Left 4 Dead

Left 4 Dead was released in 2008 by Valve together with Turtle Rock Studios for PC and Xbox 360, built on the Source engine. Four survivors must fight their way cooperatively through hordes of the infected from one safe room to the next, while the pacing and placement of the horde and special infected such as the Boomer, Hunter, Smoker, or Tank are controlled by a dynamic system called the AI Director. It reshuffles each playthrough of a campaign so the game stays tense even after dozens of runs, an unusual approach to replayability at the time. A signature feature became the so-called crescendo events, where a careless action by the players - such as tripping an alarm - summons a wave of enemies that must be survived before the path forward opens.

Just a year later, in 2009, the sequel Left 4 Dead 2 arrived with a new setting across the American South, from Savannah to New Orleans, melee weapons like a frying pan, baseball bat, or chainsaw, and additional special infected: the Charger, Spitter, and Jockey. Releasing the second game so soon caught part of the original Left 4 Dead's community off guard, prompting an organized boycott movement that called for the original game to be expanded instead of a new one being sold. Thanks to extensive post-launch support, further campaigns, the demanding Realism mode, and the Versus mode letting players control the infected themselves, Left 4 Dead 2 gradually became the entry the community keeps coming back to. The series helped define the co-op zombie shooter as its own genre, and years later its spiritual successor, Back4Blood, came from that same studio, Turtle Rock.