Rust Monster

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

The monster makes one Bite attack and uses Antennae twice.

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

The monster targets one nonmagical metal object—armor or a weapon—worn or carried by a creature within 5 feet of itself. The creature with the object must make a DC 11 Dexterity saving throw. On a 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 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 metal.

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