Rust Monster

Metal Detection. The rust monster can smell metal within 30 feet.

Rust Metal. A nonmagical weapon made of metal that hits the rust monster corrodes after dealing damage, taking a permanent -1 penalty to damage rolls per hit. If this penalty reaches -5, the weapon is destroyed. Metal nonmagical ammunition is destroyed after dealing damage.

Antennae. The rust monster corrodes a nonmagic metal object within 5 feet. It can destroy up to a 1-foot-square portion of an unattended object. If the object is worn or carried the objects owner makes a DC 11 Dexterity saving throw avoiding the rust monsters antennae on a success.

Metal shields or armor the rust monster touches with its antennae corrode, taking a permanent -1 penalty to its AC protection per hit. If the penalty reduces the armors AC protection to 10 the armor is destroyed. If a metal weapon is touched it is subject to the Rust Metal trait.

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

Defensive Bite. When the rust monster is hit by a melee attack made by a creature it can see within 5 feet, it bites the attacker.