Alpha Rust Monster

Iron Scent. The rust monster can pinpoint, by scent, the location of ferrous metal within 30 feet of it.

Rust Metal. Any nonmagical weapon made of metal that hits the rust monster corrodes. After dealing damage, the weapon takes a permanent and cumulative −1 penalty to damage rolls. If its penalty drops to −5, the weapon is destroyed. Nonmagical ammunition made of metal that hits the rust monster is destroyed after dealing damage.

Corrosive Breath:  [Recharge 5-6]  
The creature exhales a corrosive mist in a 30­-ft. cone from it's mouth that is is 20 feet wide.

Bite. Melee Weapon Attack: +3; Bite, reach 10 ft., one target. Hit: 16 2d12+4 piercing damage.

Corrosive Breath:  [Recharge 5-6]  
The creature exhales a corrosive mist in a 30­-ft. cone from it's mouth that is is 20 feet wide. Each creature in that area must make a DC 14 Dexterity saving throw, taking 49  acid damage on a failed save, or half as much damage on a successful one.

Antennae. The Alpha rust monster corrodes a nonmagical ferrous metal object it can see within 5 feet of it. If the object isn't being worn or carried, the touch destroys a 1-foot cube of it. If the object is being worn or carried by a creature, the creature can make a DC 11 Dexterity saving throw to avoid the rust monster's touch.

If the object touched is either metal armor or a metal shield being worn or carried, it takes a permanent and cumulative −1 penalty to the AC it offers. Armor reduced to an AC of 10 or a shield that drops to a +0 bonus is destroyed. If the object touched is a held metal weapon, it rusts as described in the Rust Metal trait.