Survive as Trevor in a Los Santos overrun by Left 4 Dead 2's infected, drawn from your own copy of L4D2.
Los Santos has fallen. You're Trevor, free-roaming GTA V's open world while the streets fill with the common infected of Left 4 Dead 2: their own models, skins, clothes and animations, and their moans and screams, all read from your own Left 4 Dead 2 install.
Single player, in GTA V Story Mode only. The mod switches itself off if you go online. It needs GTA V Legacy and Left 4 Dead 2 installed. No files from either game are copied: everything Left 4 Dead 2 brings is read from your own copy.
Tanks, Witches and Boomers are coming in a later version.