Darkest Dungeon

Maggots the size of a golden retriever. Slow moving, they are drawn by the smell of living flesh. They will attach themselves to any living creature and begin to feed. Any puncture of their thin skin releases a thick adhesive that can easily entrap attackers. 

Tar Body Maggot

Armor Class: 8 (unarmored). 

Hit Dice: 1d6 (HP4)

Attacks: First attack to attach (no damage), + 0 to hit, ignores armor. Once attached deals automatic damage each round until removed. First round deals 1 HP, second 2 HP etc. damage escalating as it chews its way further into the victim's more vital bits.

Move: 1/2 speed of unencumbered human. Can move on walls and ceilings.

Saves: 13+

Morale: Very stupid and will mindlessly pursue prey. Can be driven back temporarily by intense heat or cold.

Number Appearing: d10 x 3

Special: Hitting it with a bladed or piercing weapon releases super sticky goo. It will trap & tangle the weapon and potentially the attacker too (a la the folkloric Tar Baby). If killed, they melt into a big gluey puddle that will trap anyone stepping/contacting it. This smells intensely sweet and will attract other scavengers.

Dr. Seuss' Oobleck

Gooey-gluey

Maggot goo sticks to anything it touches. The goo can't be separated from itself. If you touch it and pull away, it will pull you back with alarming elasticity. 

Stab into a maggot and try to withdraw your sword and it will come out covered in goo like molten mozzarella. The strands won't separate from the body and the harder you pull the more the goo will rubber-band you back toward the corpse.

If you read Dr. Seuss' Bartholomew and the Oobleck as a kid or heard the folk tale about the Tar Baby, you have the idea.

Tactics

Tar-Bodies like to hide in tight spaces until they smell flesh close by. They are slow to react but once prey is detected, maggots will slither forward directly toward their target (or drop on them from above).

Maggots often appear in large numbers and the entangling deaths of the first few can open up opportunities for the followers. 

They will avoid intense heat (like fire) or cold and they can be driven back with torches or ice magic.

If their path toward prey is blocked (by a closed door for instance) they will quickly lose interest and go back into hiding. 

Encounter Design

Despite their low HP and weak attacks, Tar-Bodies can be very dangerous in numbers. 

These are meant to be a complication in already tense situations. Killing them in way that doesn't create more problems is usually the challenge. They'll be a big problem for groups that try to solve every problem with DPS.

Since they create a glue-trap wherever they die, they can also be an asset with a little creativity. 

It's great for these to appear in areas with tight tunnels or narrow bridges that the party won't want to obstruct with goo.

• Adventurers crawl down a narrow tunnel see the beady eyes of a few tar-body maggots coming toward them in the cramped confines. How do they get rid of them without blocking the tunnel?
• A corpse with obvious loot lies in the open, in the middle of a cavernous space, by the time it's possessions can be examined, tar-bodies have crawled from cracks in the walls and ceiling and converging from all sides. Getting out without getting stuck might be tough.
• The beastmen patrol is on the backfoot, our heroes are pressing their advantage when a half-dozen tar bodies appear, drawn to the commotion. Can the party afford to ignore them to finish off the beastmen?

Loot

Like other dungeon vermin, Maggots are more a hazard to be avoided than a source of wealth. 

Still, they are likely to appear in the vicinity of corpses and they won't have looted their possessions. If you have a good "Search the Body" table, it's a fair reward for driving a few tar-bodies away. 

Their glue is useful to the clever, so capturing a live maggot could be worthwhile in the right circumstance.

Discussion

Maggots fit into my series of low-power hazard monsters (along with the likes of Mummy Hands, Glue Snails & Gunpowder Moths). The goal of these is to provide a challenge other than a small stack of HP and a threat more interesting than raw damage.