Medium Monstrosity, Unaligned
- AC
- 14
- Initiative
- +1 (11)
- HP
- 33 (6d8+6)
- Speed
- 40 ft.
| Score | Mod | Save | |
|---|---|---|---|
| STR | 13 | +1 | +1 |
| DEX | 12 | +1 | +1 |
| CON | 13 | +1 | +1 |
| INT | 2 | -4 | -4 |
| WIS | 13 | +1 | +1 |
| CHA | 6 | -2 | -2 |
Traits
Iron Scent. The rust monster can pinpoint the location of ferrous metal within 30 feet of itself.
Actions
Multiattack. The rust monster makes one Bite attack and uses Antennae twice.
Bite. Melee Attack Roll: +3, reach 5 ft. Hit: 5 (1d8+1) Piercing damage.
Antennae. The rust monster targets one nonmagical metal object—armor or a weapon—worn or carried by a creature within 5 feet of itself. Dexterity Saving Throw: DC 11, the creature with the object. Failure: The object takes a -1 penalty to the AC it offers (armor) or to its attack rolls (weapon). Armor is destroyed if the penalty reduces its AC to 10, and a weapon is destroyed if its penalty reaches -5. The penalty can be removed by casting the Mending spell on the armor or weapon.
Destroy Metal. The rust monster touches a nonmagical metal object within 5 feet of itself that isn't being worn or carried. The touch destroys a 1-foot Cube of the object.
Reactions
Reflexive Antennae. Trigger: An attack roll hits the rust monster. Response: The rust monster uses Antennae.
How to run Rust Monster
A rust monster as ally is a niche play, and it is a fun one. A dwarven smith breeds them to recycle scrap. A wizard keeps one as an unusually polite familiar that lives in a copper-lined hutch. A noble has hired the party to dispose of an enemy's plate-armored bodyguard before tomorrow's duel, and the rust monster is the tool. Frame the alliance around what the rust monster does, not what it thinks. With Int 2, it does not think.
In the field, the rust monster is point-and-shoot. Iron Scent picks the metal target inside 30 feet automatically, so the party leads the creature toward an enemy's armor and lets the Antennae do the work. Two Antennae per turn at DC 11 Dex chips down a knight's plate in four rounds, which is the same fight a martial PC would need to win in melee, except the knight is doing the fighting back at the rust monster (33 HP, AC 14) instead of at the party. Use the rust monster as the third front-liner in a fight where the party is outmatched on metal-armored opponents.
The rust monster does not protect anyone. It does not parley or retrieve and does not understand the difference between friend and foe. It eats metal. The party's job is to keep it leashed (literal leather lead, no metal collar) and aimed. If a PC is in plate, the rust monster will turn on them mid-fight without a thought. State this rule at the table the first time the creature joins the party, then enforce it.
When the contract closes, return the rust monster to whoever lent it out. The party has earned a story, a wagon full of metal scraps, and one less encounter where a paladin's blessed plate matters at all.
A rust monster is a tax on metal armor and a comedy beat for the player whose magic sword just turned to flakes. Iron Scent pinpoints ferrous metal within 30 feet, so the rust monster does not wander; it makes a beeline for the heaviest armored character in the room. The fight is not about HP. It is about whose gear survives.
Multiattack opens with one Bite at +3 for 5 average piercing and two uses of Antennae. Antennae targets one nonmagical metal object worn or carried, DC 11 Dex save, on a fail the object takes a permanent -1 penalty (AC for armor, attack rolls for weapons). Armor dies at AC 10, a weapon at -5. With two Antennae per turn, plate is gone in four rounds if the rust monster keeps reach. Reflexive Antennae as a reaction triggers off any hit that lands, so a hit-back exchange shaves another point off whoever just swung.
Magical metal is immune to Antennae, which is the whole social contract of the encounter. A +1 longsword is fine. A masterwork mundane longsword is gone. Tell the players which is which before they swing, because losing an heirloom blade lands differently than losing a recent loot drop. The rust monster does not chase a fleeing party and does not understand that the scrap it just ate was the bandit captain's family blade.
At low HP, the rust monster grabs the nearest large metal object that is not being held (Destroy Metal eats a 1-foot cube without using an action against the holder) and burrows back into whatever crack it came from. The party that pursues finds a midden of half-eaten plate and a thousand small metal scraps.
Have the rust monster eat one mundane object the party did not want to lose. A coin pouch, a horseshoe, a torch sconce. Then everyone learns the lesson and the magic blades survive intact.
Stat block from the System Reference Document 5.2.1 © Wizards of the Coast LLC, used under the Creative Commons Attribution 4.0 license.