Back when God’s grandma was a little girl, Dungeons and Dragons’ focus was emphatically on the dungeons—and by “dungeons,” it meant not just dank, subterranean lockups but vast underground complexes containing entire societies and ecosystems. Player characters spent a lot of time exploring these networks of caverns, and there was little or no opportunity to pop back up to the nearest village and replenish supplies.
So when your front-line fighter got cocky and armored himself up like a Panzer IV, the Monster Manual provided a way to cut him down to size: the rust monster, whose sole raison d’être was the annihilation of plate mail when it was neither cheap nor convenient to replace.
This cheese beast lives on in fifth-edition D&D, and despite the absurdity of a creature nourishing itself on a pre-oxidized, chemically stable substance, we have to look at this unaligned monstrosity as an evolved creature, because any other explanation of its existence is just too meta.
Rust monsters are chitinous cave-dwellers (darkvision lets them see without sunlight), robust but not exceptional in all physical abilities, above-average in Wisdom and with no Intelligence to speak of. The composite portrait is of a bold, instinct-driven critter that goes after what it wants without fear—or subtlety—but has enough of a sense of self-preservation to skedaddle when threatened.
Rust monsters want to eat, not fight, so they won’t use their bite action unless cornered. Instead, they’ll primarily use their Antennae action, which “corrodes a nonmagical ferrous metal object it can see within 5 feet of it.” This means they have to close to melee engagement range; it also means they have to know the metal is there.
Thus, until a group of PCs approaches within 60 feet of it, a rust monster will move around aimlessly. Once it detects movement, it will venture in that direction. When it comes within 30 feet of an iron or steel tool, weapon, shield or armor, at which point its Iron Scent feature kicks in, it goes, “Aha! Lunch!” and moves directly toward whoever’s carrying or wearing it, then uses its Antennae action if it can get within 5 feet.
The rust monster is most strongly attracted to the largest concentration of iron. Thus, it might start moving toward a PC wearing ring mail and carrying a short sword but then come within sniffing distance of one wearing plate mail and carrying a longsword. In this case, it will change course toward the latter PC. Before the session in which this encounter occurs, total up each PC’s “iron points” to determine whom the rust monster will find most alluring: medium or heavy armor has FeP equal to its armor class minus 12; an all-metal weapon has 2 FeP; a wood-hafted weapon or tool has 1 FeP; a shield has 1 FeP (shields larger than buckler-size are typically made of wood with metal fittings, not entirely of metal); and a quiver of 10 or more arrows or crossbow bolts has 1 FeP.
(Note the wording of the Iron Scent feature: “The rust monster can pinpoint, by scent, the location of ferrous metal within 30 feet of it”—emphasis mine. This means that it can smell iron on an invisible creature, but it can’t use its Antennae action on it. Thus, the presence of an invisible, plate mail–wearing PC will boggle a rust monster profoundly. It wants the noms, but it can’t have them. It will just sit there, confused, waving its antennae in the air.)
Old-school D&D players will instantly recognize a rust monster—and the risk it poses—but if you’re a DM sending them against players who’ve never encountered them before, don’t describe their behavior as aggressive or hostile, because they’re not aggressive or hostile. They don’t “attack”—they “move toward,” waggling their antennae. For all the PCs know, they may just be curious, even friendly! That is, until it wraps those corrosive antennae around the PCs’ property and starts rubbing them vigorously against it, leaving big streaks of rust beneath.
Unless the PCs interfere, the rust monster won’t attack them, only their iron belongings. If they attack it, it has to choose between feeding and fleeing. It bases this decision on how large a meal it has in front of it. Once a rust monster has taken damage equal to or greater than the FeP of the PC it’s trying to feed off, it scuttles away, using the Dash action (it’s not nimble enough to Dodge nor smart enough to Disengage). As mentioned above, a rust monster uses its bite action only when cornered—that is, when it wants to flee but all paths of escape are blocked.
Rust monsters have a challenge rating of only 1/2, so one alone will pose a negligible challenge even to a party of level 1 PCs. More probably, therefore, a rust monster encounter will involve the PCs’ stumbling across a nest of rust monsters. (The MM disagrees, saying, “Rust monsters are rarely found in large numbers, preferring to hunt alone or in small groups,” but in that case, why bother? That being said, once your PCs are level 5 or higher, you can probably consider them to have outgrown rust monsters—at that point, they’ll have to be outnumbered 4 to 1 to consider rust monsters anything but vermin.)
Regardless of how many the PCs encounter, the rust monsters’ behavior won’t differ. In fact, they’ll all be most drawn to the same PC—the one toting the most iron—and will crowd around him or her, eager to nosh. Only if a single PC is surrounded by six of them (regardless of whether you’re using a square map, a hex map or “theater of the mind”) will any additional rust monster(s) settle for second-best, and it will take the same amount of damage to fend off each of them as it would to fend off one alone. (A rust monster settling for the second-largest concentration of iron will be driven off by damage equal to or greater than that PC’s FeP, not the higher FeP that its friends were drawn to.)
The MM flavor text notes that PCs “can distract the creature by dropping ferrous objects behind them,” which makes me wonder whether the authors have any experience at all with, say, raccoons. Dropping food for rust monsters is an excellent way of getting them to follow you. To satiate them enough for them to lose interest in your mouthwatering weapons and armor is going to take a lot of discarded iron—say, 10 FeP worth altogether.