Like Irallith, the surface of this realm is uninhabitable - at least during the daytime. Oberon was forced to cover the portal from the Nexus with an artificial mountain before it was safe to use. The inhabitants of Stygia all live either underground, or underwater - the farther down, the better.
The most famous city in Stygia is Cerberus, a port city built into the side of the continental shelf, and the only place in the realm with a drydock big enough to hold a leviathan class submarine. And Cerberus has not one such drydock, but three. Train lines connect it to the Nexus' gate, some two hundred miles inland - and longer lines connect to other cities, though these are frequently out of commission due to cave ins, giant stone moles, or other problems.
The second most famous city in Stygia is the mythical Golden Wyrm, a self-propelled burrowing construct of incredible proportions, said to operate at depths where even the strongest of rock melts and flows like water, and only "surfacing" into the cold stone above to replenish its supplies of water - once each decade.
Cybernetics are common in Stygia, from simple mechanical limbs to replace those lost in accidents, to entire rebuilt frames designed for heavy lifting and survival in unstable mine shafts. Portable air supplies are a common accessory, and massive algae vats form the heart of any major community - providing both fresh air and food. Meat is relatively rare, though coastal or aquatic communities often have several varieties of fish. Fried chicken is an imported delicacy, limited mostly to the upper class. Cheaper alternatives include the ever popular rat-on-a-stick, as well as roast giant mole, and oroborus steak.
Recently, with the introduction of several advanced joint designs recovered from Eleghost, exoskeletons have become a popular alternative to full cyborg conversion.
The other recent introduction is a few insular communities of aquatic totem people from Chalkon; the locals look down on them for their reliance on flesh over the strength of metals, but can't deny the efficiency of not needing complex rebreathers and personal heating suits just to do work in the icy depths outside. Mer-kin or mer-crazies are the common terms for these groups - regardless of exactly which totem animal they embody, or how altered they actually are.