Humans and related kindred who explore the underworld, even if blessed with the sharp eyes of dwarves, hemianthropoi and elves, face as much danger from falls, twisted ankles or other injuries in the darkness as from any monster.
If they attack a group from the surface, intelligent creatures who are familiar with surface-dwellers will usually attack whoever is mapping, or the torch-bearers. Other creatures are also likely to attack the torch-bearers. However they are likely to try to attack the torches themselves, believing them to be living things.
Some who enter the underworld have been known to first buy whole herds of swine, and drive them before them to set off traps, draw ambushes and so on.
This is sometimes successful, but has the side-effect of making the inhabitants of the underworld more aggressive. Many an adventurer has been angrily greeted as "the bastard who keeps sending me all this spam."