BX Monsters A to Z: Insect Swarm

 Here's another "monster" that is wildly underappreciated and underutilized: the insect swarm, from the Basic Rules. I don't think I've ever used one myself, neither back in the day nor more recently, which is kind of a shame. Let's get into it and see what sort of possibilities such an unconventional monster entry lends itself to.

According to the book, an insect swarm may be comprised of various types of insects (or other arthropods such as spiders), either crawling, flying, or both. (Presumably a single swarm is composed of a single type of creature, though.) The swarm has an Armor Class of 7, which is mediocre but still better than an unarmed human, perhaps because of the decentralized nature of the swarm. There's just not a coherent "body" to strike against. In fact, because of the other curious mechanics of the insect swarm, this number is completely, entirely irrelevant; it will never be used in play. The swarm effectively has 2-4 Hit Dice, which is only of slightly more relevance than its AC. A swarm doesn't make attack rolls, so it's only for the purpose of determining hit points, and even those might not be used in an encounter.

The swarm has an area of effect, with 10' x 30' given as typical, but the description tells us they may be larger, and it makes intuitive sense that they aren't fixed in shape either. Any character within that area is automatically attacked, and takes 2 points of damage per round while in the swarm. This is doubled if the character is unarmored, but halved if actively warding off the insects with a weapon or similar object. If that object is a torch, the swarm takes the standard 1d4 damage, but regular weapons won't harm it. (The description implies this damage is automatic with the "warding off" action; thus the irrelevance of AC.) A character may instead run out of the swarm, reducing damage by half. It takes three rounds after leaving the swarm to kill or brush off remaining bugs, implying that the character continues to take damage during those rounds. Frankly, that's a lot of damage; at least 3 points, and possibly 6 if unarmored -- likely enough to do in a low-level magic-user. If there's a fair-sized pool or body of water nearby, the insects can be escaped by immersing oneself in it, which kills all remaining insects on that person in a single round, though in a dungeon, that might be trading one danger for whatever's lurking in the water. 

Fortunately, swarms are fairly slow, with the crawlers moving at 30' (10') and flyers at 60' (20'). (It seems plausible that individual flying insects could move considerably faster, but the "controlled chaos" of a swarm makes them collectively slower. Fair enough.) It has a morale score of 11, probably owing to its non-intelligence and the disconnected nature of all the little insect instincts comprising it. A swarm will always pursue if angered by taking points of damage, but it can be escaped simply by moving out of its line of sight.

A swarm can be neutralized en masse by a simple sleep spell, or damaged by fire or cold attacks. Smoke drives them away, though how much is needed (surely more than the smoke from a single torch) isn't specified. I'd rule that area-of-effect attacks like fireballs and wands of cold simply exterminate the lot of them at once, rather than dealing points of damage. Though it isn't explicitly listed, poison attacks such as cloudkill should do the job too, as should AoE instant death effects like the death spell. The creature description notes that "the DM may choose to have other things affect swarms" so you're on Moldvay-sanctioned ground with those judgment calls. 

Insect swarms can be interesting in play because they require a different mindset to deal with than just "swing sword, roll damage, repeat." They function partly as monster, partly as trap or obstacle. Characters' Armor Class hardly matters; the fighter sheathed in plate +3 is as vulnerable as the thief in ordinary leather, and damage is automatic. The fighter's +3 sword won't be much help either. 

Insect swarms are natural fits for any location with a decomposing corpse or rotting garbage, where swarms of biting flies or scavenging beetles might congregate and breed. Intelligent monsters might set up traps which release swarms (and make foul-smelling lotions or ointments to keep them off themselves!) A treasure might be "guarded" by a swarm, which the party will either have to neutralize or lead away somehow if they want to get the prize. Other likely areas for insect swarms are muddy watering holes in arid lands, swampy pits or cesspools, storages full of spoiled food, rotten logs and stumps, mounds of seaweed and flotsam washed up on shores... Or stick one in a dungeon, where it's attracted and worked up into a frenzy by light sources that get too close.

It's trivially easy to modify an insect swarm to fit other environments and situations, too. Underwater? Shrimp or some other small crustacean will do. Sand fleas might be found in swarms on a beach. Moths, butterflies, or fireflies could serve the same role as normal bats, confusing characters by flying around their heads, but doing no damage. Stink bug swarms might cover characters with their foul odor, foiling stealth and attracting monsters until it wears off. If you want to get really wild, you can use the same stats and abilities, but make them magical -- lightning bugs or fireflies that inflict a thousand tiny shocks or burns on their victims! How about snow flies whose touch is chilling, and in large enough numbers, could actually freeze people solid? Go nuts with it and keep your players on their toes!