Corrosion Crawler

Metal Scent. The corrosion crawler can pinpoint, by scent, the location of any metal within 30 feet of it.

Corrosive Form. A creature that touches the corrosion crawler or hits it with a melee attack while within 5 feet of it takes 3 (1d6) acid damage. Any nonmagical weapon that hits the corrosion crawler 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 that hits the corrosion crawler is destroyed after dealing damage.

Spider Climb. The corrosion crawler can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) piercing damage plus 6 (1d6) acid damage. In addition, nonmagical armor worn by the target is partly dissolved and takes a permanent and cumulative −1 penalty to the AC it offers. The armor is destroyed if the penalty reduces its AC to 10

Steal. Melee Weapon Attack: +4 to hit, reach 5 ft., one target.  On a hit, you must win an athletics ability check contest against the corrosion crawler, otherwise the corrosion crawler steals an object you are carrying or a destroyed object you are wearing. In addition, if the item is medium or smaller the corrosion crawler is now carrying the item and may use disengage and dash as free actions this turn.

Every round that the corrosion crawler carries an object that object suffers from the Corrosive Form effect.

Whenever a corrosion crawler is carrying an object, it cannot make any attacks.  The corrosion crawler may use its bonus action to drop any object it is carrying.