Left 4 Dead 2
Extensive testing has been done to ensure this campaign is playable on all difficulties, including Realism Expert. All secondary game modes (Survival and Scavenge) are fully supported by this campaign and most mutations should work with Fatal Freight as well, so have at it!
The storyline will not interfere with any of the games and offers the completely new setting, Borden city. Struggle to survive in an epidemic apocalypse alongside with the original 4 heroes.