how would monsters adapt in a high power society?

[King of Nowhere]

I'm planning a world for a future campaign. it's a world that has evolved, both technologically and magically. a world where constructs and undead perform the menial labors, and a mixture of magic and technology is ubiquitous. it's not a full-blown tippyverse because some elements that are necessary for a tippyverse are impossible within my campaign world, but it's still a world of great power. with primary needs covered, the population can be put through a mass educational system, which generates a lot of wizards. fighter types are highly trained and covered in magic buffs.

this is not a world where monsters can threaten civilization, or even high level adventurers. even if the tarrasque itself showed up, it would be at most 30 minutes before someone managed to launch a sending to a major seat of power. and a major seat of power would be able to alert high level adventurers, who would teleport in and trounce the poor creature. the tarrasque wouldn't last one hour. other denizens would last even less.

so, I'm thinking on ways to make monsters still dangerous for people who will be decently leveled and with overblown wealth. people who will not be intimidated by any dumb brute.
the obvious route is to just increase the monster stats, but i won't do it. it would make the premise pointless. it's also not how a world would evolve. when humans invented weapons to fend off the lions, the lions did not grow bigger and stronger. they learned to avoid humans. instead, other creatures contended with us. rats thrived in our cities, and a lot of vermin.
I'm thinking along that route: the big brute monsters either went extinct, or they are confined to remote areas and natural preserves. the monsters adapted for humanoid society will be scavengers. they will rarely fight directly, they will mostly be pests, and they will survive mostly by being good at hiding and breeding fast.
at the same time, they should be something that can be a legitimate opponent. a mosquito biting you and potentially giving you a disease is not exactly an exciting encounter, unless you can swing your sword at it.

and since this is a fantasy world, most of them will feed on magic and will have magic abilities. basically, a high magic society is a great food source, which they use to fuel their special abilities.

one idea I have is a rat-sized animal that would hunt for magic items. alone, it would just bask in its aura, and perhaps cause it to work at a slightly reduced power. in a pack, it can drain all the power from a magic item, destroying it. swarms of those animals would be a nuisance in cities, and could be a problem for adventurers. they would gain mostly defensive abilities, like some resistance to magic that would make them more resistant to mass-scale extermination. something like a minor form of invisibility and resistance to seek-and-destroy attacks. society would mostly deal with the creatures by putting all the magic in closed containers, to deny them food. just like we deal with rats.

diminutive-sized parasites trying to lay eggs on your equipment, draining your spell slots, infecting you with disease... as long as you can have something resembling a regular battle encounter - with the monster's "attacks" threatening to inflict something unpleasant, and every member in the party being able to contribute to kill them - everything is fair game.

i'm brainstorming for ideas.

[John Out West]

Yeah i've been working out something similar for my futuristic-fantasy rpg, although I didn't really think it would be problem as much as a fun lore-building experience.

Minotaurs wander aimlessly in the labyrinthine sewers under the city, and the police are too busy and too few to bother investigating rumors, but instead reward a bounty for their heads.

Harpies populate the high-rises of abandoned districts, the building's owners having gone to the moon a century ago. Legally neither bounty-hunters nor the police can enter the private property, so instead warnings are given to avoid the skies in any ship less than B-class, and men are warned not to walk the streets alone during mating season.

Vampires don't exist, nor did they ever. Their only mention is written on the giant trees in the preserved elven libraries, which are far out of the hands of the pale aristocrats, who seem suspiciously keen on making sure that everyone knows that Vampires are simply a fantasy.

My modern problems include Clones who weren't given souls, and thus turn monstrous and evil, and Software corrupted by The Devil, creating an evil form of Malware intent on destroying, corrupting, and defiling the real world.

Hope that helps!

[King of Nowhere]

Yeah i've been working out something similar for my futuristic-fantasy rpg, although I didn't really think it would be problem as much as a fun lore-building experience.

Minotaurs wander aimlessly in the labyrinthine sewers under the city, and the police are too busy and too few to bother investigating rumors, but instead reward a bounty for their heads.

this would be an excellent plot for a low level party. One major problem of DMing a highly civilized world is that whatever problem arises, there are people qualified to tackle it, and it's hard to find suitable tasks while the party is still low level.

[D&D_Fan]

If everyone has the capacity for superhuman power and high magic, then the the monsters that are actually a threat would be truly incomparable titans. The strong monsters would likely be immune to all bludgeoning piercing and slashing, resistant to all magic, and immune to magic under 6th level. Have antimagic abilities, and magic attacks. Also like 400+ HP.

[NigelWalmsley]

Honestly, they probably wouldn't. If you look at the real world, the danger to any individual person from wild animals is pretty close to zero. All the animals that are dangerous to people because they are big and strong have been driven out of all the places humans live in numbers, and exist primarily at the sufferance of humans. If we wanted grizzly bears to go extinct, we could just do that, and there's nothing they could do to stop us. If the power imbalance isn't sufficient that monsters can regularly wipe out civilization, civilization will eventually wipe out the monsters. Because no one is going to willingly raise their kids in a place where you can't let them play outside for fear of being eaten by wolves.

Since you probably won't be able to figure out a setup where civilization can't kill the monsters, you'd have to look at reasons why they wouldn't want to kill the monsters, or at things that aren't monsters (e.g. empire of <your favorite fantasy evil> based off of <your favorite real life evil empire>). The obvious example of the former would be trophy hunting, but that's probably not great for building adventures around. "You guys are hunting Wyverns for prestige on magic!Facebook" doesn't have the same cachet as "you guys are hunting Wyverns because they killed the mayor's daughter".

A better possibility would be something like Dune. In Dune, Sandworms are giant monsters that are both able and willing to eat mobile factories whole, and yet people willingly come to the sandworm planet and do heavy industry there. Why? Because it's the only place in the universe where you can find The Spice, which resists aging and makes FTL possible. So if you were to postulate that Torrasques crap immortality juice, you could have adventures where players have to guide harvesting crews through Torrasque territory in order to support the demands of a growing gerontocracy.

Similarly, if monsters are far enough away that they aren't threatening day-to-day, there's not much reason to try to wipe them out. So you can have whatever adventures you want if they're set on other planes, or if you postulate that civilization is expanding, and there are places where they haven't yet slaughtered all the Wyverns. That gives you a kind of Age of Exploration or Manifest Destiny feel, where the players are expanding into unclaimed territory (and, of course, all of the uncomfortable realities where the territories those expansion took were not, in fact, unclaimed).

You could also look at the things that are deadly in modern society. A lot of those are things that don't lend themselves well to monsters (e.g. it's hard to imagine a convincing fight where the enemy is analogous to Heart Disease or Cancer). But things like industrial accidents and natural disasters. You could have fantasy Chernobyl, where the undead powering a local manufacturing facility have activated a latent necromantic intelligence that threatens to advance itself into a dark godhead and the PCs have to go shut it down. Or fantasy global warming, where rising temperatures cause the horrors from the deep to unpredictably rampage through coastal cities.

There's also, as mentioned, war. A lot of creatures that you probably wouldn't allow to run around your neighborhood seem like they'd be pretty good to drop on top of your enemies. A Shadow is a self-replicating WMD that's fairly difficult to kill even for reasonably tough opposition. Anything with AoE attacks and passable DR will scythe through low-level troops. And you don't necessarily have to postulate an actual war once you've acknowledged that various nations have their own Monstrous Weapons programs. You can have an adventure where you figure out why a blacksite experimenting with Phase Spiders went dark, or sabotage the containment on your enemy's Strategic Allip Reserve.

[Ironsmith]

Shocker Lizards are a much more common menace, since humans give them prime nesting grounds wherever they build power plants (which are also isolated enough that they can't be dealt with easily/immediately, and any attempts to do so just make them more aggressive).

Chimeras, Wyverns, and other large fantastic creatures are kept as curiosities in zoos, but they don't always react well to being caged. This presents an additional dilemma; the monsters in question are endangered species, making outright killing them a bit of a non-option.

Mimics and Oozes of all sorts infest city slums, preying on social outcasts like the homeless. Aggrieved relatives may send the PCs in to investigate a loved one's disappearance, to later learn that they were eaten by such monsters.

Doppelgangers give a whole new depth to the notion of identity theft. While they're not going to target major political figures, since that will draw way too much attention, the average person may still have to worry about them.

Devils, Demons, Hags, and Vampires are all also capable of blending into society normally, and may arise to positions of power as a result; worse, they're ripe for developing major conspiracies.

[Yanagi]

1. In the situation you're describing, big animal-intelligence monsters would probably experience a fate similar modern megafauna.


In developing, a "modern" tech and magic society there's going to be an expansion of resource use in ways that transform open wild spaces and destroy habitat. Even if in the current moment there's fully magical automated luxury communism, there's probably a few centuries of dividing up land and creating farms, consolidating farms into bigger and bigger patches, strip mining, just letting sewage and industrial waste get into bodies of water...and most importantly, just more and more people spread out over more and more space. There's going to be a tipover point where monster killing becomes mechanically mundane--like how buffalo killing just became a numbers game, or how tiger hunts in India became canned mass kill events. What used to be heroism will become either being an exterminator or working in land management.

Generations before the "present" of your setting, academic flourishing and publishing means that study and sharing of information would make monster's behaviors no longer surprising, their strengths and weaknesses widely know. Engineering plus more mass production means new paradigms in manufacturing new specialized equipment for uncommonly tough monsters. Modern educational training--and in particular, transformation of training in military and policing functions--means these problems would not be dealt with by roving bands, but with a coordinated, centralized force...and the farther along in magitech development, the more options there are on remote systems: a golem is a man shaped drone.

In real life hunting went from a necessity of subsistence and safety to a sport, but then inside of "sport" it still oscillates between a still-risky prospect done individually requiring craft and skill to canned hunts where there's an infrastructure of skilled employees that trim away most of the challenge to create a purely aesthetic experience of "the kill." And since monsters are, well "monsters" in the public imagination, killing them is going to be seen as generally good, requiring no thought. In the US people used to just shoot at buffalo out of train windows because they were seen as a nuisance, taking up space that could be full of cows.

Chances are many have gone extinct or are going extinct because there was zero incentive to keep them alive. I mean, if you're clearing a swamp to create good bottom land for rice, why would you value something so inherently hazardous, like musk creepers or hydras?

Would people factory farm the monsters that produced useful byproducts? Absolutely. In a society with many wizards, magical reagents are now commercial items and there's supply needs to be filled.

Would there be the bloodsport equivalent of bear baiting or dog fights? Yes. The cachet of these creatures as "monsters" would be something built up generation by generation, and people would be morbidly curious to witness and control that power. There might be zoos, parks, circuses. There would definitely be the equivalent of "guy with a tiger, living in an apartment" and "person who rashly collects super poisonous snakes."

Would the equivalent of conservationists turn up, to advocate for these creatures? Dunno. Naturalism and ecology would be very different things in a fantasy world where so many things seem to exist only to be threatening to people. I suspect, eventually, people would notice that the possibility of none of this thing, forever, ever again...but monsters are understood as "monsters": every single individual one evokes the kind of fear that and awe of the Tsavo lions. Extinction might be seen as win; nobody's signing that petition to preserve the wetlands for Lernean hydras.

2. Many small monsters would become urban pests.

Cities are an adaptation niche unto themselves: all that infrastructure to create year-round preferred living conditions while cranking up the population density, the enormous manifolds of enclose temperature-controlled space, the massive waste stream.

In a magic world, there's some obvious cases for "pest monster status"...things that would do incredibly well in urban environments because of their combination of needs and behaviors. Anything that can wedge into a physical space that human(oid)s create but don't completely use wins. Anything that eats what human(oid)s throw away wins. Anything that has a short life cycle and produces a lot of kids most of which die off will evolutionary adapt the fastest. Clever and social species that can acquire learned behaviors that allow ducking around human(oid) management systems.

So there's going to be stuff like dandelions and henbit...they just lucked out that people created conditions that make their adaptations even more useful. And there's going to be rats...able to live off the waste stream, small enough to fit into the gaps in living spaces, can survive a bust in their numbers and bounce back. And pigeons. There's going to be the equivalent of urbanized animals like raccoons, foxes, and crows, where they're clever enough to understand people rhythms and habits to thrive.

I mean, it's going to matter what magic urban infrastructure looks like in specific terms, but there's definitely going to be gaps. Stuff like slimes and rust monsters would do very well. If there's waste stream to magic, then things like abominations and creatures that eat magic would be like seagulls at landfills. And with modern supply chains there'd also be the monstrous equivalent of invasive species: it wouldn't be different pest monsters in different regions, but really successful types just spreading all around.

Problem is, this won't create traditional "monster" problems to be solved by adventurers. It will be a problem for animal control, or exterminators. And the most likely danger from pests monsters will be indirect: they damage physical or magical infrastructure, causing something to fail, causing injury and death.

3. Systemic failure means there would still be monsters and monstrous threats.

This is a hypothetical society with a roughly sketched out structure. By fiat it can have any shape, and problems can just have been solved. But:

It is exceeding likely that the described society will have exactly the same kinds of problems that global society has, because the magitech solutions presented don't necessarily change how people think about the world, or the holes in their perception. People with more power will accept their status as just and right, and find reasons to look down on and not help people with less power. Level 20 wizards will not necessarily feel empathy for the struggles of Level 5 wizards who just don't have the chops to do better. People in places with more accumulated resources...more of that magical infrastructure...will see themselves as having "earned" what they have, and have no interest in sharing with places with less. Identities--nationality, regionality...will continue to exist, and influence who helps whom how much, even when there no explicit conflict. And people--as individuals and collectives--will try and create hierarchies to climb and stay atop because they crave power as an end unto itself.

Magical infrastructure...magical education and golems and adventurer wizard expert terms and items...is no more likely to be distributed according to need than, say...real life distribution of food and shelter. It's not just that it would be inequal, it's that there would be a whole belief system about why that inequality was just. If there's regions (or neighborhoods) that get let services, where the infrastructure is failing, and where the human resources aren't allocated (whatever policing equivalent equal "monster control")...that's where there would still be old fashioned monsters roaming about. Even if everyone is a wizard on paper, that doesn't mean that everyone is a battle-ready wizard who has all the reagents and materials to fight every day. Failed rural regions where the center pillar of the economy crumbled and its outskirts. 'hoods and housing projects that have seen better years. Areas that haven't recovered after a war or some other kind of disaster. Any place there's a demimonde...that area of the country or city where people go to do illegal things, to operate in the shadow parts of the economy.

NIMBYism and whataboutism would still be in effect. The idea that monster mortalities were inevitable "...in those places" would turn up.

Magical infrastructure itself would include hazards. Golems go berserk. Undead are public health risks. Massive magical workings could have unintended consequences that can only be ascertained in the long term. Magic that reaches into other realms has social, even diplomatic, component, and scaling up those kinds of castings may cause problems: gods get wrathful and demanding, planar lords start playing intrigue. There would be an accountancy of cost in lives versus convenience of magic effects, and there would be people and groups willing to devalue lives relative to their convenience.

Indeed, individual magical power would likely exacerbate the ability of individuals to disrupt general welfare. In real life, individuals can acquire enough power...influence, capital, capacity for lethality (armed goods, WMDS)...that they can evade accountability or become shadow arbiters of social norms. The single most common theme in stories about wizards is that wizards accumulate individual power and lose track of other people and their humanity. In both literary settings and fantasy settings, this tends to be because individual magic power is the shortest, fastest path to hubris. Creation and destruction at will, no checks or balances because the power is literally your own and no one else's.

The folly of literary magic-users is like speed run of the folly that overtakes real people who just never have to hear the word "no." In a full magic society an archmage is still a WMD...how do you call them to account, if they decide to breach laws and norms? If they want to surround themselves with dangerous things or rashly summon things or create blasted arcane wastelands, how do you stop them? Which is all to say:

Where are the monsters? Where the most powerful people allow them to be...by active choice or just by apathy.

4. Industrialization plus magic will create new vistas in undeath.

Most monsters are just magic animals; their behavior is described as continuous with animal roles.

But undead-types are mostly the downstream products of human(oid) activity, and the more magic there is and the more people there are, the more undeath is going to permutate into new forms and new hazards.

The former--new forms--involves two separate phenomenon.

The first is that "natural" undeath is a kind of trace of the living person: ghosts with "unfinished business," or trapped repeating some loop of traumatic experience and emotion; gestalt undead that represent many people. More people, more concentrated, continuously surrounded by magic is likely to produce unusual new permutations of undeath that occur naturally. Giant urban gestalts fed by the ennui and resentment of people that lived and died as middle managers; drudgery ghosts continuing to go through the motions of their job in shutdown factories full of broken machinery; office wights bound to their desk.

(And yes I am thinking of that last one in terms of trying to "Turn Undead" Stephen Root's character from Office Space)

The second is that necromancy plus the infrastructure needs of a highly developed technological society is going to create a niche market for customized undeath. And, in particular if the society has powerful individuals with more resources and less accountability before the law, custom undeath wouldn't be about individual choice but about controlling laborers in an entirely new dimension. "Menial jobs will be done by undead and golems" kind of glides over the question of whose corpses are doing what, potentially forever. So on the low end of complexity, mechanized undeath means there's either a market for dead bodies, or a legal and social norms in which the dead are turned over to converted into mechanized undead as a civic duty.

But more sophisticated forms of undead are available--vampirism, lichdom--where personality and skills are retained, and...theoretically...there's going to be both practical and perverse incentives to reach for these. Civic-minded undeath might include weighing the option to take on a role forever by intentionally assuming sapient undeath; powerful individuals might decide to transform themselves so as to never have to release their assumed titles and authority. However...what if you had a great deal of power, an ability to duck accountability, and wanted one of your employees to not leave (not die), to keep doing their research job or their particular skilled craft? Can you make an undead that retains its skill with a lathe, to do nothing but turn out coffee table legs? Can you strip out the bits of free will that make people complain and job hunt, but keep the bits that make an excellent lab worker? Even if there were rules against doing these kinds of things, the incentive to do it would always be there.

5. Anything intelligent that could assume "normal" humanoid appearance would have an interesting historical arc.

I'm not sure if the word "monster" is being used in the broadest sense or a more refined one that excludes stuff like dragons. If it's the latter, then just skip this. If it's the former:

Relative to the described present, there's going to be a moment in the past where the clever monster types--stuff like rakshasa and fey and dragons--notice the shift in human(oid) societal structure and start to alter their own behavioral patterns. Some will choose direct resistance, some will choose flight...but the interesting possibility is those that choose collaboration.

One path for monsters in the described society is to become necessary monsters.

Again, think about the historical build up process to a modernized, urban culture--lots of bumpy patches over the last few centuries as power consolidated in a small number of socio-economic entities that monopolize violence and have the resources to create vast infrastructure that keeps the supply chain going. And at every point of the way, you have those entities competing with one another.

Governments would have uses for giant magic-resistant highly intelligent spellcasting flying personnel, both in civilian and military duties. Economic entities would salivate over the nest egg represented by a dragon hoard. Intelligence would want shapeshifters and mind readers.

Any creature with some useful inherent ability and the mental capacity to negotiate would weigh the pros and cons of allying with a humanoid side, making themselves critical to the effort. Those with especially large resources and a talent for manipulation might start their own humanoid institution, creating an entire system that functions in the modern humanoid world to conceal and protect them: in effect, the supernatural equivalent of shell corporation. For extraplanar creatures (if those are even a thing), this is doubly true: there's a path to power in identifying what this society needs that can only be derived from unusual sources in Prime reality, but they can access in the planes...the planar equivalent of "buy low, sell high."

Somewhere in here you get into a Faust meets "The Ones Who Walk Away From Omelas" calculation: if an institution can acquire more power--for generally good civic ends, or entirely selfish, power-justifies-power ends--what is the comparative worth of any given person?

And if institutional rot really sets in, you cross into a kind of Shadowrun scenario: there are the monsters built into the system, and the monsters that live in the gaps in the system, and any given individual might live their lives never encountering them...but they might be the unlucky one, or someone stuck in the demimonde, who's just considered an acceptable loss.

[Goaty14]

Well, as others pointed out, you have a high-magic society. They will attack and kill monsters that...
-They can see
-They can kill
-Are a threat

Naturally, this makes for monsters in the setting that have the opposite of these traits, or are...
1) Hidden: Evolved society cannot perceive these creatures.
1a) Hidden within society: These creatures can hide within the society, either by blending within the populace (ie dopplegangers), or by hiding where few people go (ie sewers, abandoned buildings, in the attic)
1b) Hidden outside society: These creatures can hide outside the society, usually using natural terrain, or by virtue of being *that* far away (ie burrowers, pixies, or extraplanar creatures/Far Realm)
2) Unkillable: Evolved society can't/won't kill all of these creatures
2a) Unkillable due to Strength: Elder-evil god-eater creatures that make high-level wizards soil their pants. Not many of them, else it ruins the worldbuilding, but they SHOULD exist, and are worth mentioning
2b) Unkillable due to Numbers: These things breed like rabbits! No matter what society does, these things keep popping up everywhere! (ie Kobolds)
2c) Unkillable due to Profit: Society benefits from these monsters, and avoids killing them (ie aforementioned golems/undead, slaves)
3) Are not a Threat: These monsters don't pose a notable threat to society, making hunting them not worthwhile (ie idk, anything CR 1/2 or under?)

[Yanagi]

To get specific:

Hive mind rats fit into any space available, can eat anything, but as the colony grow it becomes more intelligent. Most of the time this just means they're smart enough to stay undetected, eat surreptitiously, and hide signs of themselves. But sometimes a colony is both large enough and threatened enough to become proactively aggressive and develop elaborate plans for securing a home. Such colonies have committed murders, attempted takeovers of whole buildings, and have tried to cobble together tools and magical items to create elaborate traps and weapons to deter humans.

Conductor slimes' primary behavior is to attached to power lines and feed off voltage and latent magical energies, but they go undetected because they stretch to cover as much cable as possible--so thin as to be barely discernible to the eye. The problem arises when a slime infestation remains undetected, and individuals become excessively large or the population becomes overnumerous and they seek out other food sources in a panic.

Shadow anglers: ambush predators that can easily slip back and forth across planar barriers that create a "lure"--often in the form of a person-like shape--that beckons, gestures, or otherwise draws in people before the creature bounds across the planar boundary to attack and consume their target. They always existed, but their specific skill make them comparatively successful at hunting in an age with a great deal of prophylactic magic. The ones that target less noticeable targets, like pets, tend to be more successful, but as they grow in size they invariably switch to humans.

Portal mimics are also ambush attackers, the creatures have learned to shapeshift into a variety of forms resembling everyday items. The only consistent feature being the the mouth of the mimics corresponds to a door or other opening that a person would reach out and touch. By waiting until the exact moment of contact--a hand reaches for a doorknob, a foot lands on a manhole--they manage to pounce, grapple, and savage prey to death quickly. Rumor has it that there is a more sophisticated variety taking on the form of taxis and other kinds of public transport, but they are unverified.

Gremlins are fey that manifest in places where large pieces of machinery are concentrated. Some are merely mischevious, but some are homicidal malicious, specifically using their ability to control machines to create accidents and harm. Knockers are fey originally connected to mines, but now associated with all kinds of complex engineered structures...and they have no mischief in them, they devote all their energy and power to destroying and collapsing buildings. Poliads are generally benigh spirits who existence are connected to large buildings (generally residential buildings and offices rather than factories), but can become very dangerous if their territory is damaged or degraded. There are other, less understood spirits: the ones that live on the roadways as hitchhikers bringing little blessings and curses: the ones that build castles in the shadows of the subway tunnels, coaxing passengers to step in front of oncoming cars and become their permanent guests; the voices that whisper from electrical outlets and the figures that follow, peering at you, from billets and posters, block after block.

Contraelementals are a rare and disruptive spirit phenomenon that occur in locations with dense technology and magic overlaid. In their presence, complex objects begin to unmake, collapsing into base elements that re-compose as swarms of varying-size elementals. Items with magic charges do not simply become inert, but discharge their power as kind of expanding anti-magic field. When it first manifests, a contraelementals is only perceptible with detection magics, and resembles an expanding and contracted spiral of energy. As it grows, though it takes on both a rough humanoid shape and displays agency, moving toward and targeting objects and locations for unmaking.

A brazen head is a colloquial term for the core magic and instructions that animate a golem, but is also used to describe a particular kind of industrial accident inherent to golem assembly and programming: when the core animating instructions activate abnormally, the semi-consciousness of the golem extending to include other objects in a variable radius. In attempting to be a golem, a brazen head animates and draws together everything around it--included other programmed homonculi and people--creating a rough, chaotic, but recognizably humanoid form with whatever is available, then begins to execute a degraded version of its core functionality. In a society where entire industries operate on golem labor and golems are heavily incorporated into security, this kind of malfunction can cross from an containable hazard to a kind of magical meltdown in which entire factories and industrial zones collapses together to form a rampaging colossus.

The sheer profusion of magic in densely-populated areas leads to a kind of pollution problem. Living spells feed off the magic power of individuals and objects. Magic missiles swarm like flies. The football-sized kernel of a fireball rolls through the air, expanding like a puffer fish to defend itself. Transmogrified animals are just mundane pests that can cast a spell at will. Miasmas are magical smog with chaotic effects on those caught inside...but they seem to seek out targets rather than just settle at low points.

[King of Nowhere]

stuff

you raise many good points. I can answer some of those, because I already have a pretty good framework: I DMed a campaign in this world, now it's the same world about one century later. the previous campaign already had several elements of progress over a traditional fantasy world, now i'm further exploring those elements as the tech moves further.
"traditional" pollution and land overuse is, luckily, something that did not happen, because of druids and feys. those were able to put the concept of "sustainable development" into the general culture earlier than it did in the real world. nymphs are especially good at persuading the general public that their wilderness homes should be preserved. traditional monsters would keep exhisting in those wilderness.
magic pollution instead is going to be a thing. I already established from the previous campaign that high concentrations of magic can have dangerous side effects. also, magic is not an infinite resource; overexploitation of the thaumic field can be a problem.

blood sports with monsters already exhisted in the previous campaign: "dungeoning" is an extreme sport where groups of adventurers are sent in an artificial dungeoning against monsters for the entertainment of a paying public. I have it as an excuse to pit the party against dumb brutes (something that already didn't make much sense in the previous campaign)

Problem is, this won't create traditional "monster" problems to be solved by adventurers. It will be a problem for animal control, or exterminators. And the most likely danger from pests monsters will be indirect: they damage physical or magical infrastructure, causing something to fail, causing injury and death.

low level parties can be just that: pest control. In fact, having established that magic can mutate normal creatures into random monsters, it means that those pests cannot ever be exterminated completely. given the high amount of magic around, there will be even more of those monsters. this can be a good low level plot.
those monsters will rarely pose a direct death threat, but they may damage stuff. As in my idea of the swarm that drains your magic items.

Magical infrastructure...magical education and golems and adventurer wizard expert terms and items...is no more likely to be distributed according to need than, say...real life distribution of food and shelter.

that was already a worldbuilding point of the previous campaign. I established in the "lived happily ever after" end of the previous campaign that the situation improved everywhere, and I have a hard time imagining that 100 years later there would still be places where it would be normal to get eaten by a random monster. just like even in the poorest places on earth it is not commonplace for people to be eaten by lions.
those places have other problems, though. crime - the kind that is low-key enough to not send in high level people to deal with it - can still be an adventure hook. paladins generally volunteer to protect those places.

The folly of literary magic-users is like speed run of the folly that overtakes real people who just never have to hear the word "no." In a full magic society an archmage is still a WMD...how do you call them to account, if they decide to breach laws and norms?

there were still enough high level resources to deal with a rogue murderhobo in the previous campaign. an archmage kidnapping people en masse for experiment would be taken down fast enough.
on the other hand, an archmage that was sponsored and protected by a powerful nation, or other powerful organization, could enjoy much less accountability. some of the evil nations were basically kept together by those powerful evil people who had to bunch together to protect themselves from high level heroes. some of the most blatant cases were dealt with in the previous campaign, but that is by no means an obstacle to recreating the same dynamics.

"Menial jobs will be done by undead and golems" kind of glides over the question of whose corpses are doing what, potentially forever. So on the low end of complexity, mechanized undeath means there's either a market for dead bodies, or a legal and social norms in which the dead are turned over to converted into mechanized undead as a civic duty.

the previous campaign already had this in a less widespread form. there's basically a market: you can sign up a paper, when you die your body is turned into an undead and a compensation will be paid to your descendants. Less scrupolous powers would do that to all their citizens.
what happens when the demand cannot be satisfied by the natural death rate? potential plot hooks...

But more sophisticated forms of undead are available--vampirism, lichdom--where personality and skills are retained, and...theoretically...there's going to be both practical and perverse incentives to reach for these. Civic-minded undeath might include weighing the option to take on a role forever by intentionally assuming sapient undeath; powerful individuals might decide to transform themselves so as to never have to release their assumed titles and authority.

there's even a guy who keeps working from the afterlife. A skilled administrator whose status can be roughly compared discworld's vetinari, he did not try to go undead because it was too risky, but his underlings keep planeshifting into the afterlife to ask him directions . nobody tried to take his place because events from the previous campaign made clear that nobody has the skill to handle it.

However...what if you had a great deal of power, an ability to duck accountability, and wanted one of your employees to not leave (not die), to keep doing their research job or their particular skilled craft? Can you make an undead that retains its skill with a lathe, to do nothing but turn out coffee table legs? Can you strip out the bits of free will that make people complain and job hunt, but keep the bits that make an excellent lab worker? Even if there were rules against doing these kinds of things, the incentive to do it would always be there.

wouldn't that be a bit expensive to get just one worker? especially because you already have cheap automated workers. you'd have to do that to people with an excellence job to make it worthwhile, and to keep them good at their job you'd have to leave a good chunk of their intelligence... i wonder what could possibly go wrong?
could be used as plot point for the rebellion of the ghost office workers.

Relative to the described present, there's going to be a moment in the past where the clever monster types--stuff like rakshasa and fey and dragons--notice the shift in human(oid) societal structure and start to alter their own behavioral patterns. Some will choose direct resistance, some will choose flight...but the interesting possibility is those that choose collaboration.

already started. the previous campaing ended with a dragon elder trying to organize a movement to fight the humanoids... and getting trounced. hard.
the remaining dragons choose cooperation. many of them got jobs in human societies, and the same goes for many other magical creatures.
dragon pride still runs strong, though. while none of them believes anymore that they should be revered as gods and offered sacrifices, many still think that they should be A-class citizens because of their greater abilities. And some did not like how six humanoids and one golem defeated the three most powerful of their kind, and will want a rematch.

To get specific:

The first is that "natural" undeath is a kind of trace of the living person: ghosts with "unfinished business," or trapped repeating some loop of traumatic experience and emotion; gestalt undead that represent many people. More people, more concentrated, continuously surrounded by magic is likely to produce unusual new permutations of undeath that occur naturally. Giant urban gestalts fed by the ennui and resentment of people that lived and died as middle managers; drudgery ghosts continuing to go through the motions of their job in shutdown factories full of broken machinery; office wights bound to their desk.

Hive mind rats fit into any space available, can eat anything, but as the colony grow it becomes more intelligent. Most of the time this just means they're smart enough to stay undetected, eat surreptitiously, and hide signs of themselves. But sometimes a colony is both large enough and threatened enough to become proactively aggressive and develop elaborate plans for securing a home. Such colonies have committed murders, attempted takeovers of whole buildings, and have tried to cobble together tools and magical items to create elaborate traps and weapons to deter humans.

Conductor slimes' primary behavior is to attached to power lines and feed off voltage and latent magical energies, but they go undetected because they stretch to cover as much cable as possible--so thin as to be barely discernible to the eye. The problem arises when a slime infestation remains undetected, and individuals become excessively large or the population becomes overnumerous and they seek out other food sources in a panic.

Shadow anglers: ambush predators that can easily slip back and forth across planar barriers that create a "lure"--often in the form of a person-like shape--that beckons, gestures, or otherwise draws in people before the creature bounds across the planar boundary to attack and consume their target. They always existed, but their specific skill make them comparatively successful at hunting in an age with a great deal of prophylactic magic. The ones that target less noticeable targets, like pets, tend to be more successful, but as they grow in size they invariably switch to humans.

Portal mimics are also ambush attackers, the creatures have learned to shapeshift into a variety of forms resembling everyday items. The only consistent feature being the the mouth of the mimics corresponds to a door or other opening that a person would reach out and touch. By waiting until the exact moment of contact--a hand reaches for a doorknob, a foot lands on a manhole--they manage to pounce, grapple, and savage prey to death quickly. Rumor has it that there is a more sophisticated variety taking on the form of taxis and other kinds of public transport, but they are unverified.

Gremlins are fey that manifest in places where large pieces of machinery are concentrated. Some are merely mischevious, but some are homicidal malicious, specifically using their ability to control machines to create accidents and harm. Knockers are fey originally connected to mines, but now associated with all kinds of complex engineered structures...and they have no mischief in them, they devote all their energy and power to destroying and collapsing buildings. Poliads are generally benigh spirits who existence are connected to large buildings (generally residential buildings and offices rather than factories), but can become very dangerous if their territory is damaged or degraded. There are other, less understood spirits: the ones that live on the roadways as hitchhikers bringing little blessings and curses: the ones that build castles in the shadows of the subway tunnels, coaxing passengers to step in front of oncoming cars and become their permanent guests; the voices that whisper from electrical outlets and the figures that follow, peering at you, from billets and posters, block after block.

Contraelementals are a rare and disruptive spirit phenomenon that occur in locations with dense technology and magic overlaid. In their presence, complex objects begin to unmake, collapsing into base elements that re-compose as swarms of varying-size elementals. Items with magic charges do not simply become inert, but discharge their power as kind of expanding anti-magic field. When it first manifests, a contraelementals is only perceptible with detection magics, and resembles an expanding and contracted spiral of energy. As it grows, though it takes on both a rough humanoid shape and displays agency, moving toward and targeting objects and locations for unmaking.

A brazen head is a colloquial term for the core magic and instructions that animate a golem, but is also used to describe a particular kind of industrial accident inherent to golem assembly and programming: when the core animating instructions activate abnormally, the semi-consciousness of the golem extending to include other objects in a variable radius. In attempting to be a golem, a brazen head animates and draws together everything around it--included other programmed homonculi and people--creating a rough, chaotic, but recognizably humanoid form with whatever is available, then begins to execute a degraded version of its core functionality. In a society where entire industries operate on golem labor and golems are heavily incorporated into security, this kind of malfunction can cross from an containable hazard to a kind of magical meltdown in which entire factories and industrial zones collapses together to form a rampaging colossus.

The sheer profusion of magic in densely-populated areas leads to a kind of pollution problem. Living spells feed off the magic power of individuals and objects. Magic missiles swarm like flies. The football-sized kernel of a fireball rolls through the air, expanding like a puffer fish to defend itself. Transmogrified animals are just mundane pests that can cast a spell at will. Miasmas are magical smog with chaotic effects on those caught inside...but they seem to seek out targets rather than just settle at low points.

those are all good ideas. Thanks!

[Yanagi]

More generally, I think it would be interesting to play up the fey as antagonists in this setting. Particularly if you want a collective, organized antagonist.

Because high power plus plus magic means a drastic disruption of the natural world, and because the fey are in some way emanations or incarnations of aspects of the world, it would kind of make sense that a bunch of new types would...spontaneously generate...alongside all these developments.

They tend to be spell resistant, at least in lore can "hide" from people seeking them or have their own litte bubble realms attached to the world. They have a mix of enchantment-type skills and other more subtle magic such that they could slip under the high-magic radar unless being sought actively, but there are types with dangerous attack abilities too. They also forms societies and have hierarchies such that there could be leadership-class antagonist directing specific actions to meet particular ends.

Other than that: magical-industrial accidents and catastrophic failures as monsters or monster-spawning events. So instead of "natural" monsters analogous to animals, monsters are now more like the prodigies and curses of legends...direct consequences of something going out of order.

[Tvtyrant]

Shapeshifting creatures would probably be fine if they are smart enough to stay low. Dopplegangers are fine, Rhaskshasas, etc.

I imagine Mimics would quickly become smaller and human-avoidant. Turn into a trash can to eat racoons and rats, become a food bowl to get cats/pets, street lamp to attract bats and moths, etc.

[Berenger]

I imagine Mimics would quickly become smaller and human-avoidant. Turn into a trash can to eat racoons and rats, become a food bowl to get cats/pets, street lamp to attract bats and moths, etc.

Or the food bowl mimic goes straight for the food. "Strange, the cat acts as if it hadn't been fed for days.", "Alright, children, who took all those cookies?" Or perhaps mimics are bred like dogs for varying sizes, refined ornamental shapes and a taste for vermin.

[D&D_Fan]

I will brainstorm varieties of mimics for this setting.

House Mimic
These mimics have been "domesticated" by some people to serve as pest control. They appear as food, or shiny objects to lure animals, and then eat them.

Morpher
a subspecies of mimic that can transform into animals. I morpher might pretend to be a lost cat to be brought into a house, then reveal its true form and eat the family inside.

Altermold
A species of mold that is distantly related to mimics, Altermold can change the appearance of items it grows on. For example, Altermold could grow over a pit and appear to be bricks, so that whenever someone tries to walk over the mold, they fall through. It could cover a rusty iron sword, and make it look brand new.

[Wizard_Lizard]

I feel that a lot of the intelligent races would either hide, or integrate themselves with the society.
Goblinoids, orcs, all of the playable monstorous races would probably be more or less accepted into society, albeit probably with a little discrimination. Granted, Goblins and Kobolds, etc. can have their own heroes and high powered ones of their races.