My absolute favorite thing in the world is when games existences somehow mirror their names. Seriously. I have very few other interests. I'm an incredibly boring person. But anyway, just as Duke Nukem Forever took forever to come out, Call of Duty is nothing if not dutiful in its adherence to a yearly release schedule, and Toilet Tycoon was awful, Left 4 Dead 2 has not, in fact, been left for dead. Sure, Valve hasn't lavished it with quite as much attention as, say, Team Fortress 2, but it's certainly attempted to empower the community's efforts - first with Cold Stream, and then with full Workshop support. But those mods? They're about to get seventy five point blue banana apple sauce percent modderier, because Valve's put its Extended Mutation System into beta.
Here's how it works, via the Left 4 Dead zomboblog:
"What is EMS? It is vastly improved scripting system that allows script authors to go past modifying existing scripts and write custom script logic, spawn and control entities, and much much more. Mod authors will now be able to create deep custom experiences that can be played on any map or tailor create something for new or existing maps."
In order to demonstrate, Valve's included multiple new modes - including a spin on survival called Holdout - and other various examples that flex the toolset's mighty scenario-morphing biceps.
A handy wiki, meanwhile, further explains the ins-and-outs of all of the above and also directing the Director. Unfortunately, it offers no advice on how to avoid the unsettling existential questions that you're sure to encounter as a result. Basically, though, here's the big takeaway: "The Extended Mutation System is now flexible enough to support the creation of Mutations that rival all-new game modes in complexity."
Neato, huh? For now, though, the EMS is only just beginning to stir in its beta crypt, as Valve's hoping to both squash bugs and add new features before releasing in full. Happily, in a gesture of (potentially prophetic) kindness, Craig put together this nice list of preexisting mods to tide you over. And hey, there are already velociraptors and lightsabers. The future of L4D2 modding is bright indeed.