An old wooden staircase leads upwards.
|A bottle full of explosives is precariously balanced on the top stair. The staircase is structurally sound, but walking on it causes vibration, that cause the bottle to fall.
A DC 16 Intelligence (Investigation) check reveals that despite the motion, the staircase is solid.
If there is light, the bottle can be seen on the top of the stairs with a DC 16 Wisdom (Perception) check.
The staircase consists of 16 steps. Each time a stair is stepped upon, there is a cumulative 10% chance that the bottle falls off the top stair. The bottle then rolls down the stairs and explodes when it hits the hard floor.
A character on the stair may stop the bottle with a successful DC 16 Dexterity saving throw. If the bottle was seen, the saving throw has advantage.
Once the bottle explodes, it deals 3d6 fire damage to all creatures within 20 feet, with a successful DC 14 Dexterity saving throw halving the damage.
The trap can only be disabled by devising a way to move the bottle without mounting the stairs.