Underland Realm

There is no map—even among the subterranean cultures that dwell within—that accurately depicts all of these enormous tunnels, which range from natural caverns and dwarven mines to shadow elf cities. Adventurers journeying through this region have combat encounters, some social encounters, and many constructed terrain, natural terrain, and supernatural exploration challenges.

Terrains. Forest, jungle, laboratory, ruins, settlement, subterranean, swamp, temple, tomb, water.

Tiers. It’s unusual for these regions to be less than tier 2.

Weather. 1–16 clear, 17–25 mist.

Resonant. Adventurers gain an expertise die on sound-based Perception checks.

Unstable. When an adventurer creates a force that would disturb a non-stoneworked area (an explosion, loud sound, or shaping the structure of the cave through magic), roll 1d20. On a result of 20 the activity causes a cave-in. One passageway in the cave is filled with fallen rocks 20 feet wide.  

 Underland Realm Exploring Tier 0

1 Bridge of stones (exploration)
2–3 Aboleth thrall (monster)
4–5 Ankheg spawn (monster)
6–7 Azer (monster)
8–9 Darkmantle (monster)
10–11 Ettercap (monster)
12–19 Travel scenery  
20–21 Flumph (monster)
22–23 Gelatinous cube (monster)
24–25 Giant bat (monster)
26–27 Giant centipede (monster)
28–29 Giant fire beetle (monster)
30–31 Falling net (exploration)
32–34 Giant poisonous snake (monster)
35–37 Giant rat (monster)
38–41 Giant spider (monster)
42–43 Gibbering mouther (monster)
44–45 Gray ooze (monster)
46–48 Haze (exploration)
49–51 Grick (monster)
52–54 Grimlock (monster)
55–57 Magmin (monster)
58–60 Mimic (monster)
61–63 Ochre jelly (monster)
64–66 Lost item (exploration)
67–68 Piercer (monster)
69–70 Rust monster (monster)
71–72 Shadow (monster)
73–74 Shrieker (monster)
75–76 Stirge (monster)
77–78 Poorly-repaired tunnel (exploration)
79–80 Swarm of bats (monster)
81–82 Swarm of poisonous snakes (monster)
83–84 Swarm of rats (monster)
85–86 Troglodyte (monster)
87–88 Violet fungus (monster)
89–90 Stampede (exploration)
91–99 Social encounter   
100 Pit trap (exploration)

Underland Realm Exploring Tier 1

1 Poorly-repaired tunnel (exploration)
2–3 Aboleth thrall (monster)
4–5 Ankheg (monster)
6–7 Azer (monster)
8–9 Basilisk (monster)
10–17 Travel scenery  
18–19 Black pudding (monster)
20–21 Darkmantle (monster)
22–23 Doppelganger (monster)
24–25 Ettercap (monster)
26–27 Flood  (exploration)
28–29 Ettin (monster)
30–31 Gelatinous cube (monster)
32–34 Gibbering mouther (monster)
35–36 Goblin  (monster)
37–38 Labyrinthine ravines (exploration)
39–41 Green hag (monster)
42 Grick  (monster)
43–45 Grimlock (monster)
46–48 Intellect devourer (monster)
49–51 Mushroom ring (exploration)
52–54 Kobold (monster)
55–56 Magmin (monster)
57–59 Mimic  (monster)
60–61 Minotaur (monster)
62–63 Pests  (exploration)
64–66 Ochre jelly (monster)
67–69 Ogrekin (monster)
70–72 Phase spider (monster)
73–75 Piercer (monster)
76–78 Pit trap (exploration)
79–81 Shadow (monster)
82–84 Troglodyte (monster)
85–86 Warrior band (monster)
87–88 Wererat (monster)
89–90 Quicksand (exploration)
91–92 Wight  (monster)
93–99 Social encounter   
100 Lethal outgassing (exploration)

Underland Realm Exploring Tier 2

1–2 Ankheg queen (monster)
3–4 Azer forgemaster (monster)
5–6 Acid field (exploration)
7–8 Black pudding (monster)
9–10 Cloaker (monster)
11–16 Travel scenery  
17–18 Dead man’s fingers (monster)
19–20 Doppelganger (monster)
21–22 Cursed temple (exploration)
23–24 Drider  (monster)
25–26 Earth elemental (monster)
27–28 Dense fog (exploration)
29–30 Fomorian (monster)
31–32 Giant earth elemental (monster)
33–34 Enchanted statue (exploration)
35–36 Giant grick (monster)
37–38 Green hag (monster)
39–41 Flash flood (exploration)
42 Guardian naga (monster)
43–45 Hydra (monster)
46–48 Lethal outgassing (exploration)
49–51 Intellect devourer (monster)
52–54 Medusa (monster)
55–57 Magical overgrowth (exploration)
58–59 Night hag (monster)
60–61 Ogre (monster)
62–63 Marsh gas (exploration)
64–66 Ogre mage (monster)
67–68 Otyugh (monster)
69–70 Rockfall (exploration)
71–72 Piercer (monster)
73–74 Revenant (monster)
75–76 Rolling sphere (exploration)
77–78 Roper (monster)
79–80 Salamander (monster)
81–82 Rot grubs (exploration)
83–84 Salamander nymph (monster)
85–86 Spirit naga (monster)
87–88 Sinkhole (exploration)
89–90 Warrior band (monster)
91–92 Xorn (monster)
93–94 Thundering stampede (exploration)
95–99 Social encounter   
100 Spinning walls (exploration)

Underland Realm Exploring Tier 3

1 Bridge of sorrow (exploration)
2–4 Aboleth (monster)
5–7 Adult amethyst dragon  (monster)
8–10 Adult emerald dragon (monster)
11–16 Travel scenery  
17–19 Adult sapphire dragon (monster)
20–22 Adult shadow dragon (monster)
23–25 Behir (monster)
26–28 Caught in the crossfire (exploration)
29–31 Cloaker (monster)
32–34 Divi (monster)
35–37 Drider (monster)
38–39 Choking smoke (exploration)
40–41 Fomorian (monster)
42 Forgotten god (monster)
43–45 Guardian naga (monster)
46–48 Cursed waterway (exploration)
49–51 Medusa queen (monster)
52–54 Minotaur champion (monster)
55–57 Murmuring worm (monster)
58–61 Endless plummet (exploration)
62–64 Night hag (monster)
65–69 Ogre mage (monster)
70–73 Otyugh (monster)
74–76 Perilous cliff path (exploration)
77–79 Purple worm (monster)
80–82 Salamander (monster)
83–84 Spirit naga (monster)
85–86 Sphere of annihilation (exploration)
87–88 Troll or dread troll (monster)
89–90 Ur-otyugh (otyugh variant) (monster)
91–92 Xorn (monster)
93–94 Spinning walls (exploration)
95–99 Social encounter   
100 Malfunctioning planar portal (exploration)

Underland Realm Exploring Tier 4

1 Sphere of annihilation (exploration)
2–8 Aboleth (monster)
9–16 Ancient amethyst dragon (monster)
17–21 Travel scenery  
22–30 Ancient emerald dragon (monster)
31–38 Adult black dragon lich (monster)
39–47 God corpse (exploration)
48–55 Divi (monster)
56–63 Empyrean (monster)
64–71 Killing cloud (exploration)
72–79 Hobgoblin warlord (monster)
80–88 Salamander noble (monster)
89–95 Malfunctioning planar portal (exploration)
96–100 Social encounter