Invisible Threat is a one map campaign. This is a map which was second in Biogen Corporation campaign. Players didn't have guns and had to fight almost invisible aliens. In this version the gameplay was changed to have guns and fight usual aliens. The map was originally created by uradzimiru.
Download
http://gamebanana.com/maps/190143
http://www.moddb.com/games/alien-swarm/addons/invisible-threat
Installation Instructions