Valve has a double dose of content in store for players of its addictive co-op horror franchise. The company today revealed The Sacrifice, an add-on campaign that extends the experience of both Left 4 Dead and Left 4 Dead 2.

In addition to the campaign extension, Valve is retrofitting L4D’s No Mercy campaign for L4D2, adding melee weapons as well as new infected types missing from the first title.

The Sacrifice chronicles the events depicted in L4D2’s last expansion, The Passing, from the perspective of the original game cast — Bill, Zoey, Francis, Louis. Additionally, it provides backstory on how the old crew met up L4D2’s band of survivors. Near the tail end of the campaign, one survivor commits to a sacrifice. This time, however, the choice of who dies is entirely up to players.

Again, The Sacrifice is playable on both L4D1 and L4D2. The only difference between the two is that the former lacks melee weapons and infected that were added in the sequel. It’s a clever move by Valve, supporting both titles at once.

Look for The Sacrifice to hit Steam for free and Xbox Live Marketplace as a paid offering. Screenshots will likely arrive on Monday — the reveal comes by way of a GTTV segment that aired earlier today.

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