OSR: Strange Water - Calling All Marine Biologists

I'm working on a book of monsters for D&D-type RPGs. You can read more about it here. It's designed to be a functional at-table book. Not a moral bestiary, not a book of fictional ecology, but a practical utility-focused reference guide. A book of prompts.

The last chapter needs some aquatic monsters. Most of the creatures you'd expect are in the "Water" chapter; but "Strange Water" needs strange monsters.

Give me your weird, your odd, your forgotten creatures yearning to be known. Are you mad that the creature you spent years studying never gets any press in mainstream Dungeons and Dragons books? Did you see something odd and unique in a documentary? Tell me about it! Post a comment. Nerd out. Get technical. Sell it to me.

Or, if you don't know, send this article to your friends. The more suggestions, the better.

Dubious wisdom from a dubious source.


These aren't strict, but they're good guidelines.

Has To Be Able To Move
I don't want to include sessile (stationary) organisms. If a creature can't move, interaction is limited. This excludes sponges, most tunicates, brachiopods, etc. If it's stuck to a rock, it's out.

Has To Do Something
You don't need stats for a tree. Filter-feeders, drifting salps, fancy diatoms, etc. are out. If a D&D party can't reasonably fight it, or if it can't fight back, it doesn't need stats. Jellyfish are borderline; for D&D purposes, "Ooze" might cover their abilities.

Has To Do Something Unique
If an entry can be represented by "X, but Y", then it doesn't need a separate entry. Categorizing creatures for D&D purposes is a very Borgesian experience. You can cut across millions of  years and different time scales with a few lines. The statblock for "Seal" can cover Dolphin, Orca, and Moasaur; all function, for D&D purposes, in approximately the same way.

A fairly up-to-date list of what's in the book can be found here. If in doubt, suggest something.
Scale isn't an issue. Things will be scaled up to a suitably menacing size.

 Brest van Kempen

Current Candidates

Each chapter of the Monster Overhaul has 10 entries. Here's what I've got so far for the "Strange Water" chapter.

1. Abyssal Fish

You know the type. Glassy flesh, too many teeth, large eyes (or no eyes at all), phosphorescent patches, etc. 
2. Giant Starfish
All echinoderms. Starfish, brittle stars, sea urchins, and feather stars. Anything radially symmetrical that could slowly wander up to you, grab you with one of several arms, and stuff you into its mouth.
3. Elemental Eel
This is a weak entry, and the first that I'd cut. [Creature] + [Element] is something that can be easily templated. Wolf + Ice = Winter Wolf. Bear + Fire = Fire Bear. Do these ideas really need a separate entry? Still an electric eel is fairly weird.
4. Mantis Shrimp

They're weird, colourful, fairly well known, and have a D&D-like attack. I feel like they're sufficiently different from other entries to warrant their own section.
5. Polychaete Worm

I've got a few other Worm-like entries, but polychaetes tabulate well (lots of variants) and have some unique twists. It's not the strongest entry, but it's not the weakest either. I know calling it a Polychaete Worm is like calling it an ATM Machine, but it helps with navigation.
6. Predatory Snail

To include the classic Flail Snail, as well as surfing snails, poison dart snails, and all other sorts of snails. Could even include sea slugs, if I've got space for non-shelled variants.
7. Rotifer

A giant rotifer, big enough to swallow a human and create a vortex of water. I can't see any objection to this entry; it can also provide stats for any other aggressive filter-feeders a GM might need.
8. Tardigrade

A crowd favorite. Immortality optional.
9. Trilobite

Trilobites are famous and tabulate well. I'm going to use this as a catch-all entry for anything vaguely arthropodal that's a) not a wagon-sized Giant Crab and b) not a human-sized Giant Spider. Scaled up to approximately dog-sized.

I'd like to include a "Generic Arthropod Generator" near the Trilobite statblock. Basically, a table-based flowchart with illustrations that lets you fuse segments/tagmata and get various Burgess Shale-type hard- or soft-bodied arthropods out the other side. Necessary? Hardly. Wise? Probably not. But damn it, I want one, and I'm going to make one, even if I have to hire a consulting biologist or two.
10. ???

What should the 10th entry be? What existing entries should be revised, discarded, or improved?


  1. What about the hagfish? great name already, and they turn the water around them to a goo-like net that traps predators. Also, they have a skull, but no spine, which is pretty awesome.

    1. No jaw though which means they have to tie themselves into knots for flesh-ripping leverage. Starts at the tail and travels down to the head which being fastened to the meal tears away a mouthful as it's pulled "through" and the knot undoes itself. I've combined the knots with the mucous you mention for set piece cable-serpents. In the end the players realised that chopping something that modular to pieces was pointless and instead dumped alchemical waste on it to weaken the binding spittle.

  2. If it has to be a giant version of a real sea creature, I've got nothing, but how about a Kelpie?

    Skull headed adhesive amphibious horse covered in seaweed, tempts folk to their doom and so on!

    1. Kelpie is good, but I think it can be covered by other creature(s) + aquatic. Either the Grey Horse (for a vaguely benevolent Kelpie) or the Lamia.

  3. Hermit crab or another creep who wear carcasses as home/shelter/armor.

    Also, do you know that starfishies have a thousand small feets?

    Which category will be that creatures like planarians who you can slice one up and they become two creatures? Sounds like ooze but they aren't oozes.

    The kraken entry will have options for hypnotic color moves or something? We need color changing cephalopodes.

    1. Hrm, hermit crab is OK but not perfect.
      Starfishes are weird.
      I think flatworms don't get enough press, but on the other hand, they're just flatworms.
      Kraken has colour changing abilities, yes.

    2. Two easily converted as monsters: Armored fish like Arthrodira and Venomous fishies Stingrays and Pufferfishes.

  4. Hermit crabs can end up with algae snacks, defensive sponges and counter-offensive anemonies on their shells who in turn receive free taxi service. I had one boss creature which had some coral spires and more than one thing living in the shell too (slug for wall-crawling and cucumber for surprised semi-ranged attack). The idea was that under the encrusted grime the shell wasn't a natural cast off at all but a non-euclidean relic which explained how even the outside symbionts could only deploy themselves one at a time.

    Salt drakes were pretty fun being beefed up marine iguanas. Their snot shot incapacitated whatever it caked and drained moisture in a setting where dehydration was a pressing concern. The scalp patches where their snot sun screen had build up heavily (ie older/bigger specimens) formed fast-dehydrating salt crystal horns. Heat control should have been a more central mechanic but in my game it came down to a short sluggish period while recent swimmers basked, if they were too warm they might instead dazzle attackers while their salt screen reflected away the sunlight.

    Monster whales seem "in" these days and I've done a couple of those though more as unique horrors than a standard race. My favourites were the squid vs whale action which was actually one crazy hungry beast eating its own tentacular intestines and another based on the discredited idea that blood flow could change spermaceti temperature and so buoyancy. This dude had a swollen melon crawling with angry red vascularisation, the water sizzles a little and then the head deflates as it spews a stream of scalding oil.

    1. Monster whales are great, and I've tried to include a bunch under Whale.

  5. Mottled (purple) worms (in the AD&D Monster Manual) do not get enough love.

    1. I've got 10 colours of Wurm ready to go: https://twitter.com/Skerples1/status/1244369256680009729/photo/1

  6. Some ideas:
    -primordial soup that spawns other monsters (probably ooze mod actually)
    -magic oystery thing, like a carbuncle but better
    -cymothoa exigua
    -ghost ship
    -crocodile's kind of distinct from everything else in there
    -deep one - is that just merfolk, a cthulhu if so
    --add blue men of the minch to merfolk if you haven't already
    -anemone, but with legs
    -sea spawn from ravenloft, those are cool
    -remora, but like the mythical remora that stops ships it's stuck on

    1. -Cymothoa exigua is great. Bit difficult to gamify, but I suspect I can roll it into Trilobite or something.
      -I've got crocodiles in another chapter. They cover Gars too. Gars are properly weird. Heavily armoured, toxic eggs, etc.
      -Deep Ones could probably be covered by Merfolk or by Troglodytes.
      -Remora is in the Sea chapter and does the mythical thing, but to both ships and PCs.

  7. Finally, a real reason to use my degree

    Jellyfish as Myconid analog
    Vampire Squid that's an actual vampire
    Flatfish camouflage is neat
    Gulper Eel
    Dumbo Octopus
    Yeti Crab
    Black Swallower
    Bobbit Worm
    Hooded Nudibranch
    Armored Snail
    Sea Angel
    Sea Pig
    Salamanders before and after a cannibal metamorphosis
    Japanese spider crab
    Goblin shark
    Giant isopod
    Sea spiders
    Giant Sunfish

    1. Hooray!
      -Jellyfish as Myconid: a great idea, will add to the cross-reference list
      -Vampire Squid: I think Vampire is one of those easily templated ideas, but it could work.
      -Flatfish are great, but again, camouflage is templatable. [Fish] + [Invisible]
      -Nudibranchs and all other weird soft sluggy things are tempting, I must admit.

  8. What's weirder than a water weird?
    Hands down, the water weird is one of my favorite things to run into since seeing on the cover of some Endless Quest book back in childhood.

    1. Water Weirds are neat, but they're ideal for expansion into a broader category. [Weird]+[Water] can easily be [Weird]+[Element], so you can get Fire Weirds, Ice Weirds, etc. Stuck some similar critters in the Elemental chapter.

  9. Sea Lamprey to replace #3 Eel

    They are terrifying to look at but I think we can add weird by making each stage of it's unusual life cycle deadly. It spends some time being a parasite, as it grows. After spawning the larva dwell in the sediment (but still move around). Could we make it more powerful depending on what it preyed upon during the parasitic stage? So stuck on a fish it's just a lamprey, but spend this life stage on an aging and blind sea dragon and we get lamprey + scales + breath weapon. I like creatures that leave clues around. So, oh oh the PCs find lamprey larva in the sediment ... that means there could be adult lamprey around that were parastizing something terrifying. Like maybe your "Leeches of Paradise" were just sea lamprey that parastized a UNICORN!!!

    They also migrate!

    Cross reference to Vampire and polymorph as well?

    1. Lampreys could be combined with Eels. They sort of work the same way. I like lamprey larvae as omens, may integrate that into a table somewhere.

      I'm glad someone else remembers the Leeches of Paradise. To this day, saying "You feel good! You feel great! You feel... slightly alarmed..." is an in-joke for that group.

    2. Leeches of paradise as number 10 on this list then? They need to be in here and what could be weirder!

    3. I never play a campaign without them! I play with after school high school groups. There is something about the leeches of paradise that are distinctly "other". Once the kids encounter those they are into it! One kid took off his boots and waded in since they seemed so awesome! He didn't survive the encounter :P

    4. Oh I would love build-your-own-crazy-life-cycle type tables! Molting, mating spots, courtship displays, the weird thing they must eat before laying eggs (blood, gold, dreams, information, itself), r/K selection.

  10. Cephalopods. Giant nautilus for the armored win. Plus vampire squid, poisonous octopi, etc.

    Angler fish.

    In the end, it could come down to giant Arthropoda (crabs, lobsters, shrimp), cephalopoda (octopi, squid, nautiloids), echinoderms (starfish, sea cucumbers, sea urchins), Gastropoda (snails and sea slugs) and then those with bone or cartilage- biters (sharks, barracuda, seals, toothed whales), swallowers (whales, groupers, ambush gulpers, angler fish), venomous (spines, bites, tentacles, harpoons), etc.

    1. I feel like the only really common use case for cephalopods, in D&D, is the big ol' Kraken-type creatures, which is where they all ended up.

      Angler fish can go with Deep Sea Fish.

  11. Here's a list I compiled in response to the recent tweet. Not all the organisms might meet your requirements though:

    1. This is a _very_ good list.

    2. The "not sure what" might be a crinoid of some sort.

    3. Nope, ID'd as a blue button jelly: https://twitter.com/RebeccaRHelm/status/1364367574381826050

    4. Corrected in the list

  12. Point of order: a sea star will not grab you with an arm and stuff you into its mouth. It will mound itself up over you, flip its stomach inside out through its mouth on top of you, and slowly digest you that way, before putting its stomach back in and moving on its way. This is probably more fun at the table as well as more accurate.

    Some more ideas:
    Glaucus atlanticus (https://en.wikipedia.org/wiki/Glaucus_atlanticus) is a good choice. Pelagic nudibranch that looks like a dragon, eats Portuguese Man-o-War jellies, and steals their stinging cells without discharging them, so it can incorporate them into its own skin for defense.

    I see someone else mentioned sea angels (cliones), but I feel like I should add for people who are unfamiliar with them that they look like christmas-tree-topper angels flapping delicately through the water until a crown of tentacles explodes from their head to catch prey. (Contrast: https://www.youtube.com/watch?v=qEqNuIvwfcY and https://www.youtube.com/watch?v=wFpKFWlJuFI). They are also predatory swimming sea slugs, so you could consolidate them with Glaucus if you want.

    You say that jellyfish are borderline, but how about box jellies? They have advanced eyes that they use to steer and hunt, move much faster than other jellyfish, are nearly invisible in the water, and have stings that can incapacitate for weeks (Irukandji syndrome) or kill.

    Larvacean tunicates build cellulose "houses" much bigger than their bodies to filter food from the water around them as they swim, then discard them to the deeps and make new ones when they get too clogged with debris. You might be able to gamify this as a kind of swimming midwater spider's web scaled up to catch unwary players, but with something in it that will drop you into the deep rather than try to eat you (predatory monsters get a bit samey, I feel. Non-predators that can fuck you up just as bad injects a little more variety into monster motivations and tactics).

    I'm surprised you don't already have eurypterids ("sea scorpions" although they probably weren't venomous), some of which got big enough to need no scaling up. I guess you could stick those in the general ancient arthropod bin with the trilobites.

    On a similar note, echoing what people have already said about the old shelled cephalopods, but make sure you include the cone-shelled orthocone nautiloids and the weird half-uncoiled heterochonch ammonites alongside the classic rams-horn looking ones.

    Nemerteans (ribbon worms) capture prey by shooting out a proboscis tipped with a venomous barb called a stylet. Additional fun detail: the proboscis is stored inside-out in their body and and inverts (everts, technically) to shoot forward. Technically not a polychaete, but you could add their proboscis to your variant-making table for that entry.

    You could maybe do something with the deep scattering layer collectively as a kind of swarm monster: a layer of small fishes - individually harmless but so dense that they appear as a false bottom to the sea - which rises in a wave to the surface waters each night and sinks down with the dawn.

    Giant radiolarians as ooze variants in ornate spined shells of spun glass might have some potential.

    1. Point of order: you are entirely correct, but I'm not sure the distinction would help anyone being attacked by a giant starfish.
      "Help, help, I'm being eaten by this giant fly!" "Technically, you're being digested by it, though I suppose you could also say you're being abraded. Can you try to get a closer look at the labium... oh, and you're gone. Alas."

      I like the nudibranchs and other soft weirdos. Melibe in particular is a strong contender.

      Box/comb jellies might get combined or as Beroe. Just a big glowing bag that eats you, zippers shut, and wiggles.

      I think I can roll all eurypterids in with trilobites / general extinct arthropods. Side note: I put a song about eurypterids in "The Mysterious Menagerie of Doctor Orville Boros", if you're into that sort of thing.

      Nemerteans and their horrible proboscis thing have made their way into a few different entries.

      Swarm monsters are tricky, especially ones that you can't really fight. It's just terrain.

      I think radiolarians got a decent treatment in Veins of the Earth, and I don't want to do a duplicate entry or overlap ideas too closely.

    2. Random aside:
      I once had a DCC character simply known as "The Orthoceran" on the basis that he was the last of an ancient empire of elves who heavily incorporated shelled-cephalopod motifs. He had a long conical helmet of course.

    3. Heh, in a similar vein I had Nautiloid Gnomes in a one-shot once. Conical hats = shells. The gnome part was just tentacles wearing clothes. Their eyes were fake; their actual eyes were their ears.

  13. Cycliophora pass through ~5 morphoplogical stages, and change genders as they do. might be an interesting roleplaying situation.

    there are biologically immortal jellyfish. a fantasy one that gained intellect, or was just _around_ when Old Things happened perhaps.

    black smoker vents? as an adventure site, or a thing that spawns other monsters?

    things that live in anaerobic water. i guess there the challenge is more the environment, maybe non-magical waterbreathing doesn't work. it would be weird water itself.

    octopuses and horseshoe crabs have copper-based blood. is that gameable? does it still count as "blood" for magic purposes? horseshoe-crab-man vampire hunter?

    the Antarctic cryptid, the ningen. could possibly be a flavor of monster whale.

    1. Cycliophora are weird as heck, but is it possible to gamify that in a book of monsters? Maybe.
      Vents as an adventure site will be included, even just as a note.
      A copper-blood vampire is a very good idea.
      Not sure the ningen has enough info on it to really include in any definitive way.

    2. the ningen is supposed to be something between a whale and a human that lives in freezing waters. it steals the catches from fishing vessels.

      it's worth an image search.

  14. If you're willing to scale them up to size... I think a lot of micro-organisms could be strangely terrifying.

    Bacteria, amoebas etc.

    1. Ah, but which ones specifically (that can't be covered by Ooze)?

    2. I guess an amoeba would probably just be an ooze.

      Giant bacteria might be more like swimming rot grubs?

      Toxoplasmosis could be fun, especially if symbiotic with other predators.

    3. Charybdis might work, just make it motile and give it more mollusc features (a sentient ball of detritus, reef, tube worms, and barnacles/clams that can suck people in or jet them out.

    4. I think Rotifer will make a decent Charybdis stand-in.

  15. Are you sure about your commitment to avoid sessile creatures? Because the barnacle is an iconic little bug.

    These weird shrimp-like critters glue themselves to rocks, yes. But then they build a fortress around themselves, and from within that fortress reach out their long, feathery fingers to pluck little morsels of food from the surrounding waters.

    When scaled up, this filter-feeding behavior would become grasping arms reaching around the corners of dungeon hallways to drag prey back to a sequestered secret side-room.

    Still too sessile for your tastes? How about the fact that many barnacles are parasitic, burrowing into the skin of a fast-moving host to better help them catch lots of prey.

    And what happens if the party cracks open the barnacle's shell? Surely the little lump of meat inside would be helpless at that point? Yes, unless you incorporate the medieval belief that goosehead barnacles are merely the eggs of hateful geese. Perhaps every barnacle has something even more horrific inside.

    If I can't persuade you about the terror of barnacles, then maybe the notorious Bogleech can.

    And for good measure, here's the more uh, succint, version of their barnacle article. To quote:


    1. Barnacles are great, but as D&D creatures? What do they do? What do they want?

      I like the idea of a multi-room grasping arm thing. Parasites in general are great, but hard to D&D-ify or put on the random encounter table.

  16. Leptocephalus, although could be redundant depending on the nature of Deep Sea Fish

    1. It's a good sign when I can translate a creature's name but have no idea what it is. A good suggestion, even if it's just confirming the general glassy trend.

  17. Submarines? Can artificial things make the list? Potential for social encounters, and being made of metal would make it unique for the list. Biggest problem would be that submarines aren't all the same and they don't all have cool robotic arms.

    1. Heh, tempting. I think the Generic Space Ship covers the use cases of a Generic Submarine, but adding ROVs to the list could be fun.

  18. Hey, check out this post about the circadian rhythms (or lack there of) of deep sea creatures! https://www.reddit.com/r/askscience/comments/8xvckh/when_do_deepocean_thermal_vent_animals_sleep_if/

  19. Bobbit worms are terrifying and have gone undetected for years in household fish tanks.

    1. They'll have a place of pride in the Polychaete Worm entry.

    2. Yay! Sorry for the triple post. I promise I only clicked once.

  20. Bobbit worms are terrifying and have gone undetected for years in household fish tanks.

  21. Bobbit worms are terrifying and have gone undetected for years in household fish tanks.

  22. Lacrymaria, a form of microorganism with a teardrop shaped body and a long "neck" with a biting "head" at the end. Capable of extending that neck 7-8 times the length of its body. It alternates between resting, active, and transitionary states at ten minute intervals. During its active state, it whips the neck out wildly, extending it around corners and through holes, searching for rapidly for prey. If it reaches something, it can bite, delivering a poisonous, paralyzing venom, allowing it to pull the prey in and swallow it whole.

    I first found out about the Lacrymaria on Journey to the Microcosmos youtube channel ("Lacrymaria: Vicious Long-Necked Predators"), and commented there that a horse-sized Lacrymaria would make for a terrifying monster to encounter underwater. (I've had the idea for an all-underwater campaign for a while now). Probing coral reefs, sargasum clumps, ship wrecks, flooded caves, or sunken dungeons up to 100 feet away. Other commenters made the observation that such a creature could also work in a swamp or partially flooded dungeon as well. It's very unique as a monster, because its hunting appendage could be encountered long before its body was, if there's sufficient openings through which it can attack from afar.

    1. Great video! Lacrymaria is great, and was already on the shortlist... but I decided that, mechanically, it could probably just be a plesiosaur. https://en.wikipedia.org/wiki/Plesiosauria

  23. Too late now but an evolution of the starfish is the black hole fish and the supernovaFish

    1. Ooh! And I thought the Throwing Starfish was a good idea. I've already got the sun-theme with the Sun Dogs, but maybe I'll add a cross-reference.