I threw together a very basic encounter which can be summed up as "some faerie dragons mess with the party". They aren't hostile, just mischievous, and between the two of them they know Polymorph and Illusion.

I am now wondering how Polymorph's ruling of "an unwilling creature must pass a wisdom save or be transformed" interacts with a sleeping or unconscious player?

I feel like a spell like this, cast on a PC whilst they slept, would not allow for a direct save - they aren't really aware of it happening!

I've no intent for it to do anything but screw with the players - perhaps have the players transformed into woodland creatures and have to undertake the next hour of pursuit as animals, for the amusement of the dragons - The dragons could steal something important, with every intent of giving it back after their mischief.

I'm also curious as to how it works vs wildshape - could a polymorphed druid change form? if they change back, would they change back to their normal form, or their polymorphed form?

This started as a single-line encounter on a random encounter table; now I'm thinking of expanding this into a more elaborate encounter, where everyone is now small or tiny woodland critters for an hour! (I'm loving the idea of making them direct opposites of what their characters might want - the barbarian becomes a sparrow, the wizard becomes a badger, the monk becomes a snake, that sort of thing!)