Here, here.
Perhaps I should have said I wanted to be a separate part of the setting from the Underworld, as that's what I meant. As you pointed out, the underground being almost entirely separated from the rest of the setting as a whole is part of the problem. But while the Dead should have presence (I think they do canonically, anyway) I don't want them to be the main attraction, so to speak.
What interested me in the Underground (I'm just going to call it that from now on for simplicity's sake) in the first place were things like there being powerful demanses down there that mutate the inhabitants of the Underground into monsters, and it being the last place where races from the Primordial era dwell.
I want a world full of fundamentally alien beings and places, rather than fleetingly alien like the Wyld, or the Underworld and Autochthonia where most of the horrors still feel too human; and those beings and places possess inherent, overt agency, unlike Demons. I don't want The Underworld Expansion Pack, as cool as it is in of itself.
*strokes chin contemplatively.* True, very true.
Me.
What about making the underground like the deep roads from dragon age when there is no blight, or the warp from 40k when you don't run into chaos. The darkness beyond the firelight, the very depths of the ocean trenches. The place where those without the light of essence just disappear.
The World Below
The realm found deep beneath the surface of creation is one filled with forgotten and abandoned horror and wonder that few bother to explore.
Deeper than most are willing to dig or mine, most entrances exist because an ambitious soul desired something real that wouldn't be found elsewhere in creation, others due to simple geological movements exists the world below.
In this place, there exist no unified kingdoms outside the mountain folk, but over untold millennia in the time before, the primordials sent every failed creation in there grand project below the surface. A seemingly endless conglomerate of races and unique creatures not seen as fit in the combined aesthetic of creation, or just not fit for purpose, shoved beneath the earth by the light of the sun unconquered.
This deposition shows in its residents.
The Forgoten Creatures
The creatures found below are different enough that most would assume demon, behemoth or fae. Like anglerfish to salmon, they share characteristics with creatures of creation, but are distinct enough that one could never be mistaken for the other. This was further exasperated by the 3 spheres cataclysm removing some of the things that would have made the links to other creatures more obvious. They vary in intelligence and power, and have distinctly alien drives.
For example, one of the pests is a transparent leech that devours the sound you hear then metamorpasises, and burrows into its victim to devour the images the host sees, does so once more and shifts to its victims throat to gorge itself of any attempt the carrier makes to inform others of its presence. It finally emerges from its hosts mouth as a monkey equal to them in size and gender in formal wear as recongnised by the host.
The monkey will then spread the leeches until the original victim recovers at which point it would return to them and dissolve away leaving the clothes intact.
This was an aborted design by some 2nd Circle Deva for a creature meant to train conductors for grand musicals enhancing the relevant senses, and leaving them a dressed and skilled orator. It left its victims ravaged, insane, and never stopped spreading until it was purged, or they were all dead. As such it was simply forgotten below.
The Mountianfolk
The mountain folk fight an unending war with that which dwells below not because there is a horde waiting to overwhelm them and wash onto creation. But simply due to there need to expand their tunnel networks on a regular basis to provide additional resources, and compensate for regions lost in conflict. Furthermore, the constant growth of material provided by the pole of earth, makes charts and geological surveys as accurate as maps of the endless desert, making avoiding this danger impossible
Every hammer blow could unearth a cavern filled with new horrors that could want to despoil everything you love, just turn everyone's hair pink or some bizarre combination of the two. This situation is the daily reality of the mountain folk who carved out an existence and an empire in the first age. But with the devastation of the usurpation lovingly shared with them by the exalted host, and the damage wrought by the contagion and crusade have strained their ability to maintain the holdings against the risks they must take to retain what little they have left.
The only benefit these events have bought to them is some degree of release from the great geas. It was sworn to the exalted host, and although the realm has some claim to be the voice of the dragonblooded, and the bronze faction the sidereals, neither has any claim to voicing the solars or the lunars. Released from the burden of obedience, and nursing a grudge from the loss the geas still inflict on them, the mountian folk, the last real bearers of first age technological might, are a force that demands to be treated as equals whenever they can
There is only one mountian folk empire. Or rather, all mountain folk claim to be part of the same empire, even if no central leadership or structure exists. To all outsiders, they try to present a united front, as the yolk of species wide slavery is still felt by every single member of the race whenever there thoughts start to run against some of the drives forced onto them by the Deliberative.
Mining
The total aversion to the sun means that mines in creation rarely go deeper than sunlight can be sent, with the greatest mines of the realm containing great mirror arrays to routinely purge there caves of what lies below. The lesser light of flame is sufficient to deter the casual creature, causing them pain and lingering discomfort, but the light of day burns them in ways none of them can endure.
In the past Solars, and in the present fire-aspects regularly simply patrolled there mines, animas flaring, to avoid the hassle of installing such an array.
All forms of deeper mining require some degree of support from the mountain folk to retain long term viability. And due to the Exalted hosts need to have their craftsman lackeys with them for their projects, pockets of mountain folk can be found underground all over creation, especially nearer the older cities.
All who seek to claim the wealth of the earth have to deal with the mountian folk. Moreover, they are the only race to retain any degree of creation wide first age infrastructure, meaning that no-one in power can be sure that any abuse directed towards the Jadeborn will no anger the entire race.
Mines in shadowlands have the added benefit of providing unique materials to a necromancer capable of drawing out there wonders.
Demanses
The abandoned projects of the primordials sometimes twist a cavern into resembling their aborted existence if left alone enough, and if they grow numerous, or powerful enough. This twisting in creations natural essense structures generating demanses aspected to there preferred habitats that slowly mutated other creatures into more of themselves.
Many craftsmen and sorcerers of the First-Age regularly used such demanses to make there creations truly unique, and the shogunate found some that replicated a few arts thought lost with the fall of the solars.
Unfortunately, the caverns containing these demanses are regularly consumed by the growth and churn of the pole of earth over the coarse of millennia, or the creatures within them killed to the last causing the demanse to fade like darkness at dawn.
Variations across Creation
Nearer the elemental poles things become more interesting.
The pole of water containing the most openings to the world below under seas so deep that light never reaches the bottom. This means that it becomes difficult to distinguish what should be down there naturally, and what shouldn't.
The pole of wood provides among the few regions that the creatures can be seen on the surface, in the thickest canopies where even light gets lost.
The pole of wind containing deep frozen ravines and caverns leading straight to the deep, making long blizzards that bit more dangerous.
The pole of fire filled with the creatures most adapted to the light of flame, which means its the place where mortals get the most contact with things they rather they didn't, but where there legends are most coherent.
And finally the pole of earth, the primordials preferred dumping ground, with the dubious honour of having the greatest variety, population density and concentration of accessible forgotten demanses.
