Pale jelly, a j or j that has an engulf attack that causes illness and a difficulty rating appropriate to that. Some may be found on Juiblex’s level.
New monster ‘chicken’, added to the c monster class, but it is a normal chicken with no special attributes or powers. Eating its corpse gives some chicken related YAFM, and wielding its corpse and hitting a monster with it may make them flee; a comment in the code should indicate that this is because they are “chickening out”.
The Soul Forge: a single-level branch off Gehennom that contains a smith. The smith will take two items of the same type and forge them together into one; in technical terms this destroys one of them and imbues the other with whatever object properties the destroyed one had. They cannot both be artifacts, and if one is an artifact, it will not be the one destroyed, so the properties from the other one will be transferred to the artifact.
The smith’s price is based on how many object properties the resulting item will have, increased if it will be an artifact. In dNetHack, the smith would require payment in soul coins, but in variants without those, they may just ask for gold or some other valuable. If paid in soul coins, the requested type of coin would be random but deterministic based on the object IDs of the two items.
Add a unique lich or demon lord enemy who may appear as a side effect of raising the dead with the Book of the Dead. Not specified if this is the only way this monster can ever appear, or whether other mechanics like demon gating could bring them in as well.
Variant of Fort Ludios that contains Midas instead of Croesus. Midas has a petrification touch, but instead of turning you to stone slowly, it turns you to gold and creates a gold statue of you. Midas uses gold golems for his forces instead of dragons.
Mithril dragon, a dragon which spawns only if you (or possibly any creature on the level) is carrying over 75 aum of mithril in open inventory. It eats mithril items (and only mithril items, rather than being generally metallivorous).
Rust dragon, a type of dragon which breathes and makes melee rust attacks. It eats all types of metals. As with other metallic dragons, it is lawful.
Enhance Minetown with a bunch of new NPC denizens:
A secondary proposal is to make some of the denizens who teach you things actually useless a random amount of the time. For instance, the gladiator is a liar who’s good at spinning stories and not much else, or the aged traveler is a charlatan who gives misinformation, abusing Wisdom or sapping your experience points.
Red light, a y that has an explosion attack that causes paralysis. This is a reference to red lights in real life making you stop.
(There could also be green lights whose explosion gives a temporary speed boost, but since this isn’t harmful to the target it’s a sillier idea.)
Chromatic light, whose explosion attack causes you to see all monsters in random or consistently incorrect colors for a while.
A monster, probably in the y or e class, with an attack that steals an item (or a few items, excluding equipped gear) from your inventory and randomly teleports it somewhere on the same level. This makes object detection and detect treasure nice to use even after a level is fully explored.
Chaos dragon: a dragon with polymorphing breath, and whose scales grant unchanging. The dragon also has unchanging naturally (which raises the question of what happens if you polymorph into it, then die).
Cloth golem, which can be created by polypiling too many cloth objects.
Gelatinous ice cubes, which work mostly like gelatinous cubes but also have a cold attack.
Puffer fish, a monster with a non-poisonous bite but a passive poison attack. Its corpse conveys poison resistance pretty reliably but is always poisonous to eat, even if it’s been tinned.
Add a Craftsmen’s Guild to the game, as a method of advanced crafting (unlike the player’s simplified crafting such as alchemy crafting potions into other potions, or combining a blank scroll with a magic marker to make magic scrolls). The guild either has its own dedicated special level, or occupies part of a preexisting special level.
Metalworkers are probably the most important sort of craftsman for many characters. There could be multiple types, each specializing in one type of metal, or just one type that does everything.
New monster “aballin”, from D&D. It is a mobile j-class monster that can appear as a pool of water when not actively moving, and can possibly appear as a water elemental when moving. They can eat organic items like gelatinous cubes, though perhaps not instantly like gelatinous cubes, instead dissolving them some time after engulfing them. They are incapable of climbing stairs, and typically sit dormant imitating a pool until a monster draws near, at which point they become active.
Some type of monster that lifesaves you when you die, instead dying itself. This might be a new monster or not, probably something angelic (but not an Angel since you can get one on Astral).
A boss that, when first angered, takes no moves for 10 turns, and is invulnerable, though any damage you decide to do to it is tracked. After the 10 turns expire, the boss is given a HP maximum and other stats directly proportional to how much damage you did while it was invulnerable. If you kill it, it gives rewards also proportional to this amount of damage.
New monster “Phantom of War”, a powerful player monster style @ that appears on either the Astral Plane or elsewhere on the ascension run, but only if the player has successfully maintained pacifist conduct up until that point.
Orange mold, a F monster which has a passive sleep attack, and a small chance (equal to the other molds) of conveying sleep resistance.
Siren, a straightforward implementation of the ones in mythology. If you are within a certain range, you automatically move towards them at the end of your turn. You are protected if you are blind AND deaf AND not telepathic (telepathy overrides the protection of being blind and deaf, and in fact makes the pull even worse due to its heightened awareness).
A sentient and always-peaceful monster that you can recruit as a powerful steed, but you need to pay it a large amount of money to ride on it.
If adding a new centaur designed to be a significant threat increase over the existing ones, they should generate with good (if non-magical) armor and a good weapon, which is frequently a lance. If they have a lance, they will use it to joust you.
A rare type of mold that can be brewed into a healing potion via the Brewing Patch (assuming an implementation in which no existing molds are brewable into healing).
New monster “vapor cloud”, similar to the fog cloud, except it is made up of potion vapors and instead of doing damage, it causes the uncursed vapor effect of potions to happen to you when engulfed. (There probably has to be something preventing a vapor cloud of paralysis from paralyzing you eternally.)
When it moves over a potion, it consumes the (topmost if there are multiple) potion and changes. With 50% probability, it either switches to the potion consumed, or rolls for alchemy as if combining its existing potion cloud with the new potion (probably rerolling events like turning to water, evaporating, and blowing up in an alchemic blast, though it would be funny if it blew itself up).
Hostile vapor clouds will intentionally seek out harmful potions to consume and will avoid beneficial potions.
Polymorph vortices, a monster which has a passive polymorph attack (i.e. hitting it in melee polymorphs the attacker; hitting it with missiles polymorphs the missiles) and once per turn polymorphs either one random item from its inventory, a monster it has engulfed, or a random item from that monster’s inventory. Player-style magic resistance prevents the effects to a creature or its possessions that would otherwise be affected.
Blade vortex (v), which is a whirlwind of spinning blades. It has two engulf attacks that deal 2d4 physical damage each, and is difficulty 12. Apart from generating normally, it can be created by polypiling a sufficiently high number of blades at once similar to how golems are created. When killed, it drops an assortment of bladed weapons, mostly smaller ones like knives, daggers, and short swords, and possibly a chain or two. It can be harmed by thrown potions of water, rust traps, and rust attacks.
Another iteration of the proposal is to make its blades its inventory rather than just assumed to be part of the monster, with the following interesting implications:
A new type of worm that has 2 tails instead of 1.
Noxious sphere, a sphere similar to its siblings in that it has an active explosion attack, but the explosion is of poison and creates a 3x3 (or larger) stinking cloud centered on the sphere.
Aurochs, a “golden bull” like angelic creature that is similar to a ki-rin, but not quite as powerful. It is in the A monster class rather than q because the name sounds like “aura”.
New branch named “The Ancient Library”. It is likely accessed by portal from the main dungeon rather than stairs. It contains some spellbooks and a smaller number of scrolls, which serve as the main loot of this branch, but there aren’t too many of them in order not to be unbalancing. The level design makes it still visibly a structure, but fairly ruined with collapsed walls and such.
The branch is populated by monsters of a new species that either serve as the guardians of the library, or have moved in to inhabit it after its decline. They make for a difficult fight, and compounding the problem, taking a book will awaken and anger all of them on the current level. (Or the books spawn embedded in the walls – bookshelves, and you need to kick the walls to get them out, which triggers this.) There is no singular boss monster in the branch, nor is there any singular reward item.
If bookworms (#1793) are implemented, they also appear in this branch.
A monster, possibly called a “maid”, that cleans up the dungeon. It will pick items lying around on the floor, move chests and boxes into corners, wipe out engravings it finds on the floor, and so forth.
Sometimes Medusa’s Island is replaced by a new level Circe’s Island. Circe has a nasty polymorph touch attack (or perhaps can even cast it as a spell, at range), and may be accompanied by sea monsters Scylla and Charybdis.
Termite, a monster that eats wooden items it finds on the ground, and see them as treats if they are a pet.
A monster that can move any number of spaces in a straight line at once, but otherwise is only speed 16. It’s tough enough in combat to incentivize running out of its way or locking it behind a door.
A monster (imagined exclusively as a pet) that acts like your shadow in lit places, always being right next to you and possibly dying if it can’t be next to you, but if you’re in a dark area it is free to wander off.
Paper golems can grow up into “paperback golems”, which are at least twice as strong. When defeated they drop a couple blank spellbooks. Note however that some objection has been raised over nonliving golems being able to “grow up” in the first place.
A new R-class monster (possibly “nullifier”) whose attack cancels monsters it hits. Possibly it does some sort of object cancellation as well.
Evil librarian monster: it is peaceful at first, but considers every spell you know to be an overdue library book, and charges you a $100 late fee per book when it encounters you. If you don’t pay, it attacks. Possibly they carry some books, and if you manage to keep it peaceful, you can loan books from it. May generate with a magic marker.
Monster which is a shapeshifter, but regardless of what form it takes it always has an engulf attack. This is because it’s actually a variety of mimic in its normal form.
Shambling mounds, a cousin of the shambling horror. Inspired by D&D shambling mounds, but with the key non-D&D characteristic of having a single shambling-horror-like randomized attack. Compared to shambling horrors, they are weaker and appear earlier in the game.
Add a shadow monster, which is always next to you if you are on a lit square, including in walls (via limited phasing that allows it to be on wall spaces adjoining a walkable space but not on spaces not next to a walkable space). It is forced to the opposite side of you from nearby light sources. If you’re in a dark area, it will wander away, but it will immediately return to your side when you reenter a lit area.
Enchanter, a monster that enchants your gear when it hits you or when you hit it with a weapon, the opposite of a disenchanter. (Not to be confused with the enchanter rank title.)
Tulpas, a W-class monster that is exclusively summoned by casters, especially monk-types and Master Kaen, with several of Kaen’s appearing through the monk quest. They take on some of the properties of their caster, and leave no corpse.
Potion of astral vision, which confers temporary astral vision. To give this some use, also add some sort of being that appears on the Astral Plane and is only visible with astral vision.
A monster in the n or perhaps W classes whose main gimmick is that they will dodge or short-range teleport out of the way whenever they see an attack coming. In order to fight them effectively, you can’t be straightforward in attacking. You can use displacement, ray-bouncing, or invisibility (but they’ll recognize you’re using that pretty fast and adjust tactics accordingly).
Wandering peddler, an always-peaceful, randomly-generated monster that aimlessly wanders the dungeon. (Possibly, these could be implemented as shopkeepers that are detached from any shop, but that may be too complicated.) Chatting to them causes them to try to sell you items from their inventory, which they generate with several of. The items are mostly those eligible to generate in a general store, but with certain weight and possibly cost caps. Killing them either doesn’t drop their items, or it does but carries the normal drawbacks of murder and having to overcome a shopkeeper-quality monster.
In order to prevent farming items off them through create monster, these might have a low extinction cap of 20 or so.
Lava gremlins: a different gremlin species that reproduces in lava instead of water. It has at least one physical damage attack and at least one fire attack, and the regular gremlin intrinsic-stealing attack, except they steal intrinsics during any time that is not night.
Rainbow sphere, a monster that appears and behaves as if it were any other type of sphere with an exploding attack, but it does not die when it explodes; it becomes a different, random sphere type. There are two possibilities for how it can be killed: it could be brought down to 0 HP like normal, or it could have a limited number of incarnations, after the last of which it explodes and dies normally.
Internally, it should use newcham to handle its transformations; possibly it could be implemeted as a shapechanger capable of turning into any explodable e monster.
Snow golem, which is a ’ monster found in icy locales. Does not generate in Gehennom. It uses a spitting attack to launch snowballs (flavored as throwing them), which are an object which behaves like spat venom in that they disappear one way or another at the end of their flight. These deal damage and possibly knockback. The encyclopedia entry for snow golems uses “Frosty the Snowman” as reference material.
A new type of pudding or slime (though possibly a j or b monster) that has a high speed and good attack and splits whenever it takes damage. Its entire strategy is based on surrounding you with lots of clones that all have the same high attack as the original. Has a high monster difficulty; possibly, their attack stats and difficulty should be high enough that they will usually not show up until Gehennom.
This is unintentionally quite similar to Brogue’s fearsome pink jelly.
Add lamias, which are very similar to nagas, but have hands and consequently can use items.
Teleport vortex, a monster which moves around the level sucking up items into its inventory; items in its inventory are periodically teleported randomly around the level. If it engulfs the hero, it will do the same.
Bronze or brass golem. It would be either ’ or ’, depending on whether trying to avoid color conflicts with the gold golem.
Firefly, an a monster that is a light source, but a variable one. It ranges from being totally unlit to emitting light in a radius of 3 for a brief time. Is poisonous to eat, and with very low nutrition; has a mild bite attack. Tends to spawn in dark areas.
Internally, the deciding factor when a firefly should light up should probably use its mspec_used variable, which enables them to light up again when it hits 0. Also, cancelling a firefly should prevent it from lighting.
Disenchanters’ touch attacks can cancel individual items in your inventory (not just armor) as well as merely reducing the enchantment of armor. Or possibly add a new R monster that has this ability, and leave disenchanters as they are.
Add whales, which can engulf you. Several of them spawn on the Plane of Water.
Magpies, a bird that has a stealing attack; unlike monkeys, the stealing attack is limited to shiny objects (possibly based on their material) that are under a certain weight. Under that rule, rings, gems, and amulets are common targets of theirs. They probably have a rather fast speed, too. They should likely generate in small groups.
Magpies would have an alignment of 0 and lack the M2_HOSTILE flag, and thus may be peaceful to neutrals, but angering one makes all other magpies angry as well.
A monster that, similar to leprechauns, steals items made out of silver, and only items made out of silver.
A monster whose attacks scale in direct proportion to your Luck - the luckier you are, the more damage it deals.
Add a new type of eye with a hallucination-inducing active gaze attack. It could be called either “third eye” or “pseyechedelic”.
Glowworms, a w monster that emits light in a radius of 1. Maybe they have a blinding bite attack, but that may be taking the light theme a bit too far.
New Biologist role, which revolves around polymorphing monsters, brewing potions (probably non-alchemically), and possibly collecting samples from monsters.
Possibly, they start with a number of potions of booze, but are penalized for drinking booze; it’s intended to use for brewing with samples. They also possibly have a guidebook describing recipes (other roles can do this too, but lacking a guidebook they won’t be able to do it very well).
Starting equipment may contain:
One take on the quest: it sees you fighting the Black Mold (a mold that constantly multiplies itself and attacks with random elemental damage, including possibly disintegration, when touched). The quest artifact is the Elixir of Immortality, but this is not possessed by the nemesis; instead you have to brew it yourself using an ingredient found on the goal level and which will be destroyed by the Black Mold if you don’t get there first. The Elixir isn’t needed; the quest is counted complete if you destroy all the Black Mold.
They can get Expert in only a few skills, including knife.
They are penalized for “animal testing”, which is polymorphing or otherwise abusing living peaceful or tame monsters. Or possibly it’s okay as long as the subject isn’t killed. However, they are rewarded for other forms of experimentation.
Implement hoard scarabs from D&D. They are a x, which can hide under object piles containing at least 25 gold pieces (and only object piles like that; they can’t hide under single non-gold objects or furniture or anything; possibly they can only hide under a gold-containing pile when the gold is on the top). They have a nasty poisonous bite.
When the game wants to generate one in a random location, it either creates a gold pile for the scarab to hide under where it generates or alternatively it searches around the level for a suitable gold pile and puts the scarab there. (If it can’t find one, it may just fail to generate).
They may also pop out at you from inside randomly generated gold-containing chests, possibly as a container trap effect.
A trap or monster (or possibly change sleeping gas traps or homunculi to this) that causes “sleeping poison”: it starts a very short intrinsic restful sleep timeout, so you don’t fall asleep immediately but have a little time to react.
New Medusa-alternative level (like the Grue and Echo levels in dNetHack): Chrysaor’s Palace, containing a fair amount of water, soldiers, and Chrysaor, who wields a +10 gold broadsword and a gold shield of reflection, and whose attacks and resistances are on par with Croesus or even better.
Add clerical spellcasters that are more dangerous than shamans but less threatening than priests.
A b-class monster which is a shapechanger and may transform into any P, b or j monster.
A sessile invisible monster with a nasty passive attack.
Goldfish, a ; monster that is level 0, always generates peaceful and has no attacks. The purpose of having it is for flavoring special levels, especially quests.
Fire/flame nymph, n, which normally only spawn in Gehennom (or possibly near lava not in Gehennom) and are noticeably stronger than other nymphs. Have two steal attacks which burn any items stolen, and also a fire attack. Potentially a passive fire attack as well. They also explode into fire when killed.
Bookworm, a w monster that eats books (they end up in its stomach/inventory and aren’t destroyed, though they may lose a read charge). Levels up, gaining power and speed, for each book it eats (higher level books give it more power), eventually gaining more attacks. Possibly can also eat scrolls. Initializes as having already eaten a few books.
Satyr, a h monster that is either chaotic or lawful (either could fit), are always male, generate with booze frequently, and chase after any nymphs nearby (who flee or teleport from them). Appear in the Ranger quest frequently.
Carpenter ant, which eats wooden items on the floor and can tunnel, but only on wooden terrain (doors, trees).
Bumblebee, very like a killer bee, but its poison sting is worse. However, when it stings you, it becomes cancelled and can’t sting you again; a cancelled bumblebee additionally loses 1 HP per turn.
Fairies, a n monster similar to pixies: very small, flying, capable of hiding under objects. However, they are lawfully aligned and (?) always generated peaceful. If you chat to a non-hostile fairy, it will heal you a moderate amount (as in the Legend of Zelda) and become cancelled (a cancelled fairy can no longer heal you).
Burrowing monsters that don’t actually dig out tunnels (or phase), but instead move through solid rock non-destructively. They ‘‘only’’ move through solid rock; they won’t move into wall spaces or move into corridors or rooms of their own volition.
Will-o-the-wisps: gray or possibly bright blue y-class monsters that spawn individually, leave no corpse, and have no attacks except a passive blindness attack. They emit light radius 0 so you can see them, but no surrounding squares, across a dark area. Not infravisible, and could have teleportitis to replicate their folkloric effect of suddenly vanishing. Spawn rarely in swamp rooms and may not spawn at all in a lit area. Most importantly, they have a special AI that causes them to avoid the player and orbit around treasure (occasionally) or around hazards like monsters or traps (commonly).
Darkness elemental: darkens squares it moves through (removing the lit state, if the square was permanently lit beforehand).
Wyverns and/or wyrms that share the D glyph but don’t have breath attacks. Probably brown colored, since no dragon uses that.
Itinerant merchants who, if chatted with, offer one of several non-renewable trades. Generate on stairs, path to the opposite stairs, and once they reach them they disappear forever from the game. Generated with the requisite inventory to make all their trades. Always peaceful. Tougher than shopkeepers, since they have to defend themselves out in the dungeon. If you kill one, no more will ever spawn.
Dementors, W monsters that have some sort of blinding, despair-inducing attack. Lawful aligned. If you become completely incapacitated by one, they suck out your soul for an instadeath.
Arch-foocubi which do the same seduction as regular foocubi, but the random number they choose has a higher average than a foocubus’ 1-35, so the chance of a bad effect is greater and it’s less effective to boost your own stats. Perhaps rnd(35) + 25. They are also more grabby than regular foocubi, and will steal jewelry if you have insufficient gold.
Monster or damage type that un-erosionproofs gear, not very common.
Monster that steals your intrinsic invisibility (but only that, unlike gremlins).
Some sort of late-game nymph monster that steals items. Helps mitigate an underlying issue: some of the early-game threats like stealing are completely absent in the late game, with no particular reason for that being the case.
Pixies, an n monster that can hide under objects. Chatting to them will make them say “Hey! Listen!” as an Ocarina of Time reference.
Mosquitoes, an a-class monster which uses an AD_DRHP attack, which replenishes the mosquito’s health by some fraction of the damage it dealt you.
Slime mold as a new F monster, which is guaranteed to drop a slime mold comestible on death.
Silver golem, which naturally causes silver damage to silver haters, and drops silver items on death. Should be pretty rare to avoid giving the player their pick of silver items.
Demon lord who has no real special abilities, but has a very, very high physical damage output. Or give one of the existing, uninteresting demon lords a high physical damage output.
New demon lord with a fire attack comparable to Asmodeus’ cold attack (this is actually already implemented, but is unused). It seems weird that no demon lords are really associated with fire.
New demon lord who spellcasts, favoring touch of death and summon nasties. However, the only thing it summons and gates in are incubi and succubi.
Rat king (a randomly generated monster, not the SLASH’EM unique Rat King), based on the real-life rat king. It is a r, not much faster than a rock mole, but has several bite attacks (possibly scaling the amount by monster level). Each bite has a small chance of conferring disease. Its level corresponds to the number of entangled rats, so a slashing attack could sometimes cut off one rat, which then appears on the floor as a new rat, and makes the rat king lose a level.
A monster that gets more powerful every time an attack misses it.
A level 5 or 6 divination spell that creates a tame speed 0 monster. This new sort of monster gives you permanent vision in a radius around it if it is tame.
Psuedodragons that are tiny D-class monsters. They may be the starting pet of Wizards, but don’t grow up. Instead, they have some kind of telepathic/psychic/magic-enhancing passive ability that scales with their level.
Add spheres that give buff effects and deal no damage to whatever they explode at. They target tame monsters if tame, and hostile monsters if hostile. There is a spell that summons them.
If monsters with area-of-effect passive attacks are implemented, these monsters should perhaps be eyes instead of spheres, and have the buff effect as their passive, and no explosion attack.
Add stunning and confusion spheres, which cause those effects when they explode.
Electric spider that, when it stands on a web, can electrify that web and all contiguous ones, dealing shock damage to anything in it.
Alchemic golem, which is basically made out of potions. Its melee attacks expose you to random potion or potion vapor effects. Killing it drops a few random potions and causes more random vapor effects. It can spontaneously explode when it hits something, with a greater chance when you hit it in melee. The explosion obviously also causes multiple random vapor effects to the area, and scatters potions around (by throwing them, so they tend to smash if they hit anything).
Luggage pet for Tourists. Has very high carry capacity and doesn’t drop items of its own accord. Fights like a normal pet. Either it is immune to damage but has a low maximum level to compensate, or if overwhelmed, it doesn’t die but instead turns into a normal “luggage” object that will eventually reanimate.
The common suggestion along these lines has been to give tourists the Luggage as their starting pet, but the common criticism of that idea is that it makes Tourists far too powerful from the outset. Instead, a possibility is that you can trade the Platinum Yendorian Express Card to Twoflower in exchange for the Luggage. (The Card then vanishes, so you can’t arrange Twoflower’s death and get both).
Checkerbug: like a grid bug, but can only move diagonally.
Bluff monster: a shapeshifter that is biased towards powerful forms and nasties, but it actually retains the weak attacks and HP and AC of its regular form.
Poisonous tree frogs, which only generate in trees. Applying darts or arrows to them can coat the darts or arrows with poison, but the poison can also get on your hands.
Drop bears: possibly t-class monsters (or z-class, if the SLASH’EM zouthern animals monster class is used) that hide on the ceiling, generating next to or near trees. They are unimpressive in combat, except for the nasty amount of damage they deal with their initial dropping attack. Can only go back to hiding when next to a tree.
Phoenixes: birds with fire resistance and who may automatically resurrect on death, or else revive from their corpse/ashes similar to trolls.
Thunderbirds: birds with lightning breath.
Rocs: large, rideable, fast and powerful birds.
Swarm monsters which are invulnerable to weapon attacks (or else take extremely reduced damage from weapon attacks, making weapons a last resort) and instead must be hit with area-of-effect attacks. Given the relatively low availability of area-of-effect attacks, the swarm should go down in one or two hits from one.
New monster “master grid bug”. When they zap you, you get a status effect that forces you to move only orthogonally like a grid bug for some time.
Worker ant that restores walls that have been dug out.
Special level or branch that is populated by dinosaurs and an intelligent reptilian humanoid called sleestaks. There is an artifact somewhere in here that is guarded by the dinosaurs and sleestaks.
Smoke cloud monster (v), has a mild fire damage attack and a suffocating smoke attack. Generates in fiery areas.
Flesh-eating scarab beetles: they may reproduce every time they bite you.
Add pegasi, lawful u-class monsters that fly, have a slightly higher base level than a warhorse, and speed around 25. Might be tameable with vegetables, or only blessed vegetables, or one certain veggy food, though.
In variants that give Geryon a lair, implement his two-headed dog Orthus.
A t-class monster that generates on a wall space and pretends to be part of the wall, then attacks/eats you when you step next to it.
A flying monster with a displacement attack, which attacks from water in order to displace you into water. Also a bull monster that charges at you, and will either displace you if you pass a Dex save or gore you.
Yellow lights possibly have a light radius of larger than 1. Also add white lights, white-colored y-class monsters which have a light radius of 3 or 4, with a blinding gaze attack and no explosion attack (unless keeping the explosion attack is too thematic to the y class, in which case it should blind for a long time).
Add in master lichens as a real monster, which are only a bit more threatening than normal lichens (maybe speed 2 or 3, have a passive sticky attack like mimics) but have an increased difficulty so that they generate deeper in the Dungeons, and are a source of nonrotting corpses. Possibly, the passive sticky attack should also be given to regular lichens.
Chemistry golem, which spawns with some potions that it will either throw at you or quaff.
There should be one major monster of each alignment that is ungenocideable: Archon for lawful, deep ettin for neutral, arch-lich for chaotic.
Bards as a monster, who spawn in a group of same-race monsters with a (sometimes but not usually) magical instrument, and they keep their distance and play songs to buff their allies.
Dragon with slowing breath. The effect stacks with previous effects of slowing breath.
Sphere conjurer: a monster who creates hostile spheres.
Barbed orcs, which are more deadly than regular orcs.
Add a dogcatcher, who will come and impound your pet if it misbehaves (stealing from shops, killing Minetown citizens) and you don’t get it leashed before a certain amount of time has passed.
A monster that stands helpless if you can see it with normal vision (not infravision, or telepathy, or detect monsters or anything else), and attacks viciously if you cannot.
Wind dragons, a cyan D whose breath is a wind beam: it doesn’t damage you, but blows you backward, and can’t be reflected. Their scales grant flight, which now confers resistance against knockback and Newton’s Third Law.
Buff late-game undead, because undead’s significance in Gehennom tends to be pointless since they aren’t scary by the time the adventurer gets there.
Net gnomes - wield tridents and have a sticky attack (the net), which causes subsequent attacks to hit automatically
Roditaur: new humanoid monster that has an oversized rat’s head and four arms. Has a powerful bite attack and four weapon attacks.
New monster: green cube, which combines the abilities of a green slime and gelatinous cube: both sliming and paralysis.
