Rust Monster Shell

While wearing this armor, you gain a +1 bonus to AC. In addition, you can use an action to magically coat the armor in rusty flakes for 1 minute. While the armor is coated in rusty flakes, any nonmagical weapon made of metal that hits you 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 you is destroyed after dealing damage. The armor can't be used this way again until the next dawn.