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

Bite. m +3, reach 5 ft. Hit: 5 (1d8 + 1) Piercing damage.

Antennae. The rust monster targets one nonmagical metal object—armor or a weapon—worn or carried by a creature within 5 feet of itself. dex DC 11, the creature with the object. 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.

Destroy Metal. 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-foot Cube of the object.

Reflexive Antennae. An attack roll hits the rust monster. The rust monster uses Antennae.