Background, with potential spoilers: In the Netflix show Stranger Things...

...there is an otherworldly creature known as the Mindflayer, which can telepathically control creatures in our world as long as the portal to its world is open. Once it has a creature from our world under control, one of the things it can do is "melt" that creature into more flesh to add to its preferred monstrous avatar.

If we skip the psychic stuff, there's still a pretty interesting creature there. To qualify as a Mindflayer monster, an organism should

  1. Have a reproductive / feeding stage in which it acts like a non-contagious infection spread by physical contact with an adult monster (and possibly other ways). Once a victim is infected, the organism alters its host's behavior to seek out an adult monster and increase the chances of other hosts being infected. Eventually, the host is killed and its body digested to convert into monster flesh.

  2. Have an adult stage with limited shapeshifting abilities. Specifically,

    • any small piece of monster flesh should be capable of moving around on its own for some amount of time and seeking out other flesh-blobs with which to merge; this need not be indefinite, but should be long enough to be useful.
    • monsters should be able to divide into smaller pieces to squeeze through grates, squish under doors, etc., and rebuild themselves on the other side, while maintaining at least enough memory to continue with whatever goal they have in mind for doing the self-squishing (high intelligence not required).
    • given sufficient mass, monsters should be able to assume at least one complex ambulatory form, with legs and a body held up off the ground.

Ability to shapeshift into any additional forms (besides blob, walker, and pieces just cut off of a walker) is not required.

So, how could such a creature work, and how could it evolve?

A list of all Anatomically Correct questions can be found here: Anatomically Correct series.

  • 4
    $\begingroup$ This question should be more specific, as the Stranger Things Mindflayer is based off the classic D&D Mindflayer (Illthids), which function quite differently. Would you mind specifying 'Stranger Things' Mindflayer in the title? $\endgroup$
    – Halfthawed
    Jul 26 '19 at 5:18
  • $\begingroup$ @Halfthawed I was trying to make that distinction with "Mindflayers monster" rather than just "Mindflayer", but I have edited the title to improve clarity. $\endgroup$ Jul 26 '19 at 14:28
  • $\begingroup$ @Halfthawed technically it seems more based off the demons of 3.5e (timeline-wise they were using "advanced" rules but still) as in 2 it fits the crying source standard (demons watching party's movements through party member's eyes using divination magic - dispelable) & the demon possession stuff in 3 (which was used in D&D but mostly with NPCs, never fully take away a players control, and great caution - they kept saying we were devil worshipers anyway - but is really spelled out in Pathfinder $\endgroup$
    – LinkBerest
    Jul 27 '19 at 4:47
  • $\begingroup$ Mindflayers are just easier to sell than demons and have at least some similarities (honestly it looks like one of my demon mini-figures from that era- the aboleth or the Bebilith) $\endgroup$
    – LinkBerest
    Jul 27 '19 at 4:50

Your Mindflayer is a religious slime mold.

enter image description here http://bioweb.uwlax.edu/bio203/2010/renner_brad/reproduction.htm

Individuals of the prey species, like individuals cells of the slime mold, have the power to go about their business as individual single cells; slime molds have swimming cells and amoeboid cells as depicted. Your individuals can have one mode or different modes as you see fit. Individuals can be persuaded by various means to join the monster just as individual slime mold cells join a feeding plasmodium: a large flowing blob of merged individuals. The slime mold plasmodium is capable of dry land movement that the single cells cannot do, and capable of reproduction. Your Mindflayer has other emergent abilities not possessed by its constituent organisms.

Cool stuff: slime molds can learn by experience, and if an unlearned plasmodium merges with a learned one, the whole thing becomes learned.


But Dussutour’s work suggests that the slime molds can sometimes pick up these behaviors through a form of communication, not just through experience. In a follow-up study, her team showed that “naïve,” non-habituated slime molds can directly acquire a learned behavior from habituated ones via cell fusion.

The religious aspect has to do with the persuasion. Also consider that like minded individuals (humans in a group, or a pack or flock) acquire abilities in the group that would not be possible for the individual. An individual might be coerced or tricked - or might be persuaded to join the group because of the promise of the abilities group membership makes possible.

  • 4
    $\begingroup$ Even if the individual is a genuine multicellular organism ‘slime mold’ is still a brilliant answer. Alter host behaviour? Plausible. Dismantle host into mobile slime form? Check. Seek out other colonies to merge with? Something they do already. Create larger, more complex form when sufficient mass of organisms exists? Hell yeah. Basically you just succeeded in convincing me that the Upside Down is a universe where slime molds dominated the evolutionary game and the Mindflayer is just a particularly successful strain. +1 $\endgroup$
    – Joe Bloggs
    Jul 27 '19 at 20:58

Your Mind Flayer isn't a single creature. Its a hive mind of small insects which branches off from ants.

There are two types. A Worker and a Queen. A Worker is born with two pre programmed objectives. Find Food, Bring it to the hive. Unfortunately, due to their size, they can't actually hunt or gather anything worthwhile. Instead they will attempt to access any creatures brain and release a chemical which gives the target a desire to go to the hive. Once the worker has brought food back to the hive, the rest of the workers will work together to break it down and process it. Feeding the meat to newly born workers. The worker who brought the food then joins the Hive.

The instructions and locations are pre programmed into the workers while they are still eggs and they are released into the environment to hopefully find their way back. If the Hive moves before the worker ants get back, they will be lost for ever as the Hive location is pre programmed into their minds.

A Queen sends out instructions to the hive. Due to the size and complexity of the hive, there are multiple queens to work in a distributed network to control and manage all the workers. This means groups of workers can be broken off from the main hive body and have roughly autonomous control as long as they have a queen in their midst giving them commands.

The queens command the hive to move using a form that has legs. This helps provide the queens protection from attackers on the ground as they are lifted far above it. It carries over from their early evolution days where they would often be attacked while travelling in clumps on the ground.

  • 4
    $\begingroup$ Worth remembering that "queen" insects aren't generally much more bright (if at all) than their worker and soldier counterparts... not much space for brains in insect heads. The intelligence of eusocial insects emerges from their co-operative behaviour. Hive minds, not monarchies! $\endgroup$ Jul 26 '19 at 5:35
  • 3
    $\begingroup$ Losing the hive? No way: follow the pheromones!! $\endgroup$
    – Joe Bloggs
    Jul 26 '19 at 8:44

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