One of Valve’s most popular IPs is back in the news, albeit for a rather unique situation. An early prototype for what would become Left 4 Dead has been leaked online and in a fully playable capacity. Going by the name Zombie City, the prototype is actually a mod that was created with Valve’s Source Engine, which in turn was used from another Valve title; Counter-Strike: Source. The mod was in fact the main inspiration for the entire Left 4 Dead series, as it is pointed out by franchise creator Michal Booth in the Left 4 Dead commentary track.

It’s leak season, apparently. The latest is a version of the Counter-Strike map that inspired Left 4 Dead. https://t.co/GUzt1DgcfP

— PC Gamer (@pcgamer) January 8, 2023

Despite the name, there are no actual zombies. Instead, players face off against AI Terrorist players who attack only using melee. Besides the series that it inspired, the mod is similar to other game modes types in other franchises. This includes Call of Duty which has a very popular mode called Infected, where players would all try to stay away from an opposing player to stay “uninfected.” Should anyone on the team get infected, they would join the other side and assist with infecting the team. Halo has also utilized this mode with Infection, and with similar rules.

Left 4 Dead of course was one of the biggest titles when it was released in 2008, taking the co-op aspect and meshing it in a zombie apocalypse setting in almost perfect fashion. Developed by Valve South, which would later become Turtle Rock Studios, the game was critically acclaimed and was enough for other studios to try and replicate their success over the years. The studio followed up with a sequel, Left 4 Dead 2 in 2009, which itself was mired in controversy right when it was announced. Despite this, it was also highly acclaimed, with many critics praising it even more than the first entry. Turtle Rock would eventually break away from Valve and release their own original IP, Evolve in 2015.