Rust Monster
Medium Monstrosity, unaligned
100 XP
Special Abilities
Iron Scent
The rust monster can pinpoint, by scent, the location of ferrous metal within 30 feet of it.
Rust Metal
Any nonmagical weapon made of metal that hits the rust monster 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. Non magical ammunition made of metal that hits the rust monster is destroyed after dealing damage.
Actions
Bite
Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 5 (1d8 + 1) piercing damage.
Antennae
The rust monster corrodes a nonmagical ferrous metal object it can see within 5 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 11 Dexterity saving throw to avoid the rust monster's touch.
If the object touched is either metal armor or a metal shield being worn or carried, its 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.
Quick Reference
Running This Encounter
Rust Monsters are best used as environmental hazards or annoying complications rather than primary threats. Position them near valuable equipment or treasures to create tension about what will happen if the party engages. Use their fear of fire to drive tactical decisions, and consider having multiple rust monsters to increase pressure on the party's resources. Their low HP makes them vulnerable to area effects, so a single casting of Magic Missile or Scorching Ray can end them quickly.
Combat Tactics
Rust Monsters prioritize attacking metal armor and weapons over grappling. They use their mobility to dart in, use their Bite or Antennae action to corrode metal, then retreat before the party can land solid blows. If threatened with fire or magic, they attempt to flee and reposition.
Environment & Setting
Ideal in caves, underground ruins, or dungeons where metal equipment is common and parties cannot easily leave. Placing them near water sources, confined corridors, or areas with limited escape routes forces engagement.