The underworld lies beneath the whole of the Dreamlands and has a few entrances to it in various places. It is dimly lit by a mysterious phosphorescence known as the "death-fire". The underworld is inhabited by a variety of horrors, the most common being the ghouls.
After a moment something about the size of a small horse hopped out into the grey twilight, and Carter turned sick at the aspect of that scabrous and unwholesome beast, whose face is so curiously human despite the absence of a nose, a forehead, and other important particulars.
—H. P. Lovecraft, The Dream-Quest of Unknown Kadath
The ghasts are a race of fearsome humanoids that live in the vaults of Zin. They are much larger than a man and have a vaguely human face, albeit missing a nose. Their skin is rough and knotty. Their senses are unusually acute; they can see in the dark and have a strong sense of smell. They hop about on a pair of hooved, kangaroo-like legs, and are swift, strong, and agile. They have also been described as lacking a forehead. Ghasts prefer to dwell in complete darkness and have no tolerance for natural light — sunlight will kill them instantly. Otherwise, the dim, pale glow of the underworld seems to cause them little harm.
The ghasts are aggressive carnivores and often hunt in packs, though they are quick to turn cannibalistic when no game is readily available. They prey mostly on the gugs, but have no qualms about eating other denizens of the underworld. Their method of attack is particularly savage and gruesome, rending and tearing apart their victims with their muzzles, paws, and hoofed feet.
It was a paw, fully two feet and a half across, and equipped with formidable talons. After it came another paw, and after that a great black-furred arm to which both of the paws were attached by short forearms. Then two pink eyes shone, and the head of the awakened gug sentry, large as a barrel, wabbled into view. The eyes jutted two inches from each side, shaded by bony protuberances overgrown with coarse hairs. But the head was chiefly terrible because of the mouth. That mouth had great yellow fangs and ran from the top to the bottom of the head, opening vertically instead of horizontally.
—H. P. Lovecraft, The Dream-Quest of Unknown Kadath
Gugs are a race of horrifying giants. They are speechless, communicating only by facial expressions.
The gugs were banished to the underworld by the earth’s gods, the Great Ones, for an unnamed blasphemy. Now they reside in a terrifying, underground city, dwelling in lofty, round, cyclopean towers. Nearby, colossal monoliths mark the cemetery of the gugs.
In the midst of the gug city, the Tower of Koth contains a stairway that leads to the Enchanted Wood in the upper Dreamlands. There it is sealed by a huge stone trapdoor with a large iron ring. Because of a curse of the gods, no gug may open that door, though no such restriction prevents a gug from climbing to the very top of the tower.
Gugs prey on the ghasts that live in the Vaults of Zin (though prior to their banishment, they had been known to devour wayward dreamers). When in sufficient numbers, ghasts may likewise prey on the gugs. Though gugs would seem to have the advantage, they nonetheless superstitiously fear ghouls. The gugs often indulge in great feasts and, once engorged, retire to their great towers to sleep.
Nightgaunts are black humanoids with bat-like wings, rubbery bodies, inward pointing horns, barbed tails, and faceless heads. They guard the entrances to the Underworld from dreamers. They inhabit the entrances on the mountain, Ngranek, and are the reason people fear to climb too high on that mountain. They also guard the entrance to the Underworld at Sarkomand and the tops of the mountains that separate the plateau of Leng from Inganok and the Giant's Quarry. They attack at night and are said to "tickle" their prey into submission. The Nightgaunts have been known to take the dreamers who are close to the entrances of the underworld and leave them in the Vale of Pnath, at the mercy of the dholes that dwell there but are never seen. Nightgaunts are very dangerous and are capable of easily taking down Moon Beasts and stealing off dreamers unawares in their travels. It is said that you are more likely to be attacked by a Nightgaunt if you think about them often.
The Night-Gaunts and the Ghouls have a relationship in which the Nightgaunts may act as steeds for the Ghouls and servants. They also serve the Outer God, Nodens. The Shantak-bird has a remarkable fear of Nightgaunts.
City of the Gugs
The City of the Gugs is a colossal, horrifying cityscape of soaring, cyclopean towers. It is the dwelling place of the gugs, banished to the underworld by a covenant of the gods. Its most prominent landmark is the Tower of Koth, which contains a legendary stairway that leads to the surface.
Close by the city is the cemetery of the gugs, its graves marked by huge stone monoliths. Ghouls often dine here; a deceased gug feeds them for almost a year.
Crag of the Ghouls
The Crag of the Ghouls is a rugged cliff in the Peaks of Thok, from which the ghouls of deeper dreamland pitch the leftover bones of their sepulchral feasts. Uncounted miles below the crag is the bone-filled "Vale of Pnath."
The Great Abyss is a realm that lies below the ruins of Sarkomand, and is possibly a massive cavern that joins with all parts of the underworld. It connects with the upper Dreamlands by a stairway in Sarkomand.
The Abyss is ruled by the god Nodens, who is served by the night-gaunts. Nodens' influence seems very languid in the underworld and does not appear to extend much beyond the Abyss itself; except perhaps to Ngranek on the isle of Oriab, whose upper slopes are guarded by his night-gaunts.
Peaks of Thok
The Peaks of Thok (or Throk) is a frightening range of towering granite mountains in the underworld.
Vale of Pnath
The vale of Pnath (or Pnoth) is a vast pit in the underworld. It is flanked by the Peaks of Thok and is mostly lightless. The vale is filled with a mountain-sized heap of bones and is "the spot into which all the ghouls of the waking world cast the refuse of their feastings" (The Dream-Quest of Unknown Kadath, Lovecraft). Enormous worm-like creatures, known as dholes, burrow through the vale. Night-gaunts often carry helpless victims to the vale, where they are left to die.
Vaults of Zin
The Vaults of Zin is a huge cavern in the underworld. It lies near the cemetery of the gugs and opens onto a large cave that "is the mouth of vaults of Zin, and the vindictive ghasts are always on watch there for those denizens of the upper abyss" (The Dream-Quest of Unknown Kadath, Lovecraft). The ghasts who dwell in the Vaults of Zin prey on ghouls and gugs, and sometimes even one another.