Left 4 Dead 2 is the latest game in Valve's catalog to clamber out of the Steam for Linux beta. Unlike last week's Half-Life 2 news however, the extra something coming along for the ride isn't VR headset support -- it's a powerful suite of customization tools. The Extended Mutation System (EMS) gives the already robust modding community additional options for crafting one-off episodes and game type variants. For a glimpse of what EMS enables, play a round of "Holdout." This new multi-map mode introduces buildable items and the concept of resources to the co-op zombie-slaying calamity. What's more, Valve said it will add the most popular EMS creations to the official servers. Maybe with this, the world can finally witness our vision of the zombie apocalypse. Yeah, it involves marmosets.