[RPG] What happens when you cast the Polymorph spell on a swarm


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?

Best Answer

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.