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I have a feeling the RAW will suggest that this work just like with any single entity (like a goblin), but affecting a swarm seems like it should not simply be an "all or nothing" consideration. While trying to divvy up the swarm into individual units would be absurd, the description of the swarm already lists a "half-strength" variant. What do people think of an interpretation (or perhaps house rule) that allows you to put half of them to sleep and force them to therefore do half-damage until the other half is awoken? I suppose the complication would be that you would now have to track each individual half's own hit points, which certainly isn't ideal. Anyone have any other ideas beyond what the standard might suggest?

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    \$\begingroup\$ Hi Mike; polling for opinions and anecdotes doesn't really fit this site's format very well. However, with a minor tweak, this question could be adjusted to fit; maybe if you ask something like "If I wanted to use this houserule in my game, would it be balanced?" or similar... That way, the houserule can be assessed rather than asking for opinions. \$\endgroup\$ – NathanS May 23 '20 at 18:48
  • \$\begingroup\$ Good call - Its not really the right question given the site's philosophy .... I'll just delete it :) \$\endgroup\$ – Mike Murphy May 23 '20 at 18:59
  • \$\begingroup\$ Well, it could be reworked to fit, but that's your call. Alternatively, there's always a forum... \$\endgroup\$ – NathanS May 23 '20 at 19:01
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RAW, sleep does not affect a swarm.

In the description of the Sleep spell, PHB p.276:

Undead and creatures immune to being Charmed aren't affected by this spell.

All swarms in the Monster Manual (p.337-339) are immune to many conditions that typically would target a single or a fixed number of creatures: charmed, frightened, grappled, paralyzed, petrified, prone, restrained, or stunned. By this interpretation, the spell should not affect any swarms.

If a player were looking to homebrew this to have an effect on a swarm however, they may look at the individual creatures stat block and consider the swarm as an appropriately sized group of the creatures.
As an example, let's take the Swarm of Rats (MM p. 339). This swarm has a standard 24 hit points total, and an individual rat (MM p.335) has a standard 1 hit point. This would seem to imply there are 24 rats in a swarm, and from that we could say the sleep spell could target the swarm, and affect those rats as separate individuals, putting to sleep each individual rat up to the full hit points of the swarm, or however many hit points were rolled with the spell cast.
This would result in potentially creating two swarms however, as the still awake rats would continue there attack, and 1 minute later the affected rats would wake up together, creating a second swarm.

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  • \$\begingroup\$ This is how embarrassing this is to me .... this actually happened within the last month in a campaign that I was running and I told that spellcaster that the sleep spell they had cast had no effect on the swarm, precisely for the reasons you state. Then I go on here and ask a stupid question about it. \$\endgroup\$ – Mike Murphy May 27 '20 at 16:34

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