Left 4 Dead came at the right time, at the height of one of zombie lore's many upswings in cultural popularity. Heck - Turtle Rock Studios' four-player, survival co-op claim to fame might have even reignited the undead fire. Opting for a hip, 28-Days-Later-approach to rabid, flesh-mongering hordes, Left 4 Dead forced us to watch our teammates' backs, punishing brazen morons who assumed they were as bulletproof as they are in any other shooter. Elite zombies with the ability to paralyze members of our squad only fueled our codependence on fellow survivors.We've seen other co-op shooting galleries like the Payday series steal from Turtle Rock's vault of online ideas, but none operate on the same uncanny, zombie apocalypse premise, and that will always make Left 4 Dead special. We'll also never forget the genesis of another idea - a computer A.I. known simply as "The Director." This omnipresent, virtual overseer was tasked with the procedural spawning of every Zombie and every horde, changing the flow of every playthrough, thus maximizing Left 4 Dead's all-around replayability. With all the endless runners out there, the Xbox One exclusive Below, and the procedural process that is currently driving the creation of PS4's No Man's Sky, self-propagating and dynamically reactive level design is a bona fide fad and a key ideology in the evolution of modern gaming technology.