Slimes can't be harmed with weapons

They don't have a solid form. Slash it with a sword, whack it with a hammer, poke it with a spear, and all you get is a wet thud.

Slimes can't be harmed by fire

They are so wet and gooey. If you throw a torch at them, it would go right out. Even a Grade A magical fireball won't do too much damage. You need a whole team of Embermages to dry a slime out enough to harm it.

Slimes are super poisonous.

Just like a poison-arrow frog, one touch and you are toast.

Every drop of slime is a slime.

If you splash goo everywhere while hitting, they'll each act as separate conscious slimes.
The drops on your armor will search the gaps.
The drops on your weapon will climb the blade.
The drops on your boots will climb your legs.
The drops on your face or hands or flesh in general will eat you and grow and eat and grow and eat and grow...

Anything touched by a slime is done for and must be thrown away or burnt (people included).

1. Like many bottom-living sea-creatures, they can assume the colouring of whatever surface they are lying on. They can make themselves rigid enough to walk on.

8 Best Camouflaged Sea Creatures https://youtu.be/8yehnrXYa3c?t=6

2. They stay dormant until a human (or creature) or group of humans is entirely on top of them then they de-solidify and stick the feet of the intruder down like a rodent glue trap. See realistic, Photoshopped image of glue-trapped dead rodents at the bottom of the answer if you wish.




3. They are self-healing and so weapon strikes are useless. They simply flow back together.




4. The intruders will eventually tire and fall thus becoming more and more entangled. The slime then ingests them and moves on, leaving only caches of weapons and valuables for other explorers to find and puzzle over.

The following is a realistic, Photoshopped image of dead rodents trapped in a glue trap. To view, pass your mouse (no pun intended) over the image.
.

.

.

.

. This space is left to avoid accidentally seeing dead rodents in the hidden picture below.

.

.

They're slimes. In most media, they're just a ball of goo. Fireball, big sword, instant kill.
Maybe they're poisonous, or corrosive, or super sticky. That's nothing a big sword and a big spell can't stop.

Would big edits be needed? Not really.

You could easily keep them "normal slimes" while making them "super scary and dangerous oh no there's one let's run away at the speed of sound". Consider the following:

1. They can change their shape, texture, and colour at will.

(Truly, that was the most innovative idea to ever hit the slime market.)

But think about it. They can seep through the chinks of any armor and eat you alive. They can morph their bodies around to dodge attacks, or just take attacks like a sponge because they're pretty much just water and goo, they don't have pain receptors, if you take the weapon out you don't just get a perfectly healthy slime but also a weapon covered in, surprise surprise, more slimes to consume your flesh.

They can change shape and viscosity to the point where they can create weapons out of their own body while using their trails to trap adventurers in place.

Plus, they can camouflage into any surface by changing texture and colour. Is that a wall or-- nevermind, it's death.

2. They could be mycelium-like entities, or can split off from the original slime and reproduce so quickly they'd overwhelm even the best fighter.




3. Or, they can disguise themselves as food or potions or something that would be taken orally (or even a healing balm for open wounds).
   Once they've fooled you into consuming them (or they could just enter through another orifice or a cut), they can take control of your body or fill your veins with slime or... choke you, I guess.

I did draw examples of how slimes could kill someone, but I'm no artist, so bear with the low quality.

Make them microbial.

What is a slime, if not a giant amoeba by another name?

Some amoeba are really dangerous, such as Naegleria. It gets in your bloodstream and then infects your brain, which is why it is called the "brain eating amoeba" in our own world. Let's see a bunch of player characters fireball their way out of that.

And if anyone says "Oh, that's just a matter of using a proper healing spell..." - it's because of people who think like that, that spell-resistant superbacteria are now a thing. You shouldn't drink a healing potion whenever you sneeze and you shouldn't stop treatment without consulting your healer just because one day you woke up feeling better.

Corrosive, Social, Intelligent Slime

• Their malleable nature means they cannot be easily defeated by conventional weapons.


• They are corrosive, causing severe damage upon contact. They can also easily navigate gaps in armor. Close combat is basically impossible against them.


• They are pack hunters. Multiple slime creatures may ambush adventurers, taking them down before they wield their slime destroying magic.


• They are not sentient, but are intelligent in an instinctual way, sometimes capable of setting ingenious traps, using tactics like camouflage or baiting.

Safest way to defeat them would be to ambush them in their lairs, with fire or ranged magic.

In addition to @kingledions answer

Slimes multiply

They eat and eat and eat and multiply and eat and eat and eat and multiply...

Slimes go everywhere

They have no form, so they can squeeze everywhere, no matter how small the gap is

Slimes are corrosive

You need to get rid of evidence? Push it into the slime and let it digest.

Say slimes are 10' cubes.

Say your party is in a 10' tunnel, and sees ahead what seems to be a wall of force, or fog, or... something. They poke it with the omnipresent 10' pole, figure it's a couple slimes ahead of them. 20' of jello.

Then their ears start to pop. They look back and see slimes in the other direction. They are trapped between closing walls of slime!

With a farting noise, the air between the slimes squeezes past the slimes as they move together. They move slowly. It will be about a minute before they close together and the party's air runs out.

So, challenge: can you find a way to get through 20' of jello in one minute? Digging a hole doesn't help as holes larger than a fist just collapse. Slaying just the one in front won't help, as the one behind will just push its corpse forward. Slaying both on one side won't help much, as the other side will still be advancing, though it will double the time that the air remains.

[Initially, since there's a whole plethora of slimes, oozes, goos and gelatinous cubes, you don't know how anything like flesh or armor will react to contact. You might be lucky and have a hand-cream ooze, which will just make your skin softer and smoother, but do you want to take that risk?]

Jumping and drowning slimes

Slimes have found a new way to deal with their opponants.
When a slime fight a group of adventurers it contract its body in order to jump to the face of one of these poor lads. The slime is approximately the size of a water melon and its inner texture is similar to water so it simply stays here (looking like a beautiful slime helmet) and waits for its prey to suffocate...

It is really difficult for adventurers to get rid of it as the slime is very very sticky and can't be removed easily. More vicious, all slime's vital organs can move freely inside of its body so it realocates them in order for them to be in contact with the skin of its prey. Now, everything that could possibly harm the slime can harm the prey !

Slimes are likely related to snails. Some snails can be poisonous and have harpoon-like appendages and in some cases tentacles to sting prey with.

https://en.m.wikipedia.org/wiki/Cone_snail

Some can be venomous and excrete dangerous toxins to ward off or kill prey. Apply one or both of these to the not-bothered-by-square-cube-law fantasy creatures usually involved in universes with slimes and they can be very dangerous indeed.

The puddle of slime is not one single creature but an aggregate of billions of highly-intelligent micro-organisms.

As already mentioned, not only are they corrosive to weapons and armour, they are poisonous to the touch.

Moreover, they are skin permeable. One droplet on your skin and they will enter the bloodstream and migrate to the motor cortex, seizing control of your motor functions and turning you into their vehicle.

A very dangerous enemy indeed.

Many ways.

This is going to be a fun ride.
For the purposes of this question, we'll assume this slime has similar qualities to the kind you find on social media (the one 7-year-olds are really into).

Make it really big

There's basically no limit to how big this slime can get, provided that it can receive food. You could have one enormous slime blob, which will basically be impossible to kill (barring some magic kill-spell). Even by conventional means, without advanced weaponry, this slime would pose a huge threat to travelers.

You could have a thin layer of slime coat every surface, which can blob together into one huge slime when provoked, provided that every unit of the slime is uniform, and that it can be cut into parts without damaging it.

This gets even more exciting, as every single unit of slime can act on its own as well, meaning that the slime could split up into thousands of virtually indestructible droplets, which can crawl through the nooks and crannies of our hero-soon-to-be-dead's armor.

Side point: Our slime is already basically invincible.

The semi-liquid qualities of slime mean that slicing or shooting it will do absolutely nothing; physical attacks are useless. Fire won't work either; the slime isn't flammable. The only way to really defeat the slime is magic, a lot of fire, or nuclear weapons.

Of course, the slime may be unkillable, but is that true for our hero as well? This brings us to our next point..

Poison

One of the most obvious solutions is poison. There are a lot of organisms in the real world (some frogs for instance) that can definitely kill a person or two easily. You can make your slime synthesize similar compounds, and suddenly that nondescript puddle becomes extremely dangerous to travelers in the forest.

You don't even need to kill the predator. Once you incapacitate them, you can suffocate them with the sheer volume of the slime (see point 1).

Camouflage

This builds off of the poison point especially, but you can have a massive slime disguised by a layer of leaves, etc. on top of it, meaning that there's virtually no way to determine what is a slime and what isn't.

Tight Spaces

The slime is basically a liquid at will, which means it can kinda go anywhere. Through gutters, pipes, cracks in walls, you name it. Nowhere is safe; if it's enclosed, the slime can just surround it and suffocate you. Slime in an enclosed space is simply horrifying, since it can just go around you and slowly close in on you.

You know what they say: When you're being consumed by a slime, no one can hear you scream.

Putting it all together

Bob the Adventurer is taking a walk in the forest. All of a sudden, he steps into a little mud puddle. No biggie, he can clean it off..

Then the slime jumps into actions. Thousands of tiny slimes spray off of the subterranean mass, crawling through Bob's armor and onto his exposed skin. He doesn't even have time to scream before the slime encapsulates him. He is slowly pulled into the underground lair of the slime hive-mind, never to see the light of day again.

Another day in the forest goes on.

Making It Useful

There's plenty of uses for the slime, given that you don't provoke its voracious appetite. For instance, need to get rid of basically any organic matter? Just toss it into the slime. Your incredibly dangerous super-slime could solve our world's trash worries, and there are doubtless other ways to exploit the slime's behavior I haven't thought of.

Another Distraction - Realizing the Slime

Your slime could use photosynthesis to get energy (remember that it can spread itself over a veeeery large area) in addition to whatever prey it can get. Effectively, it's just a huge glue trap that's also sentient and can move..

Reproducing is no problem for the slime. There's no concept of individual slimes; they don't operate discretely. Rather, they can be temporarily split, but normally operate as one hive-mind, serving its own needs.

Making the slime as smart as it is may be a little problematic, but a little handwaving may suffice. The only way for the slime to transmit signals rapidly would be chemical signaling (which would be slow for the massive slime), but that's a detail the readers don't need to worry about.

The video game "Baldur's Gate: Dark Alliance 2" makes slimes incredibly deadly through a pretty simple change. Instead of being limited to melee, they can 'spit' chunks of themselves like a slimy shotgun (though with less projectile speed) that ends up doing a high amount of damage on a good hit.

So you can take that idea and expand on it beyond the limits of a 2004 game engine. Make those projectiles as deadly corrosive as the slime itself and now you have a simple monster being quite dangerous to a fully armored knight to being outright lethal against an unarmored tiger.

Also, as long as these slime chunks travel at a relatively slower speed and they don't have the best accuracy, the prepared adventurers can still dodge them so that these slimes can still realistically be defeated (unless you don't want them to be of course).

Make slime become Sentience.

Which mean give slime intelligent as human (or human-like). With such brain power, slime can make anything human can: make tool, diplomatic, trade (between other slime or with other species).

Some ideal to start with:

• Slime is distributing hive-mind species (which individual is a cell) which have intelligent join by number of cell in one body (same idea as The Thing).




• So small slime is as smart as a dog, but a big slime can have Human intelligent.




• For human-like slime, you can based on Zac (League of Legend champion). He also have ability to regeneration after being kill by joining small slime.

Slimes really can be whatever you want them to be.

Look at D&D for example. Slimes can be a nuisance at best or a TPK at worst. It depends on the type of slime, how large it is, what traits it has, and how how aggressive it is.

Assume a slime lives in an area where it feeds on mana in the air. It won't be that dangerous to an adventurer because it won't have needed to develop a low internal pH level. Therefore, if you stick your hand in it, it'd feel like the inside of those water wigglies from when we were kids. No harm, no threat.

But, let's say the slime has had to live in an area where the things it eats have hard exoskeletons made of metal. Ignore the idea the metal could make its way into the slime's body and make it more resilient, but its internal pH could be so low that it qualifies for the term of "negative pH". This means if an adventure put their hand in THIS slime, they wouldn't have a hand anymore. It's entirely possible even that the slime or its acid could start climbing the arm to a certain degree, causing a constant burning pain as their hand dissolves away to the slime's digestive fluids.

If the slime has to actually hunt for its food instead of just wait for things to die, then you can add a layer of aggression to it where now it's not only highly acidic, but it is now super persistent. Imagine it like this: instead of just worrying about a passive pool of acid, you now have to worry about a pool of acid that will chase you relentlessly until you can kill it or it finds something more appetizing.

Now, additional traits that a slime could reasonably have:
-heat resistance because they are composed almost wholly of fluid
-cold resistance because they are composed of an acidic substance and acid doesn't freeze easily usually
-bludgeoning resistance because they're gelatinous
-asexual reproduction since they're basically giant amoebas or man-of-war jellyfish
-the ability to turn into smaller versions of themselves when "killed" by saying larger slimes are typically multiple smaller slimes clustered together and working as one in symbiosis
-incorporation of digested materials meaning it eats something and takes on properties of that thing, like an amoeba

This is just a small portion of things that allow slimes to be varied in threat and danger. It's just for this reason that slimes are much more fearsome in tabletop games, because DMs know how to customize their slimes to create threats and dangerous situations for their players, and players fall for the threats because they're often used to JRPGs where slimes do nothing.

You can watch Goblin Slayer (potentially disturbing content warning if you do choose to) and use the way the goblins are seen in that world as a means to making the slimes dangerous but seen as no big deal. Reincarnated As A Slime is another good show that has a main character born as a slime and using some of these same traits. Both of them have mangas and light novel versions if those are more your speed. (Both started as LNs, became mangas, then became anime.) Both series are good, but Reincarnated as a Slime is far better and less gratuitous in terms of graphic imagery, not to say some minor graphic imagery (like a little girl being burned alive) doesn't exist.... but it's far less egregious than what happens at the beginning of Goblin Slayer. Also, Slime has a killer soundtrack which is worth listening to any time.

Make them highly-intelligent, sapient psychopaths in addition to being amorphous. Additionally, make them habitually well-armed.

For example:

https://www.schlockmercenary.com/2000-06-12

Classic fantasy slimes aren't scary because they're only slightly smarter than celery and therefore only a hazard to unprepared or careless characters. Smart slimes that actively hunt people and learn from their mistakes are extremely scary since they could be hiding nearly anywhere just waiting for you to make a mistake before attacking.

My question is how to make slimes incredibly powerful without using magic: OK, are they extraterrestrial or of Earth? If you can imbibe them with sentience, they can be awfully powerful. Even Earth slimes. They evolved.

They wrap themselves around a person and inject hormones/poisons, etc. These can make people mad, die from constant orgasmic bliss, affect organs in a way snake venom does, cause respiratory distress, heart failure, etc.

Unless you give them more 'power', I'd think they'd just be an annoyance. Maybe people could slip on them?

Black pudding's melt everything they come into contact with unless they're hiding. How are you supposed to fight something like that? You're basically stuck keeping a bunch of empty barrels around in hope of slimes deciding to hang out in them instead of fighting you.

If you add intelligence to something like that, give them the ability to use tools with their body, and give them a reasonably long life span then you have a build that is outright broken.

Have you played the game 'Ambition of the Slime'?

I feel it answers your question quite well. At least it might give you some ideas.

(it's here on steam: https://store.steampowered.com/app/568910/Ambition_of_the_Slimes/, but I played the Android version).

In the game the slime have very limited attack and defensive abilities (and no magic abilities), but the real ability they have is to possess the bodies of their enemies. So for them to take out a party of humans trying to attack them they will possess the bodies of some of the enemy party, and use their hosts bodies to fight against the un-possessed ones. Some examples from the game of various slime 'abilities':

• ability to increase the base stats of their host (some increase speed, some increase defense, some increase magic)


• ability to teleport close to an enemy


• ability to teleport a friend close to an enemy


• ability to reduce an enemy's resistance to being taken over


• ability to fly


• ability to possess an enemy for a longer period of time (i.e., between levels)


• invisibility

Slimes also have an element (fire/water/grass) which, if it coincides with the host's element, will result in a power boost as well.

Different slimes have different abilities, and it's how their different abilities work together that make them able to take on tough enemies.

I have a story where slime took over the world.

SCP-001 When Day Breaks is a story about the apocalyptic event where ultraviolet from the sun reduces every living organism into "flesh slime". This event devastated the entire earth, even the most technologically advanced faction in the world (The SCP Foundation) cannot contain this event.

The key points why these flesh slimes take over the earth are:

Strength in Numbers

The first minute when this apocalyptic event occurred. Half of the people in the world already outside their home. They were starting to melt and turned into fleshly mass of blobs. Then, these blobs were starting to gather around and merge themselves. They grew bigger and bigger. Imagine one big flesh slime made by the entire population of Washington D.C.

Human-like Intelligence

While they're one big flesh slime made by millions of living things, they still retain their intellect back when they were humans (and animals). Only their goal is to turn all remaining survivors into slime by using any ways to expose them onto sunlight. They're capable to speak (but have a wet raspy voice). Usually, they just attack survivors outright but if they found it difficult, they just wait. They won't go to wait peacefully but instead, constantly demoralize survivors in various ways, for example, installing fear and dread to survivors, let their mental collapsed and then survivors will join by themselves to end their suffering.

Invulnerable Body

Because they are a huge amount of flesh mass. They have incredible resistance to any physical damage. This might contrast to old-school watery slime. In the story, it proved conventional weaponry cannot stop them.

Rapid Reproduction

While their reproduction method isn't involved in breeding but just expose living thing to sunlight. The point is how easy for slime to reproduce. If they can reproduce this easy, they will outnumber the major population in no time.

i see you watch tensei shittara slime deshita too,
Alright let me explain how Slime in this works, It can absorb almost everything and change it into its own power, example it absorb a firebolt magic and it as its own magic, or unleash a stronger version of firebolt.
The slime can combine two magic it learn for example Fire and Earth to make a new type of magic something like Steel Blade or Glass Blade projectile.
The potential of this combination might be limitless.

For a different take, the creature in Hal Clement's novel "Needle" was a creature of more or less fixed size, about 2 kg. It was related to viruses on it's home world in the way we're related to bacteria, so the cell size was much smaller.

The critter could slide between human cells with ease. It could set up an array of cells between the retina cells, and see whatever it's host was looking at. When the host was cut, it controlled bleeding. It snacked on foreign bodies giving immunity to most diseases.

Not all such critters were so helpful. The Hunter was looking for one of his own kind that used hosts and moved on, leaving a trail of dead or damaged hosts behind him.

Hunter has chosen the wrong host to be cop. His host is a lightly built teenager with all the restrictions on his life that a teen has.

So the story is a 'first contact' police procedural science fiction mystery.

Good book. Young adult. In passing a good Christmas gift for a teen with some interest in SF.