Valve has activated this week's Mutation for Left 4 Dead 2. It's called Four Swordsmen of the Apocalypse, which sounds like a decent band name too.

In Four Swordsmen matches, the Survivors are armed with nothing but swords. To make matters worse, you'll be faced with "near continuous streams of Special Infected." Should be a rough time but at least you'll get to make some headway on the melee-oriented Achievements.

Mutations, if you're not aware, are experimental game modes that run for a week. To play them, you need to install "The Passing" DLC.