D&D 5E Swarm of insects (spiders) -- How do you run this monster?

[Motorskills]

I had a blast with these Swarms in Salvage Operation, I happily moved them in three dimensions (up the walls / stairs / through the cargo grates etc). The PCs hated it.

 

[Tonguez]

Not being able to stab them for fear of harming your companions is kinda the point of insect swarms, swarms are designed to require ‘problem solving’ rather than stabbing, thats a feature not a bug

 

[MNblockhead]

Not being able to stab them for fear of harming your companions is kinda the point of insect swarms, swarms are designed to require ‘problem solving’ rather than stabbing, thats a feature not a bug

Actually it is a feature and a bug.

 

[Oofta]

Some of it also depends on how you choose to run them, and how you rule them sharing the same space. Some monsters like the Rug of Smothering specifically state damage transferring to the grappled PC. It's silent on targeting the swarm. Area effects would of course damage anyone in the same area, but other spells or attacks? It's a DM's call IMHO.

 

[Imaculata]

If you do background music or sound effects then something like this would be awesome:
Creeping Terror - Scary orchestral music

The obvious music to play would be this: Boris the Spider

 

[aco175]

"One does not simply walk into a swarm of spiders."

I tend to picture attacking this as scraping them off and batting at them rather than swinging a sword like you are attacking an orc. You are still using your sword and getting the right bonuses from it, but certainly not swinging wildly.

 

[Charlaquin]

I'm about to run Ghosts of Saltmarsh, which includes a few encounters with insect swarms (spiders). As I read the stat block, it occurs to me that I have no idea how to run this monster. Unlike a swarm of flying insects, the spiders need to crawl onto a target to attack it (I assume, because that's the only thing that makes sense). But I'm not sure how to describe this action without giving the players the impression that they're being engulfed in spiders and can't attack for fear of hurting themselves or their companions. How the hell do you stab a spider when it's crawling on someone, anyway?)

I don’t think anyone has really addressed this. My suggestion would be to describe the character having some spiders on them, but not all of the spiders. Think about how you narrate swarms of flying creatures. There’s never a question that you can attack the swarm without harming the character engulfed in it, because there’s no assumption that all of the creatures in the swarm are physically on the character. Some of them are, sure, but there’s an assumption that there are plenty more, buzzing or flapping around the character in a big ol’ cloud. Now translate that idea to grounded creatures. There’s probably a big ol’ pile of them on the floor at the character’s feet, some crawling up their legs, but plenty more just writhing around on the ground.

 

[Harzel]

I'm about to run Ghosts of Saltmarsh, which includes a few encounters with insect swarms (spiders). As I read the stat block, it occurs to me that I have no idea how to run this monster. Unlike a swarm of flying insects, the spiders need to crawl onto a target to attack it (I assume, because that's the only thing that makes sense). But I'm not sure how to describe this action without giving the players the impression that they're being engulfed in spiders and can't attack for fear of hurting themselves or their companions. (How the hell do you stab a spider when it's crawling on someone, anyway?)

I guess I need to make it clear to the players that the "swarm" is a monster that can be attacked, and not some sort of trap/hazard that must be dealt with creatively. But how do I do that?

(Also, the swarm has 22 HP and resistance to bludgeoning, piercing, and slashing damage, which is effectively 44 HP. That's nuts for its 1/2 CR, isn't it?)

I think others have covered your original questions. I'll just add that I think the condition immunities for swarms are overly broad. For instance, if the swarm comprises creatures that could be frightened individually, I'm inclined to allow a portion of a swarm to be frightened if subject to an AoE such as the Fear spell. (Of course, a single target effect would not do anything noticeable.) So I'd allow a portion of the swarm to be Charmed, Frightened, Paralyzed, Petrified, or Stunned if appropriate. Grappled, Prone, and Restrained seem less likely since there aren't, AFAIK, AoEs that impose those conditions.

It's probably also worth thinking about what Sleep does to a swarm. I think I'd let it take out a portion of the swarm according to the rolled HP for one round, but then have half the sleepers wake up each succeeding round as they inevitably get jostled by the rest of the swarm (unless Sleep puts the whole swarm down).

 

[Shadowdweller]

But I'm not sure how to describe this action without giving the players the impression that they're being engulfed in spiders and can't attack for fear of hurting themselves or their companions. (How the hell do you stab a spider when it's crawling on someone, anyway?)

I would go with the direct route. "It looks like the spiders could be safely attacked without risking injury to Person X." Simple, clear, doesn't really break immersion. As for describing how you stab a swarm of spiders - you probably don't. But maybe you use the flat of a dagger blade to smoosh a number of them. Yes, of course it's less effective then stabbing normally would be...that's why the creature has resistance.

 

[Harzel]

DM: You are attacked by a Swarm of insects (spiders)
Me: Spiders aren't insects
DM: You win D&D
Me: puts on sunglasses
DM: You are the DM now
Me: takes of sunglasses

Yeah, I noticed the same thing. And centipedes aren't insects, either. A couple more unforced errors that don't really matter, but, for me, contribute to the slip-shod feel of their presentation of 5e. I mean, it's shorter to just say "Swarm of spiders" and even though there is a generic "Swarm of insects", they don't leverage the implied commonality for anything.