The Echo-Lashed Causeway
This old Underdark road behaves like a listening instrument. Every step, shout, and clang travels through the stone and wakes buried resonance nodes that answer with a shuddering pulse. The pulse disorients travelers, shakes loose stone from the cavern roof, and can turn a careful march into a panicked scramble. The hazard is most dangerous when a group moves together, speaks loudly, or fights upon the road.
The Echo-Lashed Causeway
Defenses
Detection
“A ribbed span of black basalt plates crosses the cavern floor like an ancient road, but every footstep returns a half beat late from the walls. Pale fungal lamps along the route dim and brighten in a slow pulse, and fine dust trembles upward from the seams whenever anything heavy moves nearby.”
Standard Engagement
The hazard triggers when a creature moves onto the marked road section, speaks above a whisper, strikes the stone, casts a spell with a verbal component, or otherwise introduces a loud vibration into the cavern.
On initiative count 20, the road releases a Resonant Pulse. If at least two creatures are on the road or if any creature made a loud noise, cast a spell with verbal components, or struck the stone since the last turn, the hazard also triggers Stonefall Reply. If the hazard has 22 HP or fewer, the pulse becomes unstable: the first creature to fail its save each round is also deafened until the end of its next turn.
The hazard resets 1 minute after no creature remains on the road section and all active resonance nodes have had time to recharge.
Each creature on the road must succeed on a DC 15 Constitution saving throw or take 3d6 thunder damage and have its speed reduced by 10 feet until the end of its next turn. On a failed save by 5 or more, the creature is also pushed 10 feet toward the nearest seam or edge, if any.
One loose section of ceiling or wall stone crashes down in a 10-foot-square area on the road. Each creature in the area must make a DC 15 Dexterity saving throw, taking 4d6 bludgeoning damage on a failure, or half as much on a success. A creature that fails by 5 or more is also knocked prone.
The hazard is loud enough that stealth is difficult on the causeway. While the hazard is active, creatures have disadvantage on Dexterity (Stealth) checks made to move across the road at normal speed unless they move at half speed or use magic to muffle their sounds.
If a creature deliberately moves at a crawl, keeping one hand on the ground and avoiding speech, it gains advantage on saving throws against the hazard's next routine, but its speed is halved for that movement.
If the hazard is reduced to 0 HP, the road goes dead quiet for 24 hours and becomes safe to cross, though the first thunder damage or loud impact after that period reactivates it.
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