The abyss

The deepest ways, a kilometre and more beneath the surface, where the passages are older than any people and some were made into roads by builders long gone: steps worn into the rock, broken bridges over shafts, carved gates standing open. It is utterly dark and it is hot, because the rock warms by some 25 to 30 °C for every kilometre down; the air is still and dry and tastes of dust. Food is scarce, so what lives here is large, patient and hungry, and drinks from deep black lakes. The deepest caves known, in the western Caucasus, go down a little over two kilometres, and miners in South Africa work rock as hot as 66 °C some four kilometres down.

Light
Dark
Going
Slow

Going down

How warm the rock is and whether this place can grow there, under a temperate land: the deeper, the hotter.

Near the surface (50 m)
11 °C, it grows
A few hundred metres down (400 m)
21 °C, it grows
A kilometre down (1 km)
38 °C, it grows

Hear it

Hazards

  • Heat of the deep. The deeper the road goes, the hotter the rock: past a kilometre and a half, the walls are warm as bread. D&D 5e: Where it is over 38 °C (100 °F), a creature without drinkable water makes a Constitution save at the end of each hour, DC 5 and 1 higher each hour after, or gains 1 Exhaustion level. Medium or Heavy armour gives Disadvantage on the save; Resistance or Immunity to Fire damage passes it automatically.
  • Darkness. A dark so old and so complete that a lamp seems to light only itself. D&D 5e: Beyond the party’s lights the roads are in Darkness and Heavily Obscured: a creature trying to see into it has the Blinded condition. Darkvision turns Darkness into Dim Light within its range, where Wisdom (Perception) checks that rely on sight have Disadvantage.
  • Dry roads. The lakes are days apart, and some of them hold water no one should drink. D&D 5e: A Medium creature needs about 4 litres (1 gallon) of water a day. One that drinks less than half of it gains 1 Exhaustion level at the day’s end, and cannot lose it until it drinks a full day’s water.
  • Broken bridge. The old builders spanned the shafts with stone bridges. Some have stood ten thousand years; some are about to stop. D&D 5e: A creature that falls takes 1d6 Bludgeoning damage for every 3 m (10 ft), up to 20d6, and lands Prone unless it took no damage. Our addition: a DC 13 Intelligence (Investigation) check sees which slabs are cracked.
  • The weight of rock. After days without sky, the mind starts to feel the whole mountain overhead, pressing down. D&D 5e: Each day after the third without daylight, a creature makes a Wisdom save, DC 10 and 1 higher each day after. On a failure it has the Frightened condition for 1 hour, afraid of the way ahead, and repeats the save at the end of that hour.

Adventure hooks

  1. A caravan of the deep folk crosses the old roads once a year to trade with the surface, and this year it has not come. Its trade goods were promised to a wedding, and the bride’s father wants to know why.
  2. A wall of carvings shows the builders of the deep roads at work, and the last panel shows them fleeing from something. A scholar wants a rubbing of the last panel, which is past a broken bridge.
  3. A calm voice speaks from a black lake and offers the way to anywhere in the deep in exchange for a single torch, lit, thrown into the water. The torch sinks, still burning.
  4. The party comes on a cold camp, its fire long dead, with the bedrolls laid out and the food still in the packs. The diary of the camp’s leader stops in the middle of a sentence about the silence.

What may happen

  1. Stone posts stand along the road every few hundred paces, carved with signs no one living can read.
  2. A lake so still and dark it looks like a hole in the floor. Far beneath the surface, something large turns over.
  3. A carved gate three times a man’s height stands open, its doors lying flat on the road.
  4. The floor shudders, and a long grinding comes through the rock, as of something huge passing at a distance.
  5. Lights ahead: a caravan of deep traders with laden lizards, who stop and wait to see what the party is.
  6. A warm wind blows steadily up the road from below, smelling of hot stone and something sweet.
  7. A long wall of carvings shows the builders at work, at war, and at the end, walling something in.
  8. For a hundred paces the road is paved with bones set in mortar, neatly.
  9. Water drips from the roof hot enough to sting, and steam hangs in the passage.
  10. A camp with long-cold ashes, a ring of stones and three bedrolls. The packs are still there; their owners are not.
  11. A voice speaks from the dark, very calm, in a language every member of the party understands.
  12. Every sound stops: no drip, no wind, no echo of the party’s own steps. Then it all comes back.
  13. Pale threads thick as rope spread across the road from a side passage, all running the same way.

Creatures

A town here

A city can live down here, fed by what grows below. Build it in Hearth

To bring back

Bat Guano, Brutacian Lung, Bugbear Tooth, Deep Gnome Nose, Fire Beetle Glands, Mycelium, Obsidian Powder, Ooze Portion, Scorpion Venom, Sulfur Pinch, Topaz Powder, Tourmaline Powder, Troglodyte Thumb Nail, Amber Tears, Amethyst Powder, Aquamarine Powder, Black Powder, Bloodstone Powder, Chalcedony Powder, Chitine Silk, Cockatrice Liver, Crystal Powder, Drow Hair, Echo in Stone, Ettercap Mandible, Ettin Toe, Firenewt Bile, Fixed Nitre, Galeb Duhr Soul, Giant Spider Perfect Eye, Jade Powder, Jasper Powder, Jet Powder, Lapis Lazuli Powder, Magnetite Shavings, Malachite Powder, Moonstone Powder, Myconid Spores, Phase Beast Heart, Ruby Powder, Sapphire Powder, Selenite Sliver, Native Sulfur Crystal, Tanzanite Powder, Young Dragon Tonsils, Zircon Powder, Ankheg Antennae, Barghest Fang, Basilisk Throat, Cranium Rat Brain, Cyclops Eye, Drider Tongue, Flumph Hypothalamus, Grell Beak, Kuo-Toa Gill, Mind Flayer Brain, Minotaur Heart, Ogre Liver, Phase Spider Egg, Troll Bone Marrow, Beholder Eye, Deep Lantern Organ, Dragon Tonsils, Gorgon Muzzle, Oblex Slime, Oni Liver, Roper Bile, Shadow Mastodont Eye, Tainted Quaggoth Soul, Aboleth Eye, Adamantine Dragon Tonsils, Alhoon Brain, Amethyst Dragon Tonsils, Iridium Powder, Elder Oblex Slime, Purple Worm Bile, Sapphire Dragon Tonsils, Shadow Dragon Gland, Tarrasque Carapace Piece, Topaz Dragon Tonsils, Tungsten Powder, Wyvern Poison

Elsewhere below

Fungal forest, Doline jungle, Glowing meadow, Bare labyrinth, Lava tubes, Sulphur cave, Crystal caverns

This work includes material from the System Reference Document 5.2.1 ("SRD 5.2.1") by Wizards of the Coast LLC, available at https://www.dndbeyond.com/srd. The SRD 5.2.1 is licensed under the Creative Commons Attribution 4.0 International License, available at https://creativecommons.org/licenses/by/4.0/legalcode.

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