After running a few of the tiefling and cultist armies, I've started to wonder what they eat and drink. The main Worldwound campaign setting book describes a series of diseases and infections that plague any potential food or water source in the Worldwound. After all, even Purify Food and Drink isn't meant to work.
Presumably tieflings and cultists wouldn't be immune to these so what do they eat? It's not like Abyssal creatures are likely to leave some space open for pasture like devils might do. Heck, considering the Abyssal energies seeping into the land itself from the Abyss there's a good chance it wouldn't even be possible for a demon lord to achieve.
Those along the border could theoretically launch raids for food, but what about those further in? Is everyone infected by disease? Should they all be presumed to have an invisible 'regional trait' whereby they are infected / contagious but no longer feel the effects?
As was depicted in The Worldwound Gambit, you pretty much have to bring your food with you. Or resort to cannibalism, which the cultists are certainly more than willing to do. That and steal from the Crusaders who are usually smart enough to bring supplies with them.
Some Tieflings may have the alternate racial trait which allows them to eat ash and such ( I think that is from Blood of Fiends).
Also in the aforementioned novel, you see that many of the human cultists are indeed infected by the diseases and whatnot, but their fervor (or maybe fever?) pretty much gives them a 'who cares' kind of attitude. Of course, the more they can align themselves with certain demons or persons with the ability to cure (or at least stave off the effects of) their disease, the better- and the demons encourage this kind of sycophantic behavior.
It probably helps that so many of the bad guys are cultists then. Abstemiousness can also make a small amount of food go a long way. Pretty neat trick for the demons, inadvertent as it may be, since the best way to survive is to become a cleric and the easiest way to become a part of a religion that'll survive is to worship a demon lord or something similar.