Underhollows

The Underhollows are the deep places beneath Aiaos and the domain of the deepfolk, those who live hidden from the light of the sun. Further down even than the mines of the Dwarfs, countless miles of tunnels, catacombs, caverns and galleries stretch far beneath the surface, and are home to tribes of subterranean Goblins and Kobolds, and idiosyncratic kindreds such as the duergar, the sverfneblin and the Drow, and the Kua-toa of the deep waters. These are not merely tunnel-dwellers, but cultural offshoots who live almost entirely without the light of the sun, many of whom have an ingrained distrust, or even fear, of open spaces.
Many on the surface see the races of the Underhollows as degenerates in the service of Hell, and some among them of them no doubt are, but many of them serve as a bulwark against the Abyss, and the long-buried dominions of otherfolk that the surface has forgotten about. The drow and the duergar in particular have a long tradition of custodianship over things that ought to stay buried.

The Caverns

The caverns are the naturally occuring part of the Underhollows, narrow channels and vast chambers, cut through the earth by water, fire and time. Goblins and Kobolds make their homes in the caverns, building their settlements in the more open spaces using cut stone, root wood, hides and metal. Many of the great transit routes in the Underhollows run along natural river courses.
The most splendid sights in the caverns are the forecourts of the great duergar holds, where vast, limestone vaults are cleared as frontage for the hold, which itself is cut back into the stone and faced with grand porticos and galleries overlooking and dominating the forecourt. While the hold is part of the cuttings, the natural character of the forecourt is always maintained and emphasised by the architecture.

The Tunnels

The tunnels are the artificial passages of the Underhollows, incorporating large animal burrows, kobold delvings and mine workings, as well as the great highways of the duergar. Tunnels connect occupied areas, but are not themselves populated, save by animals and the occasional monster.

The Cuttings

Cuttings are artificial excavations created or used for residence or fortification, including the vaulted halls of duergar holds and the modest passages of a kobold warren. The largest and most impressive such workings are the vaults of the drow, vast chambers hollowed from the surrounding stone and reinforced with magic. The foundations of towers, fashioned in imitation of natural stalagmites, are sunk into the floor of the vault, while the walls are constructed from the excavated stone and the spoil transformed into breathable air in arcane furnaces.
Unlike the tunnels, most vaults are illuminated, but not brightly. The vaults are lit by pale, artificial suns, while dwarf holds use lambent, enchanted crystals for light.

The Darkenings

The darkenings are those areas of the Underhollows occupied and warped by otherworldly influences, including the fey, fiends and outerfolk. These areas tend ot have an organic feel, the stone eaten away or melted instead of being cut, and often have something uncanny and unsettling about them.
In the deepest parts of the Underhollows, the Fathomless Vertex breaks through from The Abyss. This passage from the material to the Abyss is held by infernal forces, but contested by the demons of Alathkthi, corrupted drow servants of the Demon Prince known as Lilloth the Netweaver, colonies of Outerfolk and guardian clans of drow and duergar.