Iron Scent. The rust monster can pinpoint, by scent, the location of ferrous metal within 60 feet of it.

Immutable Form. The monstrosity is immune to any spell that would alter its form.

Rot Skin. Whenever the monstrosity lands a hit, the targets maximum HP reduces by the damage dealt halved, which regenerates after a short or long rest. The target is killed when HP is dropped to 0.

Grafted Weapon. Mele e Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 12 (1d12 + 6)force damage.

Expose. The monstrosity corrodes a nonmagical ferrous metal object it can see within 10 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 15 Dexterity saving throw to avoid its 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.

Graft (Only works on enemies hit by EXPOSE). Melee weapon attack: +3 to hit, reach 5 ft., one target. Hit: 8 (1d12 + 2) necrotic damage. The target must succeed a DC 16 strength saving throw or be grafted to the side of the monster. The target can repeat the saving throw on itself at the end of each of its turns ending the effect on a success. Whenever the monster takes damage, half goes to the creature that is grafted.