The Silk-Run Boulder
This cavern hazard is a defensive boulder trap hidden in plain sight. A huge stone rests in a nest of reinforced silk and is held in place by a single load-bearing strand. When a creature crosses the chamber's marked path, the strand snaps and the boulder rolls or drops into motion, smashing through the passage. Because the chamber has been prepared repeatedly, the hazard can reset itself as long as the silk remains intact and the boulder is not wedged or destroyed.
The Silk-Run Boulder
Defenses
Detection
“A boulder the size of a wagon sits half buried in a nest of chalk-white silk, its surface banded with old scrape marks where it has been dragged and reset again and again. The cavern floor around it is combed smooth, and the silk glints like wet thread in torchlight.”
Standard Engagement
A creature crosses the tensioned silk line or disturbs the webbed anchor points holding the boulder in place.
On initiative count 10, if the hazard is armed and any creature is in the marked trigger lane, the supporting strand snaps. The boulder surges forward up to 40 feet along its carved path, following the chamber's slope. It can strike up to two creatures in sequence if they remain in line, forcing each to make the Crushing Roll save. The first creature hit by the boulder also causes loose stone and webbing to billow, lightly obscuring the path until the start of the next round.
The hazard resets after 1 minute if the silk strand and release path remain intact. Replacing the load-bearing thread requires the creator or a similarly skilled creature and 10 minutes of work.
Each creature in the trigger lane or within 10 feet of the rolling path must make a DC 17 Dexterity saving throw. On a failure, the creature takes 4d10 bludgeoning damage and is knocked prone. On a success, the creature takes half damage and is not knocked prone.
A creature that fails the save by 5 or more is also pushed 10 feet in a straight line and restrained by tangled silk until the end of its next turn, as sticky strands drag across its limbs.
The boulder continues moving until it strikes solid stone, a wedged obstacle, or the end of the carved chute. Any creature in its path when it comes to a stop must succeed on a DC 15 Dexterity saving throw or take 2d10 bludgeoning damage from debris and ricocheting fragments.
The trap's trigger lane is obvious only after close inspection. Crossing it without noticing the silk line activates the hazard.
The boulder is too massive to lift by mundane means, but it can be wedged with crowbars, pitons, iron spikes, or similar gear. Each successful attempt to jam it requires an action and a DC 18 Strength or Dexterity check, at the DM's discretion.
Because the chamber was prepared repeatedly, creatures that learn the lane once can treat the hazard as partially solved. After the first successful discovery, all further Perception checks to spot the trigger in the same cavern are made with advantage.
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