State of Decay's 'Breakdown' downloadable content will be coming this month. Undead Labs says the content has been submitted to Microsoft for QA testing, and should be hitting certification testing by late next week, in time for a release by Halloween.
The official site (via Videogamer) notes that things could still change, but they're attempting to be as transparent as they can. Breakdown should be available by the end of October, "assuming no catastrophes." This is all said in the context of Xbox 360 testing, so the Steam version is likely to follow later.
So what is Breakdown? A separate post from designer Geoffrey Card pitches it as a much more difficult spin on State of Decay. It will ratchet up the difficulty each time you survive a scenario, by tweaking some key variables. You'll slowly take more damage from zombies, each zombie will require more effort to take down costing you stamina, the fast zombie and feral zombie population rises, and resources and vehicle options become more scarce.