Shrinking Cloud (5e Trap)
From D&D Wiki
This trap is activated when an intruder steps on a hidden pressure plate, releasing a cloud of magical smoke that shrinks creatures caught inside it.
The DC is 15 to spot the pressure plate, as well as unusually small footprints on the floor. A spell or other effect that can sense the presence of magic, such as detect magic, reveals an aura of transmutation magic on the pressure plate.
The trap activates when more than 20 pounds of weight is placed on the pressure plate, causing it to release a 30-foot sphere of gas, centred on the pressure plate. Each creature in the cloud must make a DC 13 Constitution saving throw or be affected by the "reduce" effect of the enlarge/reduce spell for 1d4 hours (no concentration required). The cloud remains for 1 minute. A moderate wind (at least 10 miles per hour) disperses the cloud after 4 rounds. A strong wind (at least 20 miles per hour) disperses it after 1 round.
Wedging an iron spike or other object under the pressure plate prevents the trap from activating. A successful dispel magic (DC 13) cast on the pressure plate destroys the trap.