In 2004, Turtle Rock Studios were playing around with the game code to Counter-Strike Source to allow an increase in the bot count to allow unrealistic amounts of bots on select teams. They made it so those bots could only use their knives and eventually re-skinned them to look like they were undead. They had so much fun with the concept they started a prototype named Terror-Strike and pitched it to Valve. Valve enjoyed the concept as much as TRS did and green-lit the game, along side providing aid to the development of the game. This prototype had evolved into Left 4 Dead.

In early 2008, Michael Booth, the then CEO of Turtle Rock Studios, was done running the company and sold the rights to Left 4 Dead and Turtle Rock Studios to Valve. Unknowingly, this would of caused the downfall of the art direction and difficultly that Turtle Rock in-visioned for Left 4 Dead. Valve took full control over development of Left 4 Dead around May of 2008 and redesigned the game. Valve never truly finished what needed to be done in Left 4 Dead and there was hardly any progress with the code from May of 2008 to November of 2008.

This collection of add-ons helps to make Left 4 Dead (and in this case, Left 4 Dead 2) feel a lot more like the Turtle Rock counter-part. Please keep in-mind that all of these add-ons are fan creations and do not fully reflect the original designs or detail that Turtle Rock Studios had put into their game.