Left 4 Dead’s 2008 release redefined multiplayer, injecting new life into the first-person shooter genre by emphasizing cooperative gameplay above all else. Series creator Turtle Rock Studios began tinkering with early versions of the concept on a whim, a bit of after-work fun not initially intended for public consumption. But even before the crew started toiling away on the genre-defining zombie game, creative leads had another grand idea lingering in the back of their minds, one that would later do for asymmetrical multiplayer what Left 4 Dead did for co-op.