What happens when you cast the Polymorph spell on a swarm?

Question

If a caster targets a swarm with the spell polymorph, what happens?

The polymorph spell description says:

This spell transforms a creature with at least 1 hit point that you can see within range into a new form. [...]

[...] The new form can be any beast whose challenge rating is equal to or less than the target's (or the target's level, if it doesn't have a challenge rating).

A swarm is a mass of (very) small creatures, but it is treated as a single unit in the rules.

It seems odd, but if a caster uses polymorph against a swarm, the whole swarm becomes the new shape, not just a single creature from within the swarm, right?

Is there a rule somewhere that gives swarms some kind of special consideration when it comes to single-target spells?

Answer 1

DM Fiat, but you probably cannot target the swarm.

Note that the very first part of the monster stat block says:

"Medium swarm of Tiny beasts, unaligned"

Per the Polymorph spell description, you can only polymorph a creature. So it should be ruled that the spell cannot target a swarm, or at a DM's discretion, that one beast in the swarm is changed with no effect to the rest (same AC, same hp, same everything).

As Rubiksmoose pointed out, the description text refer to the swarm as one unit. But it would be very clumsy to write out the sentences as individual beasts:

"All the individual ravens that comprise the swarm can occupy another creature's space and vice versa, and entire complement of the ravens within the swarm can move through any opening large enough for each one of the ravens. The entire swarm of beasts, nor an individual beast within the swarm, can't regain hit points or gain temporary hit points."

Related: Can a Druid Wild Shape into a Swarm or "Giant"?

Note that a Druid cannot Wild Shape into a swarm as they are multiple beasts. So it would be counter-intuitive that the reverse would work - that a swarm could polymorph into a single creature. Jeremy Crawford unofficially confirms this on Twitter here:

Wild Shape lets you transform into a single beast. A swarm is a collection of beasts, not one.

Answer 2

There are no explicit rules, but it seems as if the whole swarm would take up one new form

The rules aren't explicit, but they heavily imply that a swarm is considered one creature

It appears that there are no general rules for swarms that are provided in the rules, but some information can be gleaned from looking at individual stat blocks.

Looking at Swarm of Ravens for example we can see:

• The Swarm of Ravens is listed in one entry in the MM, with one statblock, one pool of HP, AC, abilities, etc. Nowhere does it say how many individual beasts compose the swarm, nor does it say that they are still considered individual creatures.

• The swarm is always referred to in monolithic form

  The swarm can occupy another creature's space and vice versa , and the swarm can move through any opening large enough for a Tiny raven . The swarm can't regain hit points or gain temporary hit points.

• The swarm has damage resistances and condition immunities different from those of normal ravens, is listed with a size of Medium (Ravens are Tiny) and does considerably more damage than a group of individual ravens would. This means that this is treating this group of creatures as an entirely new creature.

All these things show that all the rules we have for swarms treats them as one single entity not as a collection of smaller creatures. Thus, it is highly reasonable to extend that to targeting as well.

Given that the rules treats them as one creature, single target spells should treat them that way as well

So, since the heavy implication in the rules is that a swarm is treated as a single creature, single target spells should affect a swarm just as they would in the case they were targeting a normal single creature. In the case of polymorph, the swarm should then be transformed, as a group, into on single new form. So where you started with a swarm of rats (for example) you would end up with a single cat instead.