|The Underworld ... cold, wet, full of ghosts.|
(The Elder Scrolls: Skyrim)
The Book of the Dead by White Wolf will give you the most information on the underworld including possible enemies like Kerberoi, powerful ghosts and even geists.
What are the Kerberoi, you ask? Well they are guardians from the different realms who violently assault anyone who violates the rules of that realm. This is in keeping with several mythologies where people must know the special rules in order to pass safely through the underworld. Simply being alive in some of those domains breaks those rules - or requires the enforcement of others.
And geists? Well, they are a mutated and ancient soul who has come to embody the death they have experienced and who sometimes claim mortals in exchange for the mortals' resurrection. They then whisper in the mortals' ear, show them the death that lies around them, and offer much greater powers than those generally found in spellbooks. Such possessed mortals are called Sin Eaters as they are often pushed to help the dead rest. As Sin Eaters retain their racial and path-based special abilities and merit options, they are often quite powerful indeed (see Geist: Sin Eaters rulebook for more details).
In Fianyarr, the default assumption is that the underground passageways all eventually lead into the underworld itself (barring the magma strewn pathways, of course). In keeping with this theme, there are a few things to hink about.
First of all, caves that link to this vast underground labrynthe will quickly develop myths surrounding them unless they are far away from mortal settlements. Therefore have a think about which caves or holes have strange myths associated with them - some may be accurate and some will be wildly inaccurate. If no one comes back from entering a certain hole then it could be a verge into a wound, a nasty spot in the hedge or the a gateway to the underworld. If those who do come back are somehow change it could be because they were claimed by a spirit, replaced by a fetch, or possessed by a ghost. It's not easy to figure out the answer. Don't feel you have to give the game away by telling them right away. Give them clues and then force them to investigate the mystery to know more.
Secondly, why don't the ghosts simply leave? Perhaps they can but there are guardian creatures sitting at the entrance to repel them or perhaps the ten races must create wards and barriers to turn away former loved ones and force them to pass on lest their world become overwhelmed by the dead? Perhaps a little bit of both? It might just be that the dead can't leave unless certain conditions are met - such as summoning rituals or certain days of the year or conjunctions of the sun and stars. How might this affect ceremonies and celebrations on those days?
Thirdly, if the characters delve into the caves think of the subterranean worlds below and turn to resources on caving and cave horrors when running adventures. Natural hazards like dangerous gases, falling rocks, cave ins, and pressure changes could affect the living even if are of little consequence to the dead.
Finally, what else might live down there with the ghosts? What happens if a pack of wolves descend and evolve to exist down there? What about werewolves? What about a group of people who hid from bandits and got lost down among the passageways? What if ghosts interbred with the living to create whole cities of dhampyr (or something else)? What sort of ruins might exist in its depths? What sort of magical items or talismans?
Oftentimes you can get inspiration simply by taking a fantasy trope, situation or creature from a different system or world and come up with something truly unique by placing them in a land of death. Even drow start to look fresh when you consider that their queendoms would now sit in the lands of the dead. So feel free to pick up your Pathfinder books, your fantasy novels, or even certain post-apocalyptic modern novels and wonder how that particular element might work in this strange new world.