Alongside the 25th Anniversary Update released last week for Half-Life, Valve also gave Counter-Strike a fresh coat of paint with its own update. However, in doing so, Valve also gave Left 4 Dead fans a bit of a present in the form of the first prototype of the co-op zombie shooter that originated as a Counter-Strike mod.

Per X user Gabe Follower, Valve’s update for Counter-Strike, which gave players the ability to “support many of the features from the Half-Life 25th Anniversary update”, also included the earliest form of Left 4 Dead: a mod which was referred to as “Terror Strike” when it was in development.

Gabe Follower went on to say that a user named “The One Epicplayer” was able to port the zombie_city map and did the bot navigation mesh, giving Counter-Strike players the additional treat of being able to see what Turtle Rock Studios had come up with during their brainstorming in the mid 2000s.

The mod goes like this: The terrorist with the bomb must make it to the house and plant the bomb. All the while, they have to deal with zombies (or the Counter Terrorists in this case), which will respawn after being killed. Once the bomb is planted, every zombie will respawn and a horde will come for the terrorists. The terrorist team must endure the horde by killing all of the zombies. The round is won when all zombies are killed while the bomb is planted.

To see the mod in action, check out the video by YouTuber ilovethevopo. And if you’re interested in checking out the mod yourself, you can snag Counter-Strike for dirt cheap right now until November 28th.

Valve accidentally compiled and pushed earliest Left 4 Dead prototype called "Terror Strike" into the latest CS 1.6 update and community figured out how to play it. pic.twitter.com/llVkYCip69

— ‎Gabe Follower (@gabefollower) November 23, 2023