The Assassin Bug comes to us from the 1981 Fiend Folio, a large collection of fan-submitted monsters from the very early popularity of Dungeons and Dragons, as it was taking a foothold in the United Kingdom and the greater Commonwealth of countries. This particular creature is one of a very small number that have seen an official update to the 5th edition of the game in Mordenkainen's Fiendish Folio Volume 1: Monsters Malevolent and Benign. This is an interesting product from Wizards of the Coast—first off, “volume one” kind of implies more volumes, which we have not seen, and it was made as part of a charity drive for the Extra Life foundation. But the Mordenkainen titles for 5th edition are... weird. Mind you, so is the original Fiend Folio and the fact it was done and ready two years before it eventually saw print, and it was, I am pretty sure, the first ever third-party product made for the game, since the UK branch of TSR became Games Workshop, and none of these creatures were from Dragon or Dungeon magazine, they were from White Dwarf magazine in Britain.
Another interesting thing about the updated listing for the Assassin Bug is that they did the same thing they did later with the Hadozee species from Spelljammer, but in this case, it didn't blow up in their faces because they didn't involve anything more problematic than the original listing included. It’s pretty much a bit of page filler you can completely ignore, and they removed a bit of important and much more useful lore from the first edition version, which I think adds a lot to the monster, as you will soon see. So, grab yourself a tasty beverage, we are about to get deeply nerdy.
The loathsome, egg-bloated, buzzing, annoying, food-ruining, and disease-ridden blowfly in a vaguely humanoid form—and far more aggressive. Not just nature's disgusting little garbage eaters, but active predators of other humanoid species. This is the D&D Assassin Bug, not to be confused with the actual assassin bug, a real insect and quite interesting in its own right, but of course, a D&D version would have to be giant or demonic or something.
Assassin Bugs are known to infest some areas of Faerûn such as the Rimwood Forest of Cormanthyr. Occasionally they will reach the plains of Cormyr and the woods of the Dalelands, and there is an account of an outbreak in Sembia that saw a significant amount of forest burnt to the ground by elves, so you know it must have been hellaciously bad—basically like humans setting fire to their entire town, which is also known to happen with these revolting monsters. Despite what the freefolk of these lands will tell you over a pint in a warm tavern, they are not fly demons; there is nothing infernal about them. However, I can tell you they do inhabit the 500th layer of the Abyss, the dense nightmare jungles belonging to the goddess Kali. Trust me, you don’t want to go there.
Nobody really knows where they came from. There is speculation that some crazy sect of nature cultists—and I refuse to call them Druids—most likely a cult of the Beastlord, Malar, the god of hunting, stalking, bloodlust, and bestial savagery. But perhaps it was a cult of Kozah, an ancient sect dedicated to the Storm Lord Talos, focused on his portfolio of destruction. However, I am skeptical because they have always engineered natural disasters, for example, working behind the scenes to incite the elves to use their incredibly potent song magic to wreak enormous destruction, such as the Sundering or the flooding of the Vilhon Reach and the end of the psionic cities of the sword. It’s all speculation though, as I said, nobody really knows. Even Kozah serves a purpose in the grand scheme of things; there is no sense in punishing gods for just being what they are.
No, I am more inclined to think this is tied to the god Malar, who seeks the success and dominance of the most adaptable and strong life forms. After some extensive study of the Assassin Bugs, I can tell you exactly why I think this, and provide one hell of a fun improvement to the design of the monster in the process.
First, some fun facts. The Assassin Bugs don’t have ears—they detect sound through sensitive bristles, so burning the head of an Assassin Bug will scorch off the delicate hairs that it uses to detect sound and will deafen them. Unlike flies, they don’t have an extending proboscis. They have a mouth with fangs and a rasping tongue. Also, they don’t have the soft pulvillus pad on the base of each foot or on their hands, which makes it unlikely they will climb walls or tree trunks with the ease of a tiny insect, though they can certainly climb and fly over any hazardous terrain. This compensates a little for their horribly slow walking speed of 10 feet per round, but they are not well adapted to walking around on two legs. Don’t worry, I have an explanation for that as well—let’s SCIENCE this thing!
Thanks to a formula called Kleiber's Law, we can use the rough measurements of the mass of the insectoid monster and a common bluebottle fly to predict how long the Assassin Bugs would take to grow from egg into a full adult. We know the Assassin Bugs reproduce once every two months. However, it takes a total of 2.8 years to grow to full reproductive adulthood. This contradicts the lore and observations of the speed at which the early stages occur, so we know, for a fact, that these are not completely natural creatures—they are supernatural monstrosities. Otherwise, it would take 68 days for eggs to hatch after being implanted in a victim. Larval growth would take a total of 1.4 years, and they would remain in a pupae form for 1.16 years before emerging fully developed and ready to hunt and breed. There are a few fantasy world solutions to this problem. I won’t go into great detail; I will simply pick the one that is the most favorable to interesting plots and monstrous variants.
So, the idea here is that once the Assassin Bug maggots consume the host’s heart and the soul departs, they enter a kind of transitional state. At that point, they’re not fully bound to the Material Plane anymore—they exist partially in the Border Ethereal. This gives them access to energies that let them grow rapidly, far beyond what normal biology would allow. They can feed on the essence of the host, absorbing not just physical matter but something deeper, like the host’s life force or soul residue. That’s what fuels their quick transformation into their juvenile form.
While they’re in this dual-plane existence, they’re vulnerable because Ethereal predators could pick them off, so there’s a real danger during this phase. This forces them to grow quickly, using the essence they’ve absorbed to rapidly develop into their next stage.
What’s especially creepy is that the maggots don’t just consume the host—they also absorb traits from them. This is why the juvenile Assassin Bugs resemble humanoids rather than insects. They’re mimicking the species they’ve infested, adapting their forms to match the physical traits of the host. If they kill a dwarf, the juveniles might emerge stocky and strong. If they feed on an elf, they might be more agile. And in this juvenile stage, their bodies are somewhat malleable—they have a limited form of shapeshifting. It’s not full-on doppelganger-level, but their forms are still flexible enough that they can adapt to their host’s traits as they develop. As they grow, these traits harden, and they settle into their more insectoid adult form.
The heart plays a crucial role because it’s not just a vital organ—it’s a gateway to the host’s essence. By devouring the heart, the maggots tap into a kind of soul energy that triggers this magical, rapid growth. It’s not just physical nourishment—it’s metaphysical. That’s what allows them to break the normal rules of biology and grow so quickly after the host’s death.
And when these juvenile bugs emerge, they could even retain fragments of the host’s memories or instincts. This would make them disturbingly familiar to anyone who knew the host. You could encounter a juvenile Assassin Bug that has some of the same mannerisms or tactical instincts as its victim, which would make it both terrifying and eerie. They’re not just mindless monsters—they’re tied to their host, which makes them unpredictable and even more dangerous.
So, the limited shapeshifting in their juvenile phase helps explain why they look more humanoid early on, but as they mature, they lose that flexibility and become the more insect-like creatures they truly are.
The Assassin Bug will inject one to three eggs into a still-living host. These hatch immediately, meaning they have been inside the adult Assassin Bug already for at least a couple of months. This means you can loot a dead Assassin Bug female for her stored eggs. I'd say she probably has no less than ten, and they are not going to hatch unless they are put inside another living creature. But it may be possible to extract a compound from the egg-storing organ of the dead females that can be made into a non-magical potion to prevent implanted eggs from hatching inside a host treated with the potion. The eggs are four to five inches long and oval-shaped, with a deep blue color. They stink and make fantastic fish bait. They are also highly prized by Troglodytes, who love the smell, Bugbears, which get some sort of a buzz off them, and Trolls... Oh, Trolls just LOVE to eat the eggs and maggots of Assassin Bugs. They are known to infest themselves with the things, as they regenerate so fast the maggots can't kill them. The infested Troll will dig the maggots out of its flesh and eat them like a horrific snack. Yeah, no thanks, I'll stick with some fruit leather and Lembas bread for my trail snacks.
There are more peculiarities to these revolting creatures. The Assassin Bug maggots share traits with their distant cousins, the Rot Grubs. It's plausible that the two species are in some way related. While Assassin Bug maggots grow larger and far more deadly, there’s something to be said about how both infest the body and burrow into flesh. It’s worth noting that Assassin Bugs have been observed in proximity to colonies of Rot Grubs—whether this is some kind of natural symbiosis or coincidence remains unclear, but it makes sense that these parasitic species might share territory or even cooperate in their gruesome life cycles.
If you’re looking to run a darker or more terrifying campaign, Assassin Bugs could be part of a larger ecosystem of parasitic, body-invading monsters, including Grick hatchlings or even more esoteric creatures like intellect devourers. Think about it: Assassin Bugs could play a key role in spreading these other species, serving as the “carrier” or initial invader, weakening victims before more insidious creatures finish them off.
Also, there is the interesting observation that Assassin Bugs have a deep connection to Trolls. As mentioned, Trolls not only enjoy eating Assassin Bug eggs and maggots, but they’re also biologically resistant to infestation, healing far faster than the maggots can harm them. This has led some scholars to hypothesize that there might be an ancient link between the Assassin Bug's creation and the regenerative abilities of Trolls. It’s possible that a forgotten, primal magic ties the two creatures together, potentially dating back to the time when the gods shaped these monstrous forms. Perhaps Malar or another savage deity granted the Assassin Bugs their reproductive prowess specifically to challenge creatures like Trolls, testing the limits of their regenerative powers in the natural cycle of predator and prey. This might also explain why Trolls seem to relish consuming them.
Incorporating this into your campaign could give you some fascinating plot hooks. Maybe a tribe of Trolls has learned to cultivate Assassin Bugs, letting themselves become infested as part of a gruesome trial of strength and regeneration. Imagine a Troll Shaman who believes that consuming Assassin Bug eggs strengthens their connection to their god. Or, perhaps there’s a rival tribe that hunts these Trolls specifically to harvest the Assassin Bugs they carry, creating some horrifying alchemical concoctions in the process.
The Assassin Bug's life cycle also presents some intriguing options for campaigns centered around body horror. Given that the juvenile form can mimic the traits of its host, there’s a grim possibility that these creatures could "inherit" more than just physical characteristics. The idea that they absorb traits or memories from their victims introduces the terrifying possibility of doppelgänger-like creatures that remember people, places, or even skills from their prior hosts. If a player’s character were to fall victim to an Assassin Bug infestation, only for a juvenile bug to emerge with some of their memories or abilities, it would create a nightmarish situation—an enemy that literally knows you as well as you know yourself.
Alternatively, these juvenile Assassin Bugs could be manipulated by necromancers or other evil entities to serve as spies or assassins, leveraging their mimicry and memory absorption abilities for dark purposes. Perhaps a villain has devised a way to "program" these creatures, controlling them once they emerge from their hosts, using them as disposable but deadly tools. This would give the players a chance to confront enemies that are familiar yet alien at the same time, ramping up the stakes and emotional intensity of any encounter.
Finally, on the Abyssal front, the fact that Assassin Bugs inhabit the 500th layer of the Abyss, within the domain of Kali, suggests a particularly dark origin. Kali is not typically depicted in Faerûnian lore, but her influence on savage, predatory creatures fits perfectly with the Assassin Bugs’ brutal life cycle. It’s possible that some demonic influence has seeped into the Material Plane, allowing these monsters to spread their vile breed across Faerûn. This could provide an excellent adventure hook for players to investigate an outbreak of Assassin Bugs that seem to have a more sinister, demonic edge to them—perhaps enhanced versions of the standard bugs, with fiendish traits or magical abilities tied to the Abyss itself.
To sum up, the Assassin Bug is far more than just a disgusting insect-monster. With their rapid reproduction, deadly life cycle, and connections to both material and extraplanar forces, they offer a wealth of storytelling potential. From terrifying body horror to deep-rooted connections to the gods of savagery and predation, these creatures can become a centerpiece for any campaign involving nature’s darker side or the lurking threat of parasitic invasion. And with the addition of lore and potential interactions with other monstrous species, they could play a key role in creating an ecosystem of terror within your campaign world.
The listing in 5th edition is very close to the original version of the monster, they are medium sized monstrosities of a neutral alignment, they just seem very evil from a humanoid's point of view, they don't really bother other creatures, but are known to attack other creatures when no humanoids are available, we know this because of the horrific wounds found on many Catoblepas in the swamps beyond the mountain border of Cormyr, the tough death cows can withstand even such a savage parasite, don't ask me how, its amazing, also if you ever have random creatures run headlong into your camp fire, be very wary as they are probably infested with Assassin bug eggs.
The assassin bug has an armor class of 14, 55 hit points on average, they move at 10 feet per round walking or 50 feet flying, not very agile but fast on the wing, they tend to fly right at victims and slam into them, the males have darker coloring and will always attack first using a couple of bite attacks each round, +4 to hit one adjacent target, inflicting 1d10+2 piercing damage, and the target must succeed on a DC 11 Constitution saving throw or be poisoned for 1 minute. While poisoned this way, the target is paralyzed. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. The females will then appear from hiding, all assassin bugs have +4 to stealth and +3 to perception with a passive perception of 13 but advantage on perception tests thanks to their excellent sense of smell, they also have 60 ft Darkvision and are immune to being either poisoned or paralysed. Females will attack with another bite and then plunge in with a thick and sharp tipped ovipositor extending out of the end of their abdomen, also +4 to hit, the target is then infested with bug eggs which immediatly burst open with the highly motivated larva chewwing into flesh like crazy.
At the start of each of the target’s turns, the target takes 1d6 piercing damage per maggot infesting it. Applying fire to the bite wound before the end of the target’s next turn deals 1 fire damage to the target and kills these assassin bug maggots. After this time, the maggots are too far under the skin to be burned.
If a target infested by assassin bug maggots ends its turn with 0 hit points, it dies as the maggots burrow into its heart and kill it. Any effect that cures disease kills all assassin bug maggots infesting the target.
Given their need to lay eggs in a living host, assassin bugs generally flee from constructs and undead, they can often be fooled with a good illusion, but remember, one smell of a living target and they will attack.
If you liked this video, I have a lot more of them, also, this video is part of the Fiend Folio playlist, covering all the monsters in the book from A to Z. For lore from all editions of the game, welcoming all players to the hobby, I would love to have your subscription and be delighted to answer any questions you have in the comments below, meanwhile..
Thanks for listening, and as always, I will be back with more for you very soon.
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