Valve announced today that The Sacrifice, a new add-on for its co-operative zombie thrillers, Left 4 Dead (L4D) and Left 4 Dead 2 (L4D2), has been released via Steam, for PC and Mac. The Sacrifice is the prequel to The Passing and takes place from the L4D Survivors' perspective as they make their way South. In addition to advancing the story, The Sacrifice introduces a new style finale featuring "Sacrificial Gameplay" where players get to decide who will give their life so the others may live. The Sacrifice DLC for L4D1 is a separate download than The Sacrifice for L4D2. Check full article for details.