The Scavenger's Falling Stair
The stairway is not a deadly marvel of engineering. It is a cheap, effective ambush made from rope, scrap wood, and the ruin's own failing structure. When someone catches the cord, a support peg pulls free, causing the upper steps to collapse in a shower of stone and dust. Anyone who falls is left in a cramped lower landing while the survivors above may be cut off from them.
The Scavenger's Falling Stair
Defenses
Detection
“A cracked stairway descends through the ruin. Several steps are newer than the others, patched with mismatched boards and loose stone. A thin cord runs along the wall at ankle height, partly hidden beneath dust.”
Standard Engagement
A creature puts significant weight on the third step or catches the trip cord. The trap also triggers if the support peg is pulled without being safely secured.
The hazard remains dormant until a creature crosses the trapped section. When triggered, the support peg pulls free, the steps collapse, and rubble falls onto the stairway and lower landing.
Manual. The trap must be rebuilt with rope, a replacement peg, and several minutes of work. A scavenger familiar with the ruin can reset it in 10 minutes.
When the trap triggers, each creature on the stairway or within 5 feet of the lower landing must make a DC 12 Dexterity saving throw. On a failed save, a creature takes 2d6 bludgeoning damage and falls prone. On a successful save, it takes half damage and does not fall prone. A creature that fails the save by 5 or more also becomes restrained beneath rubble until it or another creature uses an action to free it with a successful DC 10 Strength (Athletics) check.
The collapse fills the area with choking dust. Until the end of the next round, the stairway and lower landing are lightly obscured. A creature that starts its turn in the dust must succeed on a DC 10 Constitution saving throw or be unable to take reactions until the start of its next turn.
A creature that moves at half speed while examining the stairs has advantage on the check to notice the trap. A creature that dashes across the stairs has disadvantage on that check, if it is allowed one at all.
If two or more creatures are on the trapped section when it activates, the DM can choose one creature to make the first saving throw. On a failed save, that creature's fall may force another creature within 5 feet to make a DC 10 Dexterity saving throw or fall prone as well.
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