Horrible, ugly, predatory creatures have been a staple of myth and legend for most of recorded history. Yet one need not look very far in the real world to find similarly suitable examples of dark perversion in the animal kingdom. Consider briefly, for example, the humpback anglerfish.
This horror has it all: a small lure to bring in hapless victims, a terrifying habitat at cold, dreary depths, the ability to distend its stomach to swallow larger prey, and a disgusting means of proliferation. On that last note – a common feature of many anglerfish species is the horrifying means by which they procreate.
It all begins with the female of the species (the large fish you imagine when you think of anglerfish) releasing some pheromones and using that handy lure to get the attention of a mate. The males of the species are tiny, pathetic little things, and among the smallest fish in the world. This allows them to spend less energy hunting for food, and more energy looking for mates. When they do detect a female ready to mate, they swim up to it and fuse their mouth to the female’s body, abandoning all autonomy for the rest of their lives.
Having mated with the female, the male no longer has need for most of its body parts, and its eyes fins and some internal organs atrophy away. The male is now simply a mindless shell that does nothing but produce sperm.
Adapting such a monstrous species to a campaign wouldn’t even be that hard. Slap on some legs and call it a day.
With that, I bring you the “Voracious Worm”, modelled after the pelican eel, another deep-sea horror first illustrated in 1883. Most scientists agree that, based on the size of this creature’s teeth, it rarely feeds on larger creatures.
A voracious “worm” is actually a misnomer, as the creature is most closely related to snakes. This monstrosity is what happens when an albino viper (about a 1 in 100 occurrence) is introduced into a highly toxic environment. Thought to have originated from a carton of specimens discarded by a frustrated alchemist into a sprawling city sewer, these creatures proved too adaptable to perish to the resident giant rats, rust monsters, and Otyughs.
Parasitic nature: While voracious worms can grow to formidable sizes, they are incredibly vulnerable in the adolescent stages of their development, and are initially only as long as a finger. The species adapted to this weakness by looking to stronger and bigger specimens for survival. A voracious wormling is born without eyes, and will, upon hatching, crawl around, searching for dead, warm, organic matter (preferably a nice corpse) to burrow into and wait. It can feed off the rotting flesh of whatever dead creature it occupies for weeks, remaining in a state of pseudo-hibernation, and waiting for a scavenger to come by that will swallow the fetid meat.
Often, none does, and the wormling perishes. But occasionally, a not-so-picky eater will stumble upon the corpse and devour it, voracious worm and all.
With its immunity to acid, and preference for dark environments, the wormling lives on in the stomach of its host, leeching off of its food, and slowly growing to fill its host’s twisting intestinal passages. When the worm grows large enough, the host starts to wither from a lack of nutrients. Hair begins to fall out, and nails begin to yellow. Dark circles appear under the host’s eyes and it feels a constant, nagging, hunger.
The worm, now fully grown, is finely attuned to the health of its host, and even has the ability to detect strong emotional swings. When its host begins to wither, the worm waits for an opportune emotional shift, and erupts, uncoiling itself through the intestines and inflicting massive internal damage. It burrows its way out through the host’s aesophagus, exiting in a spray of vomit out of a frequently too-small mouth. It will then feast on the host’s remains, and find its way to a nearby sewer, or other dark, wet environment, where it will hunt its prey using more conventional methods
One interesting mutation experienced by the worm is the ability to distend their jaws to incredible widths, swallowing down creatures many times their size. The worm will often choose to do this only after dragging its prey to a safe location, however, as after a large meal it is often unable to move, and vulnerable to attack.
5e Stat Block
The humpback anglerfish image (first in article) is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license. Attribution: Javontaevious at English Wikipedia