Rust Chamber (5e Trap)
From D&D Wiki
This trap is activated when an intruder steps on a hidden pressure plate, releasing ground up rust monster antennae from the ceiling.
The DC is 15 to spot the pressure plate, as well as rust on the floor.
The trap activates when more than 20 pounds of weight is placed on the pressure plate, causing ground up rust monster antennae to fall from the ceiling, forming a cylinder that is 30 feet tall and has a diameter of 5 feet. Each creature in the cylinder must succeed a DC 13 Dexterity saving throw, or any non-magical metal openly worn or carried rusts. Non-magical metal weapons take a permanent and cumulative -1 penalty to damage rolls. If its penalty drops to -5, the weapon is destroyed.
Non-magical metal armour or a metal shield takes a permanent and cumulative -1 penalty to the AC it offers. Armour reduced to an AC of 10 or a shield that drops to a +0 bonus is destroyed.
Wedging an iron spike or other object under the pressure plate prevents the trap from activating.