Photo: Antonin Kratochvil/VII

Dean Hall was close to death in the jungles of Brunei. It was December 2010 and the officer cadet in the New Zealand army was alone on a survival-training mission. Given only two days' worth of food for 20 days, he supplemented his diet with raw fish and ferns. He slept on a bed of sticks, and by the end of the mission he'd lost 44 pounds from his already lean frame. There were other trainees out there, and he started to plot raids on their food supplies. He thought of himself as an honorable person, but he was too hungry for honor. As he approached one man's camp, the guy spotted him and tossed him some rancid ramen. Hall boiled the noodles and wolfed them down.

That night, as water pooled around his bed and ramen roiled his stomach, he imagined himself inside his favorite videogame, a military simulation for PCs called Arma 2. He had been playing it since its release in 2009 and often spent three to four hours a day on the game. Now, lying on the jungle floor, his feverish imagination turned Arma 2 into something different. He visualized a new kind of game, one in which there were no missions, no objectives, and no ability to simply be respawn when killed. You had one life, and if you lost it, you lost everything.

Physically, Hall was a mess when he emerged from the jungle — he underwent emergency surgery for an intestinal blockage. But intellectually, he was energized by his visions and inspired to build a new Arma 2 modification. Amateur game mods have been popular since the 1990s, when players adapted first-person shooters like Wolfenstein 3D and Quake to create entirely new games. Mods can't run on their own — they operate in conjunction with the underlying game — and are usually given away online by the \0xFCberfans who code them. The high-water mark came in 1999, when two gamers turned the science fiction world of Half-Life into a terrorist-versus-soldier battle zone called Counter-Strike. It was so popular, gamemaker Valve bought it in 2000 and released it as a stand-alone title, eventually selling more than 25 million units.

A savvy programmer, Hall had created lots of Arma 2 mods, adding new weapons and vehicles to the game, and even new missions. But what he had in mind now was fundamentally different. Typically, games try not to frustrate players; if your character is killed, you aren't forced to start over from the beginning. But Hall thought this stripped gaming of emotion and drama. He wanted to reproduce what he had experienced in the jungle, something filled with agony, frustration, and fear. "I wanted it to be brutal," he says.

In a hotel room in Singapore, Hall began coding. In his new game, players would begin with almost nothing, stranded in the middle of a barren land, forced to hunt for supplies. If they were killed, they'd lose everything and have to start over. The only mission: Survive.

"Hall wanted to reproduce what he had experienced in the jungle: agony, frustration, and fear."

Eventually he hit on the idea to replace Arma's terrorists with zombies, but the undead would actually be the least of a player's concerns. Hall was designing the game as a social experiment: Every time a player logged in, they'd be pitted against other players also hunting for supplies. Players would compete for limited food, water, and weaponry, and their anxiety would make them more deadly than the brain-eaters. The gameplay re-created his feeling of isolation in the jungle, surrounded by dozens of starving strangers, any of whom might be plotting to steal his meager supplies just as he was plotting to steal theirs. Hall wanted the possibility of dying and losing everything to drive players to kill other survivors in order to steal their rations. He would call the game Day Z, a twist on D-Day.

Bohemia Interactive, the Czech company behind Arma 2, actively encourages fans to mess with its game, so there were already hundreds of Arma 2 offshoots. When Hall was building a mod, he would frequently contact a Bohemia developer named Ivan Buchta, usually with an esoteric code question. But after returning from the jungle, Hall had become bored, and this time, while working to finish Day Z in his spare time, he wrote to ask whether Bohemia was hiring. "I was impressed with the mods he'd made in the past," Buchta says. "We needed help on Arma 3, so we agreed to bring him on."

It wasn't a great deal for Hall. He'd have to buy his own ticket to the Czech Republic, take a leave of absence from the New Zealand army (which allows two-year absences), and be paid less than he had been making as a lowly second lieutenant. But he took the job; he was thrilled at the prospect of being in the birthplace of his favorite game. It was like a die-hard Star Wars fan getting hired by Lucasfilm.

Still, when he arrived at Bohemia, he decided not to tell Buchta or anyone else about what he was working on. "I knew they'd think zombies were stupid," he says. "They're all hardcore realism military junkies, so I kind of felt embarrassed that I was making a zombie mod." He decided just to put Day Z online. He figured a few hundred people would play and no one else would notice.

Within weeks, though, players flocked to the game. By the end of its first month, Day Z had attracted 10,000 users, and Hall decided he'd better mention it to Buchta. Sure enough, Buchta was dismissive. The company was gunning to finish its latest Arma release. He didn't have time for zombies.

But days later Buchta loaded Day Z to check it out. The game placed him near an old warehouse. He had a baseball cap and not much else. He quickly found an old handgun with no bullets. There were zombies in the distance and the threat of other players killing him for his gun. The scenery was just like Arma 2, but everything seemed different. "I felt really naked," he says. Most first-person shooters equip newbies with guns and enough ammo to survive a sustained firefight. Now Buchta was scrounging for bullets and counting them carefully. "I'm a professional game developer, but I was immediately scared and tense," he says. "Games don't usually work like that on me."

In Day Z, players traverse a bleak landscape, dodging zombies and one another. The only mission: Survive.

Buchta messaged Marek Spanel, who co-owned Bohemia with his brother Ondrej, and told them to try the game. The brothers had formed the company in 1999 and spent most of their lives playing videogames together. Now they spawned into the game simultaneously and discovered that they'd been placed miles apart. They began to move toward each other, dodging zombies and other players. It took more than an hour of nail-biting evasion to rendezvous. "Walking for half an hour in a game would usually be boring. But just doing that, I felt stronger emotions than I'd ever felt in Arma," Marek Spanel says.

To play Hall's game, users had to buy Arma 2 for about $20. At that point, Arma 2 had been out for three years and was nearing the end of its life cycle. Arma 3 was supposed to jump-start the company's sales, but suddenly purchases of Arma 2 started ticking up. By June, around 20,000 players were using Arma 2 to play Day Z. By early July, there were 405,000 users, and it didn't stop there. In August more than 1 million people were playing Day Z. Arma 2 was suddenly a top-selling PC game. In the three years before Hall put Day Z online, Arma 2 had sold 1 million copies. Now it sold a million more in just a few months.

The guy who'd arrived at the company as a junior designer five months before now seemed to hold the keys to its future. Bohemia, meanwhile, had raked in millions from the boom in Arma 2 sales, and Spanel wanted to build on that success. So when Hall said he wanted to turn Day Z into a stand-alone game, Spanel offered him a promotion from junior designer to project lead, the top spot.

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Hall was ecstatic. He now had a large team of people implementing his every idea. He requested a discharge from the military and started refining the Day Z world. Originally supplies appeared in the open, lying on the ground. Now those necessities were harder to find, hidden in cupboards or under beds. He also populated the game with hundreds of public domain books like War of the Worlds and Moby-Dick. Maybe players would find a quiet place to read away from the zombie apocalypse.

He had been in the Czech Republic for only a few months — he was still living out of a suitcase — but his signing bonus gave him enough money to buy a house. The problem was, he wasn't sure where he might settle. His military discharge was pending — he could go anywhere. Plus, he was powerfully interested in collecting experiences that might inform his gameplay. In 2006 he had climbed Mount Cook, New Zealand's highest peak, and thought that it would be interesting to explore the idea of coding a mountain-climbing game. He also felt he was capable of much more than the lowly 12,316-foot peak, so he decided to pay $100,000 to climb Mount Everest. "Dean is part crazy," Spanel says. "In the middle of the most successful project of his life, he leaves for two months to climb Everest."

Using a Sabre portable satellite modem he brought to Nepal, Hall approved design changes and budgets and IM'd with the coders building the game. "So here I am, at Everest Base Camp," he posted to the Day Z development Tumblr on April 15, 2013, and then, from 17,717 feet, offered a rundown of the new features his team was coding into the game: The world would look more battered, there'd be radios for communicating with other players, and the zombies' movements would be more, um, realistic. Though he'd arranged the mountaineering trip in a burst of enthusiasm, he tried to convince everyone otherwise. "Although the timing is poor for my sabbatical, it is not something planned on a whim and involves nonrefundable costs of up to $100K."

On May 16 he began his summit attempt. The weather was clear, and he made good time: By 12:01 am on the 21st, he was less than four hours from the peak. The Hillary Step, a 40-foot wall of rock and ice and the last obstacle before the summit, loomed ahead. He started to move toward it when he heard a distress call from his climbing partner up above. "There's a guy dying here," the voice crackled over the radio. "What do I do?"

Hall ascended to the dying man's position; it was a Bangladeshi climber from another group who had been left for dead. He was clipped into the same rope they were ascending, and he wasn't moving. Hall's sherpa grabbed the man's hand. It was limp; he couldn't tell if the man was breathing. "His position and posture symbolized absolute desperation and sadness," Hall wrote later in a blog entry.

They were only an hour from the summit, and they debated whether to abort their climb to try to save the man. Hall's partner was distraught — he wanted to help even if that meant abandoning the climb. Hall had no such ambivalence. "I thought I would have reacted differently, but when I looked at him, I realized that there was just nothing we could do," Hall says. "I felt really sad, but I figured he was dead or he was about to die." Hall persuaded his partner to keep moving, and they scrambled past the dying man. "It was a Day Z moment," Hall says grimly. The man died, and his body was left behind, encased in snow and ice.

An hour later, Hall reached the summit. "The sight was so breathtaking it was like being slapped in the face," he recalled on the blog. "I immediately started crying ... I've thought a lot about how to summarize that feeling, and the best I can do is to say that if there is a God, then it's like looking upon his face."

Four days later he was back in Prague, funneling the experience into the haunting, morally fraught environment of the new Day Z. The game, he hopes, will force others to confront their own humanity. It comes out this fall.