A small chest is at the end of a narrow hallway.
A horizontal blade shoots out from the wall. At the same time, the area under the blade fills with poison gas.
A DC 12 Wisdom (Perception) check notes a wide seam that runs horizontally along the right wall. Small holes are places at regular intervals under it.
A DC 14 Intelligence (Investigation) check reveals a thin, long line on the opposite wall where the paint has been removed, as well as slight discoloring around the holes.
A DC 14 Wisdom (Perception) check reveals a rectangular outline in the floor, about halfway.
When the plate is stepped on, a huge blade shoots out from the right side. All targets in the hallway must make a DC 14 Dexterity check to avoid the blade. The targets may specify whether they are ducking or jumping. If jumping, the check is at disadvantage. The target takes 8d8 slashing damage on a failure, but ends up either underneath (ducking) or on top (jumping).
One the blade fires, the nozzles underneath release a greasy, black gas. At the beginning of a target’s turn, they must make a DC 16 Constitution saving throw or take 4d6 poison damage. They must also make this saving throw if the enter the gas at any time during their turn. The gas naturally dissipates in 1d4+1 rounds.
The trap can be disarmed or changed in several ways.
The pressure plate can be disabled with a successful DC 14 Dexterity check with thieves’ tools.
The blade can’t be disabled from here, but the holes can be plugged. There are 4 holes, each requiring a successful DC 14 Dexterity (Sleight of Hand) check to disable by plugging it with something, like a cloth. Each hole plugged removes a d6 from the damage.