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Team Fortress 2: Invasion

Team Fortress 2: Invasion was an unreleased iteration of Valve Software's Team Fortress 2 after Team Fortress 2: Brotherhood of Arms. It was revealed with the leak of Half-Life 2 in 2003 - the leaked source code included the source for Team Fortress 2: Invasion and the leaked build of Counter-Strike: Source included two player models (temp_player.mdl and alien_commando.mdl) from the game. Some other images has since been found on the sites of Valve developers.

Images from Chuck Jone's website