Rust monster

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A rust monster, R, is a type of monster that appears in NetHack. It is one of the two members of the rust monster or disenchanter monster class.

Rust monsters are metallivores that are capable of swimming, but unlike other metallivores they will only eat rustable metallic items they come across, including relevant dungeon features such as iron bars - they will avoid eating other non-rustable metals, as well as the amulet of strangulation and the ring of slow digestion (if the ring's randomized appearance is made of iron).[1] A rust monster has two touch attacks and a passive attack that deal no direct damage - the touch attacks instead rust worn metallic armor, and the passive attack rusts metallic weapons used to hit them. These attacks have no effect on erosion-proofed armor and weapons.

A rust monster that eats a metallic item restores its HP up to an amount equal to that object's weight.[2] If a rust monster attempts to eat a rustproofed object, including you if you are polymorphed into a rust monster, the object is spit out, removing the proofing and leaving the monster stunned.[3][4][5] Tame rust monsters consider rustprone iron-based items to be treats.[6]


Randomly generated rust monsters are always created hostile.

The random R placed in the central room of the Castle at level creation is likely to be a rust monster.


Rust monsters are a common mid-game annoyance for players: their speed of 18 makes evasion and maintaining distance difficult, and their passive rusting reduces the amount of viable options for ranged attacks that can keep them from compromising metal armor. Fortunately, rust monsters have no monster MR, leaving them very vulnerable to wands and spells. Standard unarmored pets such as cats and dogs can more easily deal with them without fearing retaliation.

Wearing non-metal armor or rustproofing whatever metal armor and weapons you have will render rust monster attacks completely harmless. For players who lack the means to do so, unicorn horns are a solid backup weapon to use when fighting rust monsters, and Elbereth can push rust monsters back out of melee range, providing time and space to either fire wands and spells at them or else to remove any metallic armor before engaging in melee.

Even after they are no longer a danger to your own inventory, rust monsters remain an annoyance due to their tendency to eat any metallic loot lying around, often before you can get to it - this is especially undesirable on a level with bones and/or any metallic artifacts, though artifacts will resist being eaten or rusted 95% of the time.[7] Rust monsters can be used to deal with punishment in some cases by having them eat the iron ball and chain attached to you, though they will usually try to rust your armor afterward unless you use one that is not hostile, or else polymorph into one yourself.

Like other metallivores, you can also polymorph into a rust monster for the purposes of eating jewelry; though much faster than rock moles and xorns, rust monsters cannot wear armor at all, and their touch attacks are all but useless in terms of self-defense ability.

As pets

While generally unremarkable as pets, tame rust monsters are very easily fed if you do not care about picking up any further metallic items, and can be useful in some other minor cases, e.g. becoming punished or weakening the armor of hostile monsters. Be sure not to throw iron projectiles or other weapons directly at a pet rust monster!


The rust monster first appears in Hack for PDP-11, which is based on Jay Fenlason's Hack: Hack 1.21 instead has the oxidizer, which serves roughly the same role.

The rust monster is included in the initial bestiary for Hack 1.0. From this version to NetHack 3.0.10, including variants based on these versions, rustable weapons and armor subjected to erosion have their enchantment reduced; rust monster attacks could bring enchantments as low as -2 with their attacks, making them somewhat similar to modern disenchanters. Proper object erosion is introduced in NetHack 3.1.0.


The rust monster is one of many NetHack creatures that comes from Dungeons & Dragons, first appearing in the 1975 Greyhawk supplement for the original release. The rust monster and several other creatures in particular, such as the owlbear, were based on dime-store plastic toys that were used by Gary Gygax and other play-testers as monster figures, with their appearances based directly on the toys.[8]

A rust monster is a pony-sized creature that dwells underground, with a squat and humped body, four insectoid legs and thick, lumpy hide. It can smell metal from up to ninety feet away, and the long antennae that extends from under each of its eyes can instantly rust or corrode any metal they touch to the point of falling apart—its hide has the same effect on weapons that strike it, and magic items weapons have a chance of resisting proportional to their enchantment. A rust monster's diet consists of freshly-rusted ferrous metals such as iron, steel, and steel alloys, which it prefers to other metals; adventurers can take advantage of this by distracting it with suitable throwaway items.


<The rust monster> spits <foo> out in disgust!
A rust monster tried to eat an item that was rustproofed, stunning it and removing the proofing.[3][4]
Ulch - that <foo> was rustproofed!
You were in the form of a rust monster and attempted to eat a rustproof item;[5] this is followed by one of the two messages immediately below.
You spit out <foo> onto the <surface>.
You spit out a rustproofed item that you attempted to eat while in the form of a rust monster, dropping it onto the ground.[9]
You spit out <foo>.
As above, but the item is cursed and remains welded to you.[10]
The <food> smells disgusting to you right now.
You tried to eat a rustproof item while you had the food appraisal intrinsic.



In SLASH'EM, the rust monster is the first quest monster for Ice Mages and makes up 96175 of the monsters randomly generated on the Ice Mage quest.

The rust monster is also the second quest monster for Flame Mages and makes up 24175 of the monsters randomly generated on the Flame Mage quest. Many rust monsters are also generated on level creation for each floor of the quest: four each on the home level, locate level and goal level, two on the upper filler level, and three on the lower filler level(s).


In UnNetHack, a dilapidated armory is a special room that often contains rust monsters.

The random R placed in the room with the throne at level creation for each layout of the Castle is likely to be a rust monster.


In dNetHack, dilapidated armories contain 1-2 rust monsters.

The Sign of the Scion Queen Mother ward can repel rust monsters.

Encyclopedia entry

These strange creatures live on a diet of metals. They can turn a suit of armor into so much useless rusted scrap in no time at all.