An early prototype of Turtle Rock's Left 4 Dead has leaked on Steam, and it's completely playable. According to game historian Tyler McVicker, the prototype is the first time that the general public has seen early footage of the acclaimed zombie shooter. 

Turtle Rock co-founder Michael Booth has stated in Left 4 Dead commentary (as PCGamer acknowledged) that the game began as a Counter-Strike mod. But it's not every day that a game's prototype gets randomly uploaded, and let alone in a playable state.

Originally known as Zombie City (or Terror-Strike), the Left 4 Dead prototype was a mod for Counter-Strike: Source recently uploaded to the mod site GameBanana over the weekend as part of a larger mod leak. In it, counter-terrorists are bots who swarm players and only attacked with knives.

The mod's uploader, Wolfcl0ck, said they made some quality of life changes, but otherwise left the mod untouched. "It still retains the map's basic setup, essentially making it how it was supposed to look if not for engine porting issues," they wrote. 

In the aforementioned commentary, Booth said that while creating bots for Counter-Strike: Source, Turtle Rock (then called Valve South) realized that "a few of us armed to the teeth with automatic weaponry against 30 knife-wielding enemy bots was a lot of fun." 

"That basic kernel of ‘small team of friends against hordes of clawing enemies’... we soon realized that the ‘co-op vs the horde game’ had a ton of potential."

After its fall 2008 release, Left 4 Dead received critical and commercial acclaim, and Valve acquired Turtle Rock that same year. The game's campaigns were eventually ported over to its 2009 sequel, Left 4 Dead 2, and the studio was re-founded as an independent studio back in 2011. 

Left 4 Dead has since gone on to influence recent titles such as Evil Dead: The Game and Tom Clancy's Rainbow Six: Extraction. It's also heavily influenced Turtle Rock's recent title, the 2021 zombie shooter Back 4 Blood.