D&D 5E What are your tips for making mobs/swarms interesting?

[NotAYakk]

To make a mob of (4) Goblins, take the standard Goblin statblock and apply the following:
-Increase it's area size by +1. Both small and medium creatures take up 1 square, so both are bumped to Large.
-Keep any traits/skills/senses that make sense. In this case, Nimble Escape allows Disengage as a bonus action. Hide as a bonus action may work in some cases, but DM can call that as it comes up.
-Add Swarm trait: a [size] swarm of Small humanoids. Can occupy another's space, can't regain HP or gain THP.
give it resistance to bludgeoning, piercing, and slashing damage. Usually I give advantage vs spells like Hold Person or Charm Person, but do what makes sense for you
-Multiply the HP by 2 (this takes into account the resistance)
-Double the damage dice to half the number of creatures in the swarm.
--For melee attacks they have advantage against any creature who is occupying the same space.
--For ranged attacks, the target instead makes a Dex save or take dmg, or half on a success.

For Small, I'd have more than 4 in 2x2 squares; small creatures can fit in less than a 5' square. Say 6.

Second, have in vulnerability to area effect damage instead of resistance to slash/pierce/bludgeon. So a fire bolt isn't 2x damage over an arrow.

A Large swarm of Small X would be 6 goblins.

Give it 5x HP. Give it 2x damage and 2 attacks, plus creatures who move adjacent or start their turn have to make a save or take 1x damage.

Large Swarm of Small (2x2 squares, 6 creatures): 5x HP, 2x damage, 2 attacks, 1x aura, 3x range
Huge Swarm of Small (3x3 squares, 12 creatures): 10x HP, 4x damage, 2 attacks, 2x aura, 6x range
Gargantuan Swarm of Small (5x5 squares, 32 creatures): 30x HP, 5x damage, 4 attacks, 5x aura, 10x range (5x5 area)

DCs and attack accuracy would also go up as the CR does.

Mob of Goblins
Large (Huge/Gargantuan 5x5) swarm of Small humanoids (goblinoid)
AC: 13 (15 w/ shields)
HP: 35 (70/210)
Vulnerability: spells that deal damage over more than 1 square of the Mob
Swarm: can occupy other creatures spaces, cannot regain HP or gain THP, can take 2 reactions (3,4) (no more than 1 per turn)
Nimble Escape: can take Disengage or Hide as a bonus action
Pack tactics: has advantage on melee attacks against any creature occupying its space.
Knives everywhere: if you start your turn within 5' of the Mob, or move within 5' for the first time on a turn, you must make a DC 12 dex save or take 1d6+2 (5) (2d6+2 (9)/5d6+2 (19)) piercing damage. If the Mob has attacked you since the end of your last turn you have disadvantage on your save.
Actions:
Mob: Make two Shortsword Attacks (two/four)
Shortswords: +4 to hit; 2d6+2 (9) P (4d6+2 (16)/5d6+2 (19))
Shortbows: target must make a DC 12 Dex save or take 3d6 (10) piercing (6d6 (21)/10d6 (35) 5x5 area), or half on a success.

Now lets try to balance them against just raw goblins.

Defensive CR: 1/8, 1/4, 5
Offensive CR: 4, 6, 15
Overall CR: 2, 3, 10
XP: 450, 700, 5900
Adjusted XP of "raw" Goblins: 300, 1000, 4000
CR if we hit Adjusted XP: 1.5, 4, 8
N^(3/2) Adjusted XP: 367, 1039, 4525
Target offensive CR: 4, 8, 11
DPR to hit target XP: (good) / 65 / 95

Revised:

Knives everywhere: if you start your turn within 5' of the Mob, or move within 5' for the first time on a turn, you must make a DC 12 dex save or take 1d6+2 (5) (2d6+2 (9)/5d6+2 (19)) piercing damage. If the Mob has attacked you since the end of your last turn you have disadvantage on your save.
Actions:
Mob: Make two Shortsword Attacks (three/three)
Shortswords: +4 to hit; 2d6+2 (9) P (4d6+2 (16)/5d6+2 (19))
Shortbows: target must make a DC 12 Dex save or take 3d6 (10) piercing (6d6 (21)/10d6 (35) 5x5 area), or half on a success.

New CR: 2, 4, 8

and these "Mobbed" goblins are worth "as much" as just using individual goblins.

I might be tempted to add in a "break up" rule as well.

Morale Failure: If the Mob starts with 10 / 20 / 60 HP or less, it breaks into individual goblins. For every 5 HP or fraction an individual Goblin (CR 1/8) is created in the space. 1d2 / 1d4 / 1d12 of them attempt to flee each turn, the remainder fighting back.

Or even

Downsized: If the Mob is reduced below 5/35/70 HP, it transforms into a single goblin/a large mob of goblins/a huge mob of goblins. When it does so it has to make a will saving throw against a DC equal to 10+1/2 of the damage that reduced its size or become frightened.

 

[d'Angereaux]

I'm running a mega dungeons where not infrequently, the parties will run into a large number of baddies. E.g. giant rats, fire crabs, and even goblins. Also, swarms of rats, bats, and insects.

As I DM I've been finding the mobs a slog to run and the swarms boring.

I could treat a large number of rats, for example, as a swarm, with jacked up stats, but then you loose the chaos and uncertainty of having attacks happen throughout the initiative order. I've also tried to using the mob-combat rules in the DMG. The mob rules are good because they give a certain number of hits per turn, saving on lots of roles and ensuring that a mob is a threat even to characters with high ACs. But having all the creatures act on the same turn changes player strategy in a way that makes the combat more boring.

Similarly, I hat how swarms generally have a single attack, making them a danger to a single player at a time. The solution to that is multiple swarms, so, fine. But then many swarm attacks have trouble hitting high armor classes, which is not a reasonable result. Why would a swarm of tiny insects be hindered by platemail? If anything, having a lot of armor would be worse as you can easily shake out the creatures or swat at the creatures that got below your armor.

For mobs, I've gone to using tools to handle large numbers of combatants so they can attack throughout the initiative order. I've used Hero Lab, but now use Improved Initiative. But but I miss the auto hit rules of mob combat. I'm thinking that when there are a large number of combatants, I can have members choose to attack on a lower initiative order until there are enough attackers for an auto hit and they will continue to act together on the same round. That gives the sense of a character truly being ganged up on. So, their may be the chaos rush of, say, goblins, but they will start to cluster together around individual PCs.

For swarms, I've thought that in addition to using a sufficient number of swarms to create a large enough swarm to threaten the whole party and have multi attacks that I would add some special abilities for tiny creatures. First, I'm thinking that tiny creatures can ignore worn armor. Natural armor, or magic armor like mages armor, or bark skin would have full effect, but plate mail, magical or now, would offer no protection against a swarm of ants, bees, spiders, etc. Further, anyone in plate, or chain, will have disadvantage on attacks against the swarm attacking them unless they doff their armor.

Maybe that seems unbalanced, but it would make swarms worthy of fear and more interesting.

Other things to make mobs and swarms more interesting and challenging:

• Give disadvantage to perception checks when engaged in combat with mobs or swarms
  
  
• Make communication among the party more difficult with din of a mob battle or the noise of a swarm. If you want to yell or hand-signal a command to another player, you have to make a CHA (performance) check to make your message heard/seen and understood. It would still be a free action. If in a swarm of tiny creatures, make a saving throw or have your mouth filled with the creatures and take a level of exhaustion as you choke on them (or if that's to harsh, a disadvantage on all attacks, skill checks, or saves for the next round.

I'd be interested in learning other tips for running mobs and swarms in 5e.

Hey friend, check out this video I made about this exact topic.