I have an issue with swarms

Eh, I don't know about immune to forced movement. To me, that's a ruling because it depends up on the source of the movement. Shove? Nope, won't work. Thunderwave? Yes, that moves a swarm.

LordEntrails said:

Eh, I don't know about immune to forced movement. To me, that's a ruling because it depends up on the source of the movement. Shove? Nope, won't work. Thunderwave? Yes, that moves a swarm.

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That rule does account for source. In 4e, the forced movement from melee and ranged attacks are weapon attacks. Thunderwave is an area effect.

Swarms actually saw a mighty nerf between 3.5 and 5E (don't know how they worked in 4E, don't care). The difference between being immune to weapon damage and merely resistant to it is a pretty huge one. The fact that they need to ROLL TO HIT now whereas before they didn't also seems like an eminently reasonable change. And the nice tidy simulationist mechanic of their killing power decreasing when they get below half hp is just icing on the cake. I have no idea how low level parties survived encounters with swarms in 3.5.

The reason most real-world animals that swarm do so is for defensive reasons. It makes it much harder for predators to attack in the confusion of a swarm.

The first thing I would do with most swarms is give disadvantage to all attacks against a swarm unless it is an AOE spell/ability.

Next, it depends on what the swarm is. If controlled by an intelligent being (e.g. bats controlled by a vampire) or if the swarm has its own intelligence (e.g. cranium rats) I may give some tactical combat advantages. Generally that would simply be multiattack and increased to-hit bonuses.

Keep in mind that some swarms should make obscure site and in some cases give the blinded condition.

The area of the swarm should be treated as difficult terrain. We had "frog crossing" signs in central Wisconsin, not for the frogs sake, but because hitting hundreds of frogs while driving down a highway could be deadly to the driver as much as the poor amphibians. I would imagine that trying to walk among hundreds of biting rats would be difficult.

For ground swarms, treat it as slippery, make a dex save at the start of each round or fall prone.

Make a fear check when first encountered. I'm not scared of insects, but I've been chased by bees and have sat on anthills. Neither were true swarms, but it ruined my day. You survival reactions kick in and it would take experience, training, or high mental fortitude to keep your calm in a true swarm.

For insect swarms, you could have them cause breathing problems. Locust swarms are horrible to be caught in and they will eat almost everything but the only instances of locust swarms killing people are from breathing problems. Perhaps impose a level of exhaustion for every round in a swarm. With stinging insects you can requires con saves or get the poisoned condition.

So, going to your bat example. First, we'll assume that this is a natural swarm of a species of bat that evolved to be aggressive.

1. Frightful. Any creature in the swarm must make a DC 12 Wisdom safe or become afraid. They can make a new save at the start of each round. Once a save is made, that creature is immune to fright from this kind of swarm for the next 24 hours.

2. Heavily Obscured. Anyone within the swarm has the blinded condition. This affects normal vision, dark vision, truesight, and blind sight (unless through echolocation)

3. Hard to move safely. Anyone in the swarm moves as if in difficult terrain. This includes flying creatures.

4. Deafening. Anyone in the swarm is deafened by screeching of hundreds/thousands of bats flying around them.

5. Hard to hit. All attacks against the swarm are made at disadvantage, whether from inside or outside the swarm, unless they are AOE attacks.

6. Chance of rabies. For every successful attack by the swarm there is a 10% that the hit creature will be subjected to a rabid bite. The creature must make a DC 15 saving throw or contact Rabies. After 24 hours the creature will lose one point of Wisdom and one point of Constitution and will continue to lose a point of Wisdom and point of Constitution every 24 hours until either reaches zero, at which point the creature dies. Lessor restoration will cure the disease.


This makes a bat swarm a memorable encounter without simply giving it more attacks or easier to hit attacks.

UngeheuerLich said:

Swarm: A swarm is considered a single monster even though it is composed of several Tiny creatures. Most single swarms are Medium, but some can be larger.
A swarm takes half damage from melee and ranged attacks. It is vulnerable to close and area attacks, as indicated in the monster’s stat block.
A swarm is immune to forced movement (pull, push, and slide) effects from melee and ranged attacks. Close or area attacks that impose forced movement affect the swarm normally.
A swarm can enter or move through an enemy’s space; this movement does not provoke opportunity attacks. An enemy can enter a space occupied by a swarm, but the space occupied by the swarm is considered difficult terrain, and doing so provokes an opportunity attack.
A swarm can squeeze through any opening large enough to accommodate even one of its constituent creatures. For example, a swarm of bats can squeeze through any opening large enough for one of the bats to squeeze through. See the Player’s Handbook for squeezing

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The main differences I can see from 5E are ...

1. Vulnerability to close and area attacks.
2. Immunity to forced movement effects from melee and ranged attacks.
3. Moving through an enemy's space doesn't provoke opportunity attacks.
4. The swarm gets an opportunity attack on an enemy that moves into its space.

Everything else is already in 5E, as far as I can tell. Points 1 and 2 would be hard to model in 5E, since the distinction between melee and close attacks doesn't really exist, although probably there could be an allowance for vulnerability to damage dealt in an area attack. And of course, forced movement is much less of a thing in 5E. Points 3 and 4 would be pretty simple to port over, though.

MNblockhead said:

. . . make a sex save at the start of each round or fall prone . . .

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That sounds right.

To me, it was the single attack of a swarm that was most troubling, it just doesn't have the right feel. Now sure the attack is described as "Bites" implying that if the attack is successful there will be multiple bites as part of the single attack - but that really feels like they're working around the whole multi-attack feature for an unknown reason?

I dunno - it just seems mechanically weird as is Advantage and multi-attack seemed the right way to go about it.

robus said:

To me, it was the single attack of a swarm that was most troubling, it just doesn't have the right feel. Now sure the attack is described as "Bites" implying that if the attack is successful there will be multiple bites as part of the single attack - but that really feels like they're working around the whole multi-attack feature for an unknown reason?

I dunno - it just seems mechanically weird as is Advantage and multi-attack seemed the right way to go about it.

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I understand it as the damage from all the many bites, scratches, stings, etc. Agreed that using plural bites would make this clearer.

robus said:

To me, it was the single attack of a swarm that was most troubling, it just doesn't have the right feel. Now sure the attack is described as "Bites" implying that if the attack is successful there will be multiple bites as part of the single attack - but that really feels like they're working around the whole multi-attack feature for an unknown reason?

I dunno - it just seems mechanically weird as is Advantage and multi-attack seemed the right way to go about it.

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Splitting the attack in 2 is cool - it lets you have the swarm sweep through the party attacking a couple PCs as it moves. That's a nice touch.


I'd skip the advantage, though. I just don't like making it an always on thing. Pack tactics can be mitigated by the players if they want to bother, which to me seems just a smidge more interesting (tactical even) . . . I'm talking myself into giving the swarm a sort of pack tactics, though, like if two or more swarms join up, them they get advantage.

Swarm Trait: Can attack once for each creature in its area.