Just as announced, the update allows for the Versus mode to played in the Dead Air and Death Toll campaigns and it also adds a new Survival mode in which you have to battle waves of brain thirsty zombies – something which, as you will see, is very, very difficult.
Also, if you still haven’t purchased this wonderful game yet, you can do it now for a lower price, since Valve seems to be ready for celebration: $39.99 for the entire package!