experience they had seen before at the cinema.

The zombies in the World War Z film move almost as one; the hordes swarm like a torrent of water, flowing down city streets and engulfing anyone caught in their path. When they hit a dead end, such as the high walls of a fort, they begin to collect in pyramid-like piles, growing in size until they can overwhelm the ramparts and spill over to the other side. In the original game, you can face more than 500 zombies at a time, and a recent Horde Mode XL update doubled that size to more than 1,000.

“Our team wanted to recreate the most iconic stuff from the film, and that meant we needed hordes of zombies swarming the player,” Saber Interactive’s Chief Creative Officer Tim Willits says. “It was definitely a big technical challenge. We had to build our Swarm Engine to power these massive battles with hundreds of zombies attacking.”

Whereas Turtle Rock started with a foundation familiar to zombie fans and built a unique structure on top, Saber Interactive knew that it would stand out from other zombie games by getting closer to the film, as the source material stood out in the subgenre thanks to its swarm behavior.

“The team spent a lot of time tweaking the system while simultaneously comparing it to the film itself, making sure we were doing our best to get that 'feeling' right,” Willits says. “The first time I saw the enemies pyramid up and climb together was amazing—it’s so impressive, and there’s nothing else like it out there.”
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That’s not to say Saber Interactive didn’t also have to contend with potentially misaligned expectations. The team had to keep “the special zombie types and the swarms true to the movie and the World War Z license,” Willits says. “We had to balance zombie variety with ‘zombie-realism,’ keeping it accurate while also varied and challenging.”

The perfect amount of whelm

A challenge for many game developers is finding the sweet spot of complexity. Throwing too much at a player can cause them to become overwhelmed by stimuli, making it difficult to prioritize incoming threats; too little can cause them to become bored. This is a particular challenge for action shooters such as Back 4 Blood and World War Z, thanks to the sheer volume of enemies players must contend with. When facing literally hundreds of zombies, knowing where you should focus your attention can quickly become difficult.

It’s also very difficult to make a game that caters to any player’s skill level, particularly a shooter. “The first-person shooter genre is super mature,” Yanez explains. "You have players who have been playing shooters for 30 years, so you have players who are not overwhelmed with [a hundred zombies], and then players who really, really are.”

With such a wide spectrum of player skill to contend with, both Back 4 Blood and World War Z offer different difficulty levels. But behind the scenes, that means the developers have to adjust hundreds of numbers to hit the right sweet spot for each notch on that scale.

“Horde tuning is not easy,” Yanez says. Every horde is constructed from a different arrangement of basic and special zombies, with some pairings creating a much more significant challenge than others. The job is made even harder when tuning hordes as the game is still being developed. Yanez recalls one time when testers flagged that some of the basic zombies' behaviors and pathing weren’t functioning properly.
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The team fixed the bugs, pushed out the internal update, and almost immediately started getting comments from the testers like: "'[The basic zombies] are properly getting to you and properly hitting you,' ‘they are a major threat, more so than the specials,' ‘they're relentless,’” Yanez recalls.

Despite extensive testing during development, Turtle Rock still had to release a string of hotfixes for Back 4 Blood after launch, further tweaking the game’s zombies. You can read how fine some of the changes were: reducing ability cooldowns by less than a second, for instance.

“It's difficult,” Yanez says. “Internally, we have a much smaller player pool, so once you put the game out, you're going to have a million people playing it, and then a million different voices telling you [if something is] too overwhelming or not overwhelming.”

Willits says the World War Z team also prioritized not overwhelming the player. “Key to achieving this balance was our AI Director system,” he says, “which orchestrates the game's pacing and difficulty.” Like the one in Back 4 Blood, it dictates when to spawn waves of enemies and what kind of special zombies to throw at a group—“the intensity and difficulty can adapt to the group's skill level,” Willits says.

In order to keep players focused on the gunplay, Saber Interactive chose to shift much of the game's complexity to moments outside of a run. In World War Z, you can level up your class and weapons, and choose different perks to alter the behavior of your character and their guns. However, you make those choices before the start of your run, so, within the crisis of a horde attacking, the emphasis is on “rapid action and decision-making,” Willits says. “This fast-paced gameplay means that players are not bogged down by resource management and, instead, can focus on winning the battle.”

A matter of perspective

“For World War Z, the decision to make it in third-person was to fully capture the dramatic scale and intensity of our massive zombie swarms,” Willits says. You can see the impact of this choice whenever a horde attacks you. By having the camera at a distance from your character, you don’t just see the first row of the swarm that’s charging you down; you can spy the sea of undead behind them, too. It also means that as you run away from a horde, you can swivel the camera about and watch them gain on you as you try to sprint to safety.

“This perspective not only enriches the visual experience, but also offers a tactical advantage by allowing players to maintain awareness of their surroundings, including threats that might be sneaking up from behind,” Willits says.

When discussing Back 4 Blood, Yanez reached a similar conclusion: “It's a lot more complicated in first person because of the narrow view. It can be frustrating if something is hitting you and you don't see it or hear it.”

Turtle Rock employs several systems to prevent frustration. For instance, rooms you’ve explored are invisibly flagged as empty. “We try to make sure that there isn't a clown car of spawning enemies coming from a room that you've cleared,” Yanez says. The audio team developed a more complicated system to make players aware of zombies sneaking up on them.

“As [a special] comes closer, sound is prioritized to make sure you can hear the AI coming, even if there's a ton [of enemies] around,” Yanez says. ”It was a pretty complicated system to get right, especially when you have high AI counts and potential threats.”

The main challenge comes from assuming what a player wants to be focused on; they may be well-aware of the zombie approaching from behind and need to hear what the zombies in front of them are up to. “They may not hear a [sound] from something that they actually feel is a threat because we've dumped that sound based on this off-camera thing,” Yanez explains.

As you can see, a significant amount of thought goes into the zombies in video games. What works in one title may frustrate players in another. What defies expectations in one game may be a step too far in another. The developers of each zombie game have to develop complex technologies to ensure the experience fits the challenge they’re trying to build, even when they share a genre. So, the next time you see yourself facing down a wave of the undead, spare a thought for the developers who have spent years fine-tuning those hordes to be a particular threat to you.

Back 4 Blood and World War Z are both available on the Epic Games Store.