If you're a fan of Valve's work, you probably know that one of the company's most popular and beloved games, Left 4 Dead, started as a mod for Counter-Strike. The original developers, Turtle Rock, created a mod for the shooter named "Zombie City", which may be the earliest known prototype of Left 4 Dead to date.

Left 4 DeadImage credit: deviantart.com

As modders and dataminers report, in the recent Counter-Strike 1.6 update, Valve "accidentally" added an early prototype of Left 4 Dead called Terror Strike. Through manipulation and rituals, enthusiasts managed to launch this mode.

In the role of "zombies" are the special forces soldiers, who reappear on the map a short time after death in a map called zombie_city. A group of terrorists needs to carry a bomb to a house. After setting the explosive device, all the "zombies" suddenly appear on the map — this is the "horde" (you can hear the same monster howl).

Victory is counted if the terrorists survive the "horde" attack or kill all the "zombies" before the bomb is set.

Considering all the hype around Half-Life and Counter-Strike lately, one might think this is just another fun release from Valve in honor of some of its games, but it seems not. The code dump for this prototype appears to be completely accidental.

As former Valve employees admitted, the early prototype of Left 4 Dead was sharply criticized by studio head Gabe Newell, as no one really knew how to balance games with asymmetric multiplayer and a PvP element. Eventually, the entire approach to the project was reconsidered, shifting the central focus towards close team cooperation.

Main image: Reddit