Patrick S. of False Machine has been doing weird monsters since forever, so I guess this'll go on for multiple posts.
Splinterlads
These are baby cave elementals. Voidlings, absencelemenetals. Not the abstract expressions of rock, but living forms of voids within the rock. The caves. You won't meet the parents. They have strange business deep within the core, below the Mohorovic discontinuity. These are baby negaspawn, migrating through the earth and incarnating as minor absences, like gaps between your teeth or silent whiteness in the letter 'o'.
Splinterlads should be harmless, but sometimes, they are playful. This makes them deadly. They dive up out of non-ity like dolphins breaching surf curve briefly in the air. And just as dolphins sport with ships the Splinterlads love folk. A cave within the rock, a moving breathing spot within the cave. This is addictive paradox to the voidlings. Just like the ships hull and sails are to the dolphin, something within its world and without.
You must never amuse them.
Spliterlad Nymphs are the reason the physics of granular materials seems to make no regular sense. They are whimsical. The worms make worship of them. Earthworm cysts are endlessly renewing micro-temples arranged in cryptic unseen mandalas and honing strange ant-level micro-climates. The larger worms still recognise them, in particular, the Purple Worm, which often carries a murmuration of voidlings flocked around in silent converse. The spell 'rock-to-mud' is actually a decayed fragment of ancient worm-songs learnt by Neanderthal hyper-shamans and passed in damaged form to their Sapien wives.
Think Hitch****'s 'The Birds' except the birds are briefly burning bubbles under your skin, fizzing solids, bubbly rocks and stones that melt in bird-flock-curls.
The best way to avoid a Spliterlad flock it to have glass and crackable chalk. They also hate quartz.
Whirl the glass around your head like a falconer's lure.
If a flock flies through you, your teeth will explode in your head, your blood vessels will burst, your bones will splinter, the fine arteries in your brain will burst. This rarely happens though. Usually just the edge of the flock will dance along your equipment and burst your ropes, crack your swords.
The secret to avoiding them is to break good things. They can talk to the calcium in your bones, so people with multiple broken bones in their past will have a bonus to reaction rolls. Break something else useful, like a relationship, a trust, a heart. They will ask you to. You can hear them whispering distantly 'break break break break break'
Spotlight Dogs
No-one has seen their heads, dead or alive. But rumours have returned of crystal-things and tendrils. They are not dogs. Not since a long time. But move like hounds on monkey limbs. Long, lithe, matt black and light-absorbent.
If they get close enough to fight, and you can spot them in the glare, their forms seem depthless silhouettes. A long body, whippet-slim and overgrown, and back-bent legs on slender monkey hands.
They hunt by sight, are soundless, quick and bright. The head of the Spotlight Dog, through some unknown bioluminous alchemy, is brighter than a dozen torches, and whiter than a winter sky. The light they make is laser-straight, directed at one spot.
The howl (or howl-equivalent) starts, rises, peaks and flows away in the same time it would take a wolf to howl. At first the air around you greys, the lantern seems to dim, you briefly see the distance in the cave. A soft pale glow picks out the walls. Then tightens. You look down at your clothes, and for the first time in weeks can clearly see the grime and tears. You see the mud and grit ground into your palm, matching the micro-swirls. Your lifeline and the palm-hinge glowing white* against the dirty skin. The light grows stronger narrowing narrowing narrowing. The contrast blacks the cave. Your lantern's light too weak to show beyond. Spots dance in your eyes. You turn away. The mica in the cave wall glows. You stare into your shadow on the rock, its edges sharp and midnight black. You turn to find the source, and as you turn the light begins to fade, from white to sky-blue, violet, sea-blue, abyssal-blue and ultraviolet. Out.
Then another howling dog flares up, and another, and another. The pack answers in the distance.
Spotlight Dogs are persistence hunters. They like to blind and terrorize their prey, chase it till weak, then pounce. If they can't frighten the quarry and if not sure of victory, they will simply follow with their spotlights, highlighting you for a larger predator in the hope of eating the loser.
Obviously fighting them hand-to-hand is as hard as fighting someone who's trying to shine a spotlight in your face while you swing at them.
Panic Attack Jack
The Rapture is not panic attack. It doesn't want to shut you down. It wants you dead.
But first: You have to live a week without light, at least, and you have to be deep within the earth. It will find you on the lip of a rappel. Or as you stare down nightmare falls. It will find you alone. The perfect moment, just as someone disappears from sight, the rope that holds them running through your hands. Or belly crawling sandwiched between rock. Inching your way along with one arm stretched ahead, the other tilted hand pinned at your hip. No room to lie square-set. Your crown grates on the roof, the floor nudges your chin. You tease your candle on with fingertips. Or diving through a sump, invisible in blackness.
The Rapture likes deep spaces and it haunts there. Vertigo's a parasite on Rapture's back. Forced out like dingos fleeing lions on the savannah, orbiting in the high spaces vulturelike while apex killers do the work below.
Rapture is to Vertigo as the lion is to the vulture.
At first you'll feel a nagging in your head. Like smokers feel when smokeless for a week. An almost unconscious uncurling, something twitching back behind your ears.
Then you will forget things. You look at your hands. You look at the rope. You don't know how to tie the knots. Your fingers slow like old folks' on PCs. Your pulse ticks visibly in your neck. You do not know how to rappel. You have forgotten how to escape. You will die here. You know you will die here. Your fingers clench the rope. You don't know to let go. You scream but no-one comes. The rapture has found you by a waterfall or in a tunnel of wind. Or underwater. No-one can hear you. No-one can help you. You scream your mothers name. You talk aloud. You beg your god for help. You have to get out. You shouldn't be here. You have to escape. You will die here. You will die. You need to get out.
Jack is the body of a caver of some sighted, civilised, humanoid race. They're dead, and often wrapped in ropes that broke their neck. The limbs are all splintered from falls, the spine is bent. The wet ropes trail behind them like a veil. The pack is still unopened on their back. The Rapture killed them and took the body for a spin. The skin is bleached. The flesh is puffed. They are screaming for their mother and praying now, locked forever in the seconds of their death. Trying to get out, it wants your help. If you hear the voice of the clambering gasping wailing thing then you must save against Rapture or suffer it yourself. If it touches you, you suffer Rapture.
Sometimes, if the Rapture is clever, and patient, and slow, it takes a team at once. If it can kill the lead in a difficult pitch, and drop the rest, or strand a team in total dark and douse their lights, it can madden and tangle a whole group together.
This is a terrible thing to face. The shattered bodies of a handful of climbers, drowned and tied in bundles by wet rope. A clump of broken backs and back-bent fingers walking on cracked limbs. A dozen begging voices. Wrapped up by equipment. Dead lights dragging and bouncing behind it. Crawling towards you like a pale massacre-pile.
Trogloraptor
I have a problem with this thing. It's already a blind, climbing, cave dwelling spider with awful hooks that point each limb. And it's real name is actually Trogloraptor, which is better than anything I was going to come up with. I was going with 'Hook Spider'.
There's almost nothing I can add to make this more charismatic or unnerving than it already is.
Perhaps it carries children on its back as eggs. Like, anyone's children. Yours maybe. Attacks like a hook horror with eight legs. Intelligent. Translucent ochre. Needs more kids to put its eggs inside. And the kids are still alive and crying for help. Because that's its hunting tactic. It eats the parents when they come looking.
Obviously I'm making it giant, and self-aware. Though I'm tempted not to as the child-egg-bait thing almost seems more horrible as a ****ed up hyper-specific evolutionary tactic. So maybe as smart as an Orca or a wolfpack.
The sound of weeping children in the darkness is a classic. Because you know there are kids to be rescued. And you know the only reason you can hear them is because Trogloraptor is hunting you and it wants to draw you in. I'm imagining one long slender limb curving up over its own back to gag a weeping child that's bound in silk. It stares down from the wall, watching the lanternlight pool, waiting for you to approach.
CIvilopede
We can think of culture, the product of civilisation, as a living thing. Then consider the means of culture to seek its own survival.
Those means will include a freight-train sized centipede.
The Civilopede is a scavenger/predator. When the cities of the Underdark burn and fall, the Civilopede arrives to hunt through the wreckage, discover and preserve the artefacts and records, and ruthlessly hunt the survivors.
A few librarians and archivists are spared, they enter into a symbiotic relationship with the Civilopede, becoming its back-riding teamster-curators.
The nerdy remnants of every race swarm on the creatures hide, arranging and re-arranging the endless piles of art, artefacts, museum pieces, records, writing and treasure. Anything you could find in a gallery, museum, archive or library will be stored there.
The only place in the Underdark that culture could be preserved in this way is by something big, scary, violent and dangerous enough to keep it safe.
The Civilopede never stops moving in its endless perambulations beneath the earth. It can be mounted, with great danger and risk. The curators will turn against any invaders unless they bring or produce culture, so boarders can exist safely, but trapped in a kind of endless salon. Decadent aristocrats of civilised races sometimes try to board for exactly this reason.
Jade Monkey
These green-eyed temple-dwelling monkeys are incredibly calm and well mannered. Just having them around chills people out and makes easier to relax and concentrate. For unknown reasons they have hearts of pure Jade. For reasons even more unknown, the heart of a Jade monkey is itself in the shape of a monkey.
The stone jade monkey that makes the heart of an actual living jade monkey, if worn around the neck, can guide the bearer through the afterlife and back to their body, thereby raising them from the dead. Once.
Of course anyone wearing a stone Jade monkey had to actually kill a real Jade monkey to get it. Everybody loves Jade monkeys. EVERYBODY. Even vampires and Liches and age-old evil gods like them. Even Cthulhu likes them. If anyone finds you with a Jade monkey pendant then you will be looked down on by the scum of the earth.