Aggressive. As a bonus action, the rust monster can move up to its speed toward a hostile creature that it can see. Ore Scent. The rust monster can pinpoint, by scent, the location of precious metals within 30 feet of it. Trampling Charge. If the rust monster moves at least 20 feet straight toward a creature and hits it with a gore attack on the same turn, the target takes an extra 11 (2d10) piercing damage. If the target is a creature, the target must succeed a DC 14 Strength saving throw or be knocked prone.
Gore. Melee weapon attack: +8 to hit, reach 5 ft, one creature. Hit: 18 (3d8+5) piercing damage Corroding Antennae. The rust monster corrodes a nonmagical metal object it can see within 5 feet of it. If the object isn’t being worn or carried, it destroys a 1-foot cube of it. If the object is worn or carried by a creature, the creature can make a DC 14 Dexterity saving throw to avoid the rust monster’s touch. If the object touched is either a metal weapon or a metal armor being worn or carried, it takes a permanent and cumulative −1 penalty to the AC it offers or to damage rolls. If a weapon’s penalty drops to −5, it is destroyed. Armor reduced to an AC of 10 or a shield that drops to a +0 bonus is destroyed.