Poison Gas Trap (mechanical trap)
A narrow slot just below the shelf can be detected with a DC 15 Perception check. A DC 10 Investigation check determines that there isn't a sure way to deactivate the trap, but succeeding by 5 or more on the check reveals that standing back some distance and using an object to remove the key might avoid whatever effect it has. A burst of sour-smelling, yellow gas sprays from the slot. Creatures adjacent to the pillar must succeed on a DC 15 Constitution or become poisoned for 1d4 hours.