Rust Monster

The rust monster can pinpoint the location of ferrous metal within 30 feet of itself.

The rust monster makes one Bite attack and uses Antennae twice.

Melee Weapon Attack: +3 to hit, reach 5 ft.. Hit: 5 (1d8 + 1) piercing damage.

The rust monster targets one nonmagical metal object—armor or a weapon—worn or carried by a creature within 5 feet of itself.
Dexterity Saving Throw: DC 11, the creature with the object.
Failure: The object takes a -1 penalty to the AC it offers (armor) or to its attack rolls (weapon). Armor is destroyed if the penalty reduces its AC to 10, and a weapon is destroyed if its penalty reaches -5. The penalty can be removed by casting the Mending spell on the armor or weapon.

The rust monster touches a nonmagical metal object within 5 feet of itself that isn’t being worn or carried. The touch destroys a 1-ft Cube of the object.

Trigger: An attack roll hits the rust monster. Response: The rust monster uses Antennae.