Underworld entities that couldn't care less about you or your interests.
1. Bouldermen: great rocks cursed w/startling intelligence, telepathic, sessile, ambitious long-term planners but must wait for forces of nature/incredibly strong creatures to effect travel, if smashed become intelligent heap of stones
2. Uurx the Impervious, immortal giant w/thick, lustrous coat of crimson fur, the softest, coziest, cuddliest imaginable, cannot be harmed physically or by magic, totally non-violent, roams the Underworld in search of non-existent mate
3. The Invisible Spectators: loiter near lairs of lethal monsters and terrors hoping for adventurers to happen by for a dust-up, issue rousing applause during melee (always supporting the monsters) but admire fierce combatants, in event of TPK they respectfully return uneaten portions of bodies to surface
4. Pure intellect embodied in colossal fungal bloom filling vast chamber, essentially death-proof, pulsates a good deal, automatically seizes the minds of any sentient beings w/in 150' but readily discards as boring/pointless, powerful telepathic broadcast just makes very rude comments in very loud psychic voice
5. Time bats flick back and forth in timestream, hover hummingbird-like, no interest in surroundings whatsoever, brain-scrambling trans-dimensional echolocation accompanies random arrival/departure.
6. Semi-divine giant black cave swallows w/feathers like plate armor swoop in, issue stunning wing-buffets, pin adventurers w/preternaturally mighty feet, seize all coils of rope (prefer 50'), gently release and its off to improve the nest
7. The Demi-material Horde: cast out of native dimension, indistinct humanoid shadows armed to the teeth w/shadow weapons, drill constantly, await arrival of sorcerer-messiah to make them fully material at which point they will destroy everything
8. The cold fires: levitating balls of blue flame roam the Underworld collecting information, will trade rumor for rumor which they dispense w/dispassionate objectivity, if proffered rumor seems lame, off they go on their way
9. Dejected deity, giant-size human w/head cradled in hands, blazing halo, gleaming platinum lute broken and cast aside, from time to time shakes tremendous fist at cold uncaring cosmos but otherwise unwilling to share feelings, acid tears endanger any nearby
10. Gigantic cave urchins extend their neurotoxin-pumping quills whenever approached by anything, an uncontrollable reflex, so no one ever gets close enough to hear their whispered poetry
11. The miles-long eyeless serpent keeps most of its mass down a bottomless pit, titanic head lolls about mouth agape, probably a good idea to throw some food in there as one passes by, many Underworld residents toss in their dead
12. Diseased demon-lord wracked by chills, exploding boils, streams foul oily sweat, projectile vomits poison at random intervals but still unbelievably powerful, must eat of divine flesh for cure, unless you've got some you may wish to get the hell out the way