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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]
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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]

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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.

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    \$\begingroup\$ Are you basing this on the stat block being, in the MM, each creature has a stat block? \$\endgroup\$ – KorvinStarmast May 6 at 15:58
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    \$\begingroup\$ This answer claims that it is the RAW answer, but it cites exactly zero rules. \$\endgroup\$ – Thomas Markov May 6 at 18:52
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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.

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    \$\begingroup\$ Can you find another source than the now-unofficial tweets? At the very least, you might point out that the tweets aren't official rulings but might speak to intent. The site you link to is an unofficial compilation site and has no official affiliation with D&D, Crawford, or the rules. \$\endgroup\$ – G. Moylan May 6 at 16:54
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    \$\begingroup\$ Probably best to change the URLs to the direct twitter rather than through the 3rd party website. \$\endgroup\$ – NautArch May 6 at 17:16
  • \$\begingroup\$ Did not know Sage Advice wasn't considered official, but as suggested, but I did change them to the direct tweets and clarified who said what. \$\endgroup\$ – Victor B May 6 at 18:18
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    \$\begingroup\$ @VictorB The "Sage Advice Compendium" is an official, public document used to adjudicate and interpret the rules. The Sage Advice website, though similarly named for Crawford's tweets, is not official and never has been. His tweets are no longer official, too, after long history of contradictory or confusing rulings \$\endgroup\$ – G. Moylan May 6 at 18:20
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    \$\begingroup\$ @VictorB See the “Official Rulings” segment here. You can still often use his tweets, as they speak to intent, but it’s best to 1) acknowledge that in the answer, and 2) find something RAW or SAC if possible \$\endgroup\$ – G. Moylan May 10 at 5:15
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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.

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  • \$\begingroup\$ You should cite relevant evidence/experience to support your claims. See this meta: What are the citation expectations of answers on RPG Stack Exchange? \$\endgroup\$ – V2Blast Apr 28 at 22:51
  • \$\begingroup\$ @v2bl I did cite the rules. See paragraph 3. It just is an unpopular reading of the rules. It happens, I am ok with it. Beeps answer, in contrast, cites no rules, but is popular with many upvotes. And no mod advising reading basic SO expectations. \$\endgroup\$ – Yakk Apr 28 at 23:03
  • \$\begingroup\$ Or by cite, are you meaning quote? Quoting that much text wouldnpt be legal, as it is a long list, and it is the absence of swarm from the list that is of note. \$\endgroup\$ – Yakk Apr 28 at 23:08
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    \$\begingroup\$ You don't need to quote the entire list - what I mean is that (for instance) you say "the rules for creatures", but not which rules/where they can be found (e.g. book and page number/section). Likewise, your answer makes a number of other claims but doesn't necessarily cite any particular rules that support those claims. \$\endgroup\$ – V2Blast Apr 28 at 23:14

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