Counter-Strike: Condition Zero

Counter-Strike: Condition Zero is a first-person shooter video game and the sequel to the original Counter-Strike. The game was released in 2004 on the GoldSrc engine. Condition Zero features a multiplayer mode, which features updated character models, textures, maps, and other graphical tweaks. Unlike other Counter-Strike games, Condition Zero also contains a single-player mission pack with the player unlocking maps and more efficient bots as they pass certain requirements for each map while playing as a Counter-Terrorist.

Unfortunately, this game has no official FGD because the maps were done with a different level editor that apparently did not require any sort of file. It also appears that this editor was not limited to 30 characters in a classname string, so some entities are unusable inside Hammer.