Rust Monsters are one the few original monsters in D&D that didn't fade into obscurity. Probably because of their ability to turn James' favourite metal items into dust.

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Rust monsters are basically large bugs with two equally large antennae. The Rust Monster, with the antennae, rubs an unspecified chemical on the metal that degrades it and turns most of the item into dust.

  • Rust Monsters affect iron and iron-based alloys.
  • It's not neccessary for the item to start to turn into dust immediately, but it should lose structural integrity at a rapid pace, so as to prevent the adventurer (James) from smacking the poor critter in the head with that sword before it ceases to be one.
  • Since, along with Mordenkainen's Disjunction, these creatures were invented for the sole purpose of striking fear in the hearts of adventurers, they don't (have to) follow the conventions of evolution.

How could Rust Monsters effectively erode iron-based metals?

  • 1
    $\begingroup$ Hot acid? Remember those experiments back in school? But if you want to destroy a decent sword in mid swing, you might need a lot of firepower, intense (unrealistic) heat (laser eyes ftw) might be the solution here ... depends on how much time you've got $\endgroup$
    – Raditz_35
    Oct 20, 2019 at 20:18

1 Answer 1


Ok, so here’s the thing, in order to rust metal that quickly, you’re going to make it burst into flames. However, since we’re trying to strike fear into the hearts of daring adventurers, this is a big bonus!

As for how to do it, its actually fairly easy to explain. See, contrary to its name, oxygen is not the best oxidization agent we have. Enter fluorine, which oxidizes so strongly that it can cause water to burst into flames. So, your rust bug obtains some fluorine, and since we’re talking about D&D you can use a little handwavium to explain how they get pure fluorine and keep it from burning them alive, and they store it in a viscous, gel-like substance in special organs at the base of each antenna. On command, the gel is pumped through the antennae and out special pores. The gel sticks to whatever it hits and rapidly begins to evaporate, exposing the deadly fluorine within. The gel, and whatever it has touched, bursts into flames as the fluorine begins oxidizing everything around it. By the time the flames die down the adventurer has dropped his weapon/stripped off his armor and ran away screaming. When they come back later to retrieve their items, all they find is a pile of rust.

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    $\begingroup$ Hmmm. Setting the sword on fire kind of bypasses the "weaken structural integrity part". It wouldn't be healthy to breathe in the fumes either. Only if it could be done with something less extreme... $\endgroup$ Oct 20, 2019 at 20:32
  • $\begingroup$ Yeah, the fumes would be, let’s say highly unpleasant. The big takeaway here is that the less extremely this is done, the longer it will take. $\endgroup$
    – Nick
    Oct 20, 2019 at 20:36
  • $\begingroup$ Whatever that has flames and fumes should be enough. $\endgroup$ Oct 20, 2019 at 20:51
  • $\begingroup$ The fumes would be deadly. Using fluorine as a weapon would be highly destructive of most things in the vicinity including humans, rust monsters, swords and even concrete. $\endgroup$
    – Slarty
    Oct 21, 2019 at 8:25
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    $\begingroup$ True that, honestly I think the only way for the rust monster to survive using fluorine would be to either have a biology based around it, or to constantly stay coated in some kind of very non-reactive gel/slime, like whatever they would use to deliver the fluorine to the target. Otherwise everything is going to go boom real quick. $\endgroup$
    – Nick
    Oct 21, 2019 at 13:42

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