In Apocalypse, Typhus and friends had a formation that consisted of Plague Zombies. This has now been translated to the new Chaos Space Marine codex in a specific format.
With Typhus, the chaos cultists can be converted in to plague zombies. This has advantages: fearless and feel no pain being chief among them. That said, there are disadvantages: slow and purposeful and the inability to take optional upgrades that would usually be open to regular cultists.
One of the problems that has already been flagged with the wording of Typhus' rules concerns what counts as "upgrades". Specifically, whether increasing the number of zombies in a unit counts as an optional upgrade. I don't see that this counts as an option. An option is usually taken as being one of the listed equipment swaps (and so forth). Hence, huge blobs of plague zombies seem feasible!
For under 1000 points, one can now form the core of an army from Typhus and hundreds (well, perhaps up to 200 and something anyway) plague zombies. This is quite incredible really! Even if the plague zombies are weak, the feel no pain rule will keep many alive for long enough to get in to close combat to cause headaches, and to go to ground on top of a valuable objective. Not too many armies have the firepower to take down more than one hundred models ... and even in close combat, death star units can only take out up to tens of models in close combat per turn. Take thunder hammer terminators: a squad of 5, at worst, would take out 10 per turn. Against several tens of zombies, they're going to be locked up, tarpitted and whittled to death eventually (yes: even terminators roll 1's for saves, despite recent experiences!).
Depending on points limits, this leaves room for several fast attack, heavy support and elites choices (perhaps alongside some plaguemarines, or Nurgle daemon allies as well?). Building such an army feels appealing to me -- I've not done such a "horde" army in a while. And making it competitive seems like a neat challenge -- so I'll try to post a speculative army list once the codex is actually released. Is this flavour of plague zombie army speaking volumes to other people as well? Or is it simply my Nurglesque mind that has been corrupted?!