Talking about this, is there any good source describing underworld mortal comunities?
(I am thinking to port Alhambra to Exalted for a game someday).
Actually, the 2e corebook says that mortals can totally chow down on Underworld food, as long as they don't mind being stained with necrotic Essence that will pretty much guarantee that their souls will rise as ghosts upon death, barring necromantic intervention to correct the spiritual imbalance. I'm not as certain about living inhabitants of the Underworld, but I think you could do neat things with that idea and there was definitely something in the corebook about pallid, waxen-skinned mortal Neverborn cultists who have never seen Creation's sun.
If nothing else, having there be small Underworld cultures would encourage further development of necromancy and necro-thaumaturgy, if only to explain how they deal with all the yidak. I think I had a writeup somewhere...
I'm working on something in this vein, actually. I call them Ghouls, to take up the niche of sentient corporeal undead.
Cliffs notes: Mortals can eat the food of the Underworld, but if they do it too long or spend enough time in the Underworld, they will eventually start being
of it, instead of only in it, and stop being of Creation. Meaning, acquire appropriate mutations, especially 'Creature of Death', cosmetic mutations, and an inability to digest the food of Creation, less commonly assorted poisons and being hard to kill short of decapitation or burning or the chunky salsa rule and the like, that will however leave them quite drained and hungry. Also sometimes they get mental problems such as addiction to human flesh or berserk rage. So a deathlord's plot could totally be to get a city hooked on Underworld grain that only he can supply.
Meat, coming from dead creatures, can count as food of either Creation or the dead, to give Ghouls in Creation a way to survive (badly, usually. Meat in premodern societies tended to be expensive.). Exception is the flesh and blood of the living, this always counts as food of the Underworld, and eating too much of it will turn you into a Ghoul too.
This means that there's quite a few tribes of Ghouls living in Creation too, pragmatically cannibalistic hunters and fishers in the north, ritual island cannibals in the West, vampiric bat people in the East that live off of the blood of animals, sewer rats in Nexus living in places too contaminated and poisoned for anyone else surviving off feral street animals and in lean times dug-up corpses and the occasional hapless passerbys.
It also provides an interesting in-setting reason of a taboo against and consequence of Cannibalism. Besides the Hungry Ghosts, that is.
Normal 'Lesser' Ghouls are merely mutated humans, they can still exalt. They also, however, have a path to enlightenment centering on consumption and cannibalism ('Greater Ghouls'), with Lunar-alike but weaker attribute excellencies and cannibalistic face-stealing, controlled self-mutation, some mental offense, far expanded poisons, or other more exotic effects. They lose both the ability to exalt and to leave a ghost though (assuming they had the latter to begin with), as both their hun and their po fuses together into their body.
Of note also, the difference between ghoul and zombie is somewhat blurry, rule of thumb is if it's mindless and a corporeal creature of death, feel free to treat it as zombie even if it started as a ghoul that lost itself, say by being killed and rising again too often.
In the Underworld, having far less power than even the weakest ghost, most ghouls will represent something of an underclass. Still, a greater ghoul should be a match for an equal-essence ghost. There's also ghoulish animals; Vultures, wolves, giant flies and spiders, and so on.
Inspiration are the Pomegranades of Hades, Ghouls and the Red Court Vampires from Dresden Files, Dark Souls, Skinwalkers and Wendigos of native american myths, and so on.