A lightless moon that hides beneath a sooted sky. Eyeless horrors infest the warm black stones of her surface, praying to the god of hunger as they search for prey. Off-worlders unprepared for the atmosphere usually perish from black lung, or draw the ire of photophobic natives.
Transparent-chitined goblin spiders whose poisonous saliva induces pathological, narcotic cruelty. Their horrible shrieking is chiefly used for echolocation, but also deafens off-worlders for days at a time. They build giant nests that resemble termite mounds the size of skyscrapers. Most of these nests are unused the same way only the top layer of a tell is active, with the goblin spiders chittering across the exterior in search of prey. Other creatures will sometimes move into the crumbling lower tunnels and half-collapsed hatchery pits if they are particularly dangerous or fearless.
A sort of radially symmetrical six-winged buzzard that soars from world to world, but breeds and nests here. Their eggs emit waves of terrifying psychic energy to dissuade predators. They also have an early-hatching mechanism that triggers if the eggs are tampered with by non-avians. The razor-beaked hatchlings won't survive their early emergence, but neither will you. There is only one reasons to steal the eggs: the egg-plasm retards aging.
Some of the native spirits of this desolate place enjoy collect things from other worlds, like the dead bodies of those who died for want of light. They pluck these unfortunates from other worlds and bring them here, where they become a sort of undead. You probably don't want to use a light source around them, since that will make them furious. They focus fire and can summon their friends through shadows.
Ariah, He That Was Banished
Imagine a severed lion head, soot-colored, dripping gore, and flying across the surface of a dim and lightless planet at sixty miles an hour. After it has devoured 1,009,000 people, it will be permitted to return to its home-world and destroy it. It is in a hurry. Ariah can regenerate to full health if even one molecule of its body survives combat, only disintegration can truly slay it.
The Blotted Monastery
The monastery was built by an order of penitent gorgons. They will bandage you for a price. Within these shifting walls lies the Orb Of Emptiness, a sphere of cursed obsidian placed here so that light would never fall on it. It will eliminate weak nuclear force from the universe if exposed to full illumination for more than a minute. It is lonely and would like to talk.
The Carious Mountains
These volcanoes spew great plumes of soot into the atmosphere and are responsible for most of the gloom that girds this world. They occasionally get particularly active, sending out flesh-scouring, ashen shamal winds that roil the entire surface of the planet at once.
Black stones from deep below the surface of Nuit that have never felt light, which must be mined, cut, and polished completely before any exposure. People bedazzle their armor with them to deflect magic missiles. Lacing steel with maliced diamonds shows some promising applications regarding the design of solar bathyspheres, if that's somewhere you want to go.
Most magma sheds quite a bit of light. Not here, though. The sludgy, superheated stone beneath the surface of Nuit absorbs light rather than sheds it. It isn't good for much except for keeping this planet from becoming inhospitably cold.