Rust Monster (CR 3)

Medium Aberration
Alignment: Always neutral
Initiative: +3 (Dex); Senses: darkvision, scent, Listen +7, and Spot +7

AC: 18 (+3 Dex, +5 natural), touch 13, flat-footed 15
Hit Dice: 5d8+5 (27 hp)
Fort +2, Ref +4, Will +5
Speed: 40 ft.
Space: 5 ft./5 ft.
Base Attack +3; Grapple +3
Attack: Antennae touch +3 melee
Full Attack: Antennae touch +3 melee and bite -2 melee
Damage: Antennae touch rust, bite 1d3
Special Attacks/Actions: Rust
Abilities: Str 10, Dex 17, Con 13, Int 2, Wis 13, Cha 8
Special Qualities: -
Feats: Alertness; Track
Skills: Listen +7 and Spot +7
Advancement: 6-8 HD (Medium-size); 9-15 HD (Large)
Climate/Terrain: Any underground
Organization: Solitary or pair
Treasure/Possessions: None

Source: Monster Manual

Rust (Ex): A rust monster that makes a successful touch attack with its antennae causes the target metal to corrode, failing to pieces and becoming useless immediately. The size of the object is immaterial - a full suit of armor rusts away as quickly as a sword. Magic armor and weapons, and other enchanted items made of metal, must succeed at a Reflex save (DC 20) or be dissolved.

A metal weapon that deals damage to a rust monster also corrodes immediately. Wooden, stone, and other, nonmetallic weapons are unaffected.

A rust monster can scent a metal object up to 90 feet away, dashing toward the source and attempting to strike it with its antennae. The creature is relentless, chasing adventurers over long distances if they still possess intact metal objects but usually ceasing its attacks to devour a freshly rusted meal. A clever (or desperate) adventurer can often distract a hungry rust monster by tossing it some metal objects, then fleeing while it consumes them.

The creature targets the largest metal object available, striking first at armor, then shields and smaller items. It prefers ferrous metals (steel or iron) over precious metals (such as gold or silver) but will devour the latter if given the opportunity.