The Subtenebrae is the name for the living underground that comprise the lands underneath the surface.
The world underground is divided into two categories, “living” lands and “dead” lands.
Living lands have water, and and an ecosystem. The rocks and the stone in these sections are still growing and moving, with magma vents and geothermal areas relatively common. Water in these areas provides for mineral deposits, stalactites and stalagmites, and sediment flows. The water in these areas also provides life to various plants and fungus, many of which give off a bluish, purplish, or greenish bioluminescence. You can tell when you have reached the Subtenebrae by the dim lighting, or so they say. On occasion, stretches of deadlands separate portions of the living lands from each other, and can be considered a lifeless wasteland.
Deadlands are normally relatively near the surface, and are the type of underground caverns normally encountered by surface dwellers. Deadlands have relatively little water, and virtually no geothermal activity. Some pockets of living rock can be found along access routes to the deeper portions, and some mighty mountain ranges of living rock break completely through the deadlands and thrust themselves into the air of the surface.
Both areas of the Subtenebrae are subject to changing geography. The “true” subtenebrae is more likely to have access points collapse, rivers change course, or magma block off a passage than the deadlands, as locations shift in relation to each other.
Significant locations and major terrain features normally stay in the same relative position to each other, but travel routes that were viable yesterday may be gone tomorrow. Some locations have major cavern systems or underground rivers leading to them, and these areas have become more well known and or infamous in the lore of the underground.
Rumors and legends abound of the multitude of mysterious creatures that live primarily underground.
Goblins: live in caverns and tunnels systems relatively near the surface. They straddle the divide between the surface and the underground. Living and dwelling in one, and raiding and scavenging from the other.
Kobolds: are the miners and diggers of the underground races. Their skills, while not as sophisticated as the dwarves, allow them to sniff out and excavate ore, producing metal to trade with the surface, or that they use to create crude weapons and armor for goblin and ogre armies.
Dire Corbies and Hook Horrors:
Duergar: are distant cousins to the more civilized hill and mountain dwarves of the surface. Mountain dwarves may dwell deep, but never as deep as the duergar. The duergar are slavers and are one of the more common threats of the underground. They live in isolated communities in large caverns with very dimly lit fortresses built in the fashion of surface keeps and castles.
Morlocks: are degenerate humans long lost from the world of light, and have regressed through years of subterranean dwelling into ravenous, barely thinking beasts of the endless night. They no longer remember the civilized lives their ancestors led, although many morlock tribes still dwell in the shattered ruins of their ancient homes. Some morlock tribes have been subverted by unfathomable powers from beyond the darkness.
Dark Creepers and Dark Stalkers: