How are swarms considered in terms of number of creatures?

Question

There is, in Adventurers League Dnd, a monster statblock with the following ability :

At the start of his turn, every living creature within 100 feet must succeed on a DC 15 Constitution saving throw or they lose 5 hit points and [he] regains 5 hit points.

That monster also has an ability to spawn Swarms of Rot Grubs (Medium swarms of Tiny Beasts). I am thus wondering how those two abilities interact.

More generally : how are swarms considered vis-à-vis creatures ?

1. Considered as a single creature [here, would be a single CON save]
2. Considered as many creatures (how do you say how many?) [here, would be X CON saves]
3. Considered as no creature [here, would be 0 CON saves]

Answer 1

Swarms are (implicitly) treated as singular creatures

All of the information we need to mechanically handle a given monster is provided in its stat block:

A monster's statistics, sometimes referred to as its stat block, provide the essential information that you need to run the monster.

Stat blocks will also explicitly provide special saving throw information when it is present.

Next, we can look in the Sage Advice Compendium on page 16 for a confirmation that swarms are indeed groups (in an answer about the conjure animals spell):

... swarms are groups of creatures.

However, given that swarms are listed as their own stat blocks in the Monster Manual, we must consider that those stat blocks are treated the same as any other, and would therefore make saves, attacks, etc. in a similar fashion to obviously singular entities. A swarm's stat block represents multiple creatures but is treated as a single creature.

Importantly, note that the stat block doesn't detail how many creatures are in a given swarm, only that the attacks can differ when the swarm drops below half its starting HP (as detailed in the attacks for a given swarm). We could say that the HP represents how many creatures are in a swarm, but that would only be an interpretation, not a rule, and shouldn't change how we handle rolls.

In short: While Swarms clearly are groups of creatures, they are listed as stat blocks in the Monster Manual, et al., so treat them the same as any other singular creature with a stat block.

In your specific scenario, we go with option 1:

Considered as a single creature [here, would be a single CON save]

Answer 2

Swarms are counted as a singular creature, but they have a few differences to non-swarms.

1. They are able to occupy another creature’s space (and vice versa)
2. They are unable to regain hit points or gain temporary hit points
3. They are able to move through a space large enough for a singular creature of their size (tiny in this case)

Of course, you could always play it differently, if you like, but RAW, the swarm would only make one saving throw.

Answer 3

There is a wrinkle to answering your question that I think should be addressed as well. While swarms, in your case, would be considered a singular creature, they are also majorly considered a group of creatures.

I have noticed many people debate and ask if a swarm is considered a single creature for purposes such as a Druid's wildshape(1), or Conjure Animals, which Jeremy Crawford (1), Mike Mearls (2), and Christopher Perkins(3) has spoken out on this(2) matter adamantly(3) stating that they are a swarm and not applicable as an individual creature.

However, in your case, you are asking about saving throws and such, which falls into the category of how the swarm interacts with the world and how they are formed. In this case, we must look at them, according to a tweet by Jeremy Crawford,(1) as many animals bound together. As such, they have a single HP to calculate, and not a collective of them, so they would work as if they are in a symbiotic relationship where they cannot exist seperate.

Likewise, hitting a swarm with a spell, like Burning Hands, would hit them as a group, not allowing stragglers.

Answer 4

Swarms are not singular creatures. Swarms are collections of creatures.

This is clear from every single text that describes what a swarm is, and what a creature is.

The rules for creatures list a set of types, which do not include swarms. Swarms state they are collections of creatures.

A spell that says "target eyeball become blind" would work on a human being who has an eyeball, spells that target creatures work on swarms.

In this case, "every creature" becomes a bit of a problem, as the rot grubs would consist of many creatures. How a DM handles that is going to require adjudication, but the most sane way is to slightly misapply the general rule from spell stacking; all of the effects happen at once, so only the most powerful applies, and it applies once.

In this case, it is one effect, so the swarm makes one saving throw if 1 or more of its creatures are in the region, and suffers the effect of the failure/success.

Similarly, the creatures of a swarm (while in the swarm) share HP; so spells that deal HP damage to a creature damage the entire swarm. Spells that damage every creature in an area use the "do the effect once". On the other hand, you could argue that spells that kill a single creature only kill one creature in the swarm.