In early January, a Reddit user posted an emotional story about waking up on a beach and befriending a fellow lost soldier. But the soldier’s health began to deteriorate. And the author was eventually forced to kill his friend with the other man’s own gun to end his suffering. “His voice gone, I sat there staring at my monitor and began to cry,” the Redditor wrote. “I’ll never see that friend again and I miss him very much.”

"God damn," wrote a commenter. "Alright I’m getting this game."

The writer was playing DayZ, a zombie apocalypse multiplayer PC game that sold its 1-millionth download last week, less than a month after its Dec. 16 release. That release is only the game’s early-access alpha version, which developer Dean Hall will be enhancing and improving for most of the next year before launching it in beta. But even at this stage, the reason for DayZ’s enormous success is becoming clear—the game play leads to a degree of psychological tension and emotional response that players report never before experiencing in a computer game.

DayZ game makes you feel every murder you commit