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Are you a bad enough dude to
Alien Swarm is an update of an old Unreal Tournament 2004 mod, made by the original developers in their spare time during the development of Left 4 Dead and Portal 2. Oddly enough, it's not a first-person shooter!
For some reason, rougher versions of the Aliens from before Valve hired the dev team are present and working in the game, including an Alien Queen boss.
Full sourcecode and sound definitions for a zombie enemy (described in the source code as a "Swarm Civilian corrupted by SynTek chemicals into being a crazed zombie like thing") exist in the game. Unfortunately, there are no models or sounds, and it can't be spawned in-game. The code can be found in steamapps\common\alien swarm\sdk_src\game\client\swarm\asw_zombie.cpp and steamapps\common\alien swarm\sdk_src\game\server\swarm\asw_zombie.cpp once you've installed the mod SDK.
Heaven only knows how this didn't get in the game. Everybody knows that Valve luuuurve their zombies.
Pretty much all the UI strings from Left 4 Dead 2 are present, with a find/replace to replace all instances of "Left 4 Dead" with "Alien Swarm". This leads to some amusing unused strings...
"L4D2_Game_left4dead1" "Alien Swarm 1" "L4D2_GamePlaying_left4dead1" "Playing Alien Swarm 1" "L4D2360_NotJoinable_OtherTitle_left4dead1" "Playing Alien Swarm 1" "L4D2360_JoinError_OtherTitle_left4dead1" "Your friend needs Alien Swarm\nso that you could play together\n"