How do swarms work in 5e?
I'm so used to PF swarms, and I know there's got to be a difference.
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In the simplest terms, a swarm is resistant to bludgeoning, piercing, and slashing damage, and also has a number of conditional immunities (charmed, frightened, grappled, paralyzed, petrified, prone, restrained, stunned) from being a swarm. Past that, they have the swarm trait:
Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening
large enough for a Tiny raven. The swarm can't regain hit points or gain temporary hit points.
In addition, their damage output is roughly halved when their HP total is half or less.
That, more or less, is it.
Ah. So no imunities to weapons or automatic damage to an area?
Nope, but they generally seem to have a higher than normal attack bonus (1 or 2 higher than normal for their CR), but I think this is balanced by the fact they have a reach of 0.
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They have weapon resistance, which basically just makes their HP go further. However, if you had a swarm of tiny creatures, you could just say that they had weapon immunity.
As for the automatic damage, it works out well enough to give them an attack roll. PF's conceit was that a bunch of tiny creatures would have no chance to hit anything with a high AC without some weird math, but 5e obviates that problem.
My PCs are about to go underwater adventuring, and I was thinking of using a swarm of quippers.
But our paladin has an AC of 23. That means I need a 19 to hit him with the swarm. I was expecting, before I looked into this, to be able to do some automatic damage before they killed of the peddly CR 1 swarm. Maybe I'll use something else.
You can use multiple swarms, maybe with some using the Help action?
I think swarms are meant to be used in groups. For example, an old school 3.5 or PF swarm is a 10 foot cube. In 5th Ed, that is 8 Medium-sized swarms.
Like most types and subtypes and such swarms don't have universal rules anymore but they are all statted with some commonalities, as lorathorn listed.
unless you're going to make your own swarms, you don't even need to know these, because the individual stat blocks spell them out.
In the Critter Compendium, my project for the DM's guild (which will have over 200 new monster stat blocks, most of which illustrated, if you didn't know), there will be a couple of special swarms, some of which can take humanoid forms, temporarily foregoing some of the swarm abilities in order to gain the benefits that the humanoid shape brings with it.
The Tome of Beasts has a Rat King which is kind of like a humanoid swarm. Or at least a humanoid-shaped swarm.
It's pretty creepy.
Not a bad idea with all the demons around in the underdark. :)
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Do they have that "blood in the water" ability? Or whatever it's called? The one where you get advantage on attack rolls against creatures with less than their full hit point total?
If not, it's definitely a trait that quasi-piranha could have/would have/should have.
You could also boost their Dex score and/or give them "Expertise" on attack rolls and double their proficiency bonus on attack rolls.
And maybe the paladin will be "sensible" and take off his full plate before going swimming. Even though 5th Edition doesn't apply any penalties for wearing armor and swimming (that I know of). Maybe the swim DC itself could change due to armor? Like normally it's a DC 10 Strength (Athletics) check, but 15 in light, 20 in medium, and 25 in heavy armor? I'm just spitballing here.
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The DMG has a good bit of advice on this (page 250). The short version is that you can group together the number of creatures that it would take to hit (in your case, 10), and just let them auto hit. You could extrapolate that into "distinct" swarms of 10-15 quippers, each of them being able to auto-hit until they are at half life, representing the loss of overall quippers.
Or whatever. The point is, there are options, some even supported by the DMG!
Another thing I did with my swarms.
If they hit someone that person is now covered int he swarm and drag them with them when they move. Also any attacks split the damage. So if hammer wielding fighter smashes swarm covered sam and does 20 damage , they both take 10 and then apply modifiers.
These are some great ideas! Thanks everyone!