Stone Trap (5e Trap)
From D&D Wiki
This trap is activated when an intruder steps on a hidden pressure plate, releasing a gas that will slowly turn the user to stone. The DC is 15 to spot the pressure plate but, you can see the poor souls who were turned to stone.
The trap activates when more than 20 pounds of weight is placed on the pressure plate, causing it to release a 10-foot sphere of gas, centered on the pressure plate. Each creature in the cloud must make a DC 13 dexterity save to get out before it could do serious damage. In order to free someone from the stone trap you must have someone dispel the curse.