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?