The Underhalls are the smallest of the Underground Realms and came long after the others. It only stretches throughout the Arganorh twin-continents. The Stoneborn dwarves began construction of the Underhalls when their empires on the surface were destroyed by orcs. Remaining cities on the surface, surviving but isolated, had no way of reaching each other, and for decades contact between the cities was rare. As a solution, the Stoneborn dug vast tunnels to connect the cities underground. Eventually they went even deeper, to seek more minerals and gems. From their efforts were built great halls and mining networks, which led to the founding of underground cities, which then later developed into the Under-Kingdoms, and their inhabitants became the Darkborn.

A wonder of architectural genius, the Underhalls' main roads were tall and wide, with smaller tunnels connecting to outposts and underground settlements along the way. Controlled lava travelled in canals along the underground roads, and lit them as if they were struck by daylight. But after the Darkblood curse broke out in the deeper city of Orthavir and corrupted the minds of its inhabitants (creating the Darkbloods), the Darkborn were forced to abandon several outposts, and eventually the Underhalls' roads decayed from lack of maintenance. 

Now the majority of the Underhalls is abandoned, and maps of the complicated roads are hard to come by. Expeditions venture into the Underhalls to try to retake its outposts or map its roads, but few ever return to the surface. Aside from Darkbloods, other creatures now make the Underhalls their home; some of which are very ancient and terrifying.

Three dwarven Under-Kingdoms still survive in the underground: Tonbarad below the Vardenfels Mountains, Orthazzar below the Mountains of Ash, and Kazh-Barok below the Dathrien Mountains.

Deeper, past the Underhalls, lie the Underneaths. The Underneaths is the largest of the Underground Realms; stretching throughout all the continents. Unlike other Underground Realms, the Underneaths is the one realm with the largest concentration of various fungal flora; many of which are almost as thick as a surface jungle. Light and warmth is achieved through volcanic activity from deeper down, and some caverns are lit by glowing gems and crystals. Not all of the Underneaths consists of flora, but some of the strangest plants tend to grow down there.

The Underneaths was home of the outer kingdoms of the sectizh people, and it was there they cultivated the majority of their food. Most sectizh eventually abandoned the Underneaths and went for the surface; the reason supposedly being Eldritch Gods who had enslaved the sectizh who lived further down, and wars between the sectizh turned to starvation, and eventually cannibalism. Nearer the surface the sectizh became Signis' Arachnyr servants or the Vithrid, but as the Eldritch Gods' influence spread, the Arachnyr and Vithrid empires continued to shrink and disappear.

Aedh Shai, Shadow Elves, came to the Underneaths once to escape the prosecution carried out by their former Aedh Ashlem brethren, and built smaller cities down in the deep caverns. Eventually they returned to the surface, having fled a deep darkness that corrupted their people, which they know as Shie'Cilei, "The Darkfallen Queen". Shie'Cilei's corrupted followers and worshipers are simply known by the Aedh Shai as the Shie'dari, "The Dark Children".

The Maéc, once part of the Maaré people, hid in the Underneaths to escape Tiral's banishment of the Tidewoven, and built a network of arcane kingdoms and cities connected by telemantic machines and portals. But the Maéc have long since disappeared, leaving only ruins to be found by explorers; ruins guarded by the Maéc Maegstruct golems, as well as the horrid mutations of their arcane experiments, the Maegwicht, who sometimes find their way to the surface.

Most Metallic Dragons also make their lairs in the Underneaths; coveting gems to create more of their children. They and their broods form several colonies in the many caves and continue to excavate tunnels throughout the Underneaths. Other dragons, some unaffected by the Feral Affliction that turned the majority of their kind feral, are also found within the Underneaths; having created hidden lairs for where they hide themselves, their eggs and their riches.

Other areas of the Underneaths closer to the surface are dominantly inhabited by goblins and gremlins, who often wage century-long wars against each other to fight over ancestral territory. It is believed that the first goblins came to the surface from the Underneaths. Gnomes supposedly also once called the Underneaths their home before they came to the surface.

The deepest and most unstable caverns of the Underground Realms are found in the Deepdark. It is the last realm before the Earthen Core and Worldsoul, and its darkness is inhabited by Earthen Gods and the Eldritch Gods. Enslaved sectizh are said to still be down there in their underground empires; serving the Eldritch Gods.

In truth there is no one who can say what is really down in the Deepdark, as no known mortal has been down there and survived to return to the surface. Tales stemming from sectizh texts found nearer the surface tell of a realm of magma and lava, of hardened, glowing gems and crystals that light up vast caverns. But there is supposedly also areas where the Eldritch Gods have made their lairs; caves where the walls aren't of stone, but eldritch flesh.

Past their lairs are the sanctuaries of the Earthen Gods known as the Earthen Core, where the Worldsoul is kept safe by their watch, and the prison of the fallen Creator Gaerdras was built. It is within the Earthen Core that many of the Earthen Gods became the Earthen Pillars; frozen in place as they slumber and hold up the world around them, so that the world doesn't implode into the Worldsoul.

It is believed that the Dragon Gods are somewhere within the Deepdark, where they have hid ever since the Worldblight and the Feral Affliction struck their kind. It is likely that they entered a deep slumber, waiting for when they awake to return to the surface and reclaim their lands and former worshipers.