[RPG] Help! My PCs polymorphed the boss enethe! What do I do

dnd-5egm-techniquesmonsterspolymorph

Our Druid has recently gotten access to the polymorph spell and has started polymorphing the opponents I so lovingly crafted for our (well, mostly my) enjoyment. (So far, he has been turning them into rabbits, not yet killer whales.)

Now, the canonical response to a polymorph is to reduce the polymorphed creature to 0 HPs, upon which it will transform back. Or to attempt to break the spellcaster's concentration.

However, polymorph is a 4th level spell, not a cantrip. It does not seem all that likely the target's allies know the spell, can connect the dots when their friend suddenly turns into a bunny, and start to attack the bunny or the spellcaster. It seems more likely that they will blink upon seeing their friend transformed into a bunny, shrug, write him off, and continue battling the PCs as before, which would essentially take the target out of the fight.

I have been thinking about allowing the target's allies an Arcana check each, against some reasonable DC (e.g., 14, which is 10 plus the spell level of polymorph). On a success, they recall hearing about the Polymorph spell and the two possible remedies. On a failure, they don't. In which case the target will remain polymorphed until it gets hit by a stray fireball or some such. I might even allow the target itself an Arcana check, and on a success, it will try reducing its own HPs. Well, this might at least make for some fun.

It looks to me like polymorph might be a bit overpowered against opponents who no have legendary resistance, and whose allies only consist of non-spellcasters because they will likely not know what steps to take to break the effect. Is there a better way to role-play what happens to a polymorphed enemy?

To address the concern about this being opinion-based: I fear that if adversaries often do not know how to break the polymorph spell, this will make combat more boring, since the PCs will simply open by polymorphing what (they think) is the most dangerous enemy, taking him out of the fight entirely. Also, it's asymmetric, since the PCs know how to deal with someone polymorphing one of them. What techniques could I as the DM use to make sure battles stay interesting, possibly even fun?

Best Answer

According to this answer, anyone who sees a spell being cast can use a reaction to make an Arcana check with a DC of 15+spell level, with advantage if the spell is on your spell list. Success means you identify the spell, which presumably means you also know how to get rid of it.

However, let's think about this. You're in battle, and an enemy spellcaster walks up and waves his hands and shouts some nonsense words, and suddenly your friend turns into a bunny. You probably wouldn't know to kill the bunny, but you'd definitely know that you should break the spellcaster's concentration if you want your friend back!

You've written that you think it would be difficult for the NPCs to figure out which of your party's several spellcasters had cast the polymorph spell, because several of them were all casting spells in the same round. I think you should allow your NPCs to be aware of turns, and to be aware of whose turn it is when a given effect happens. Your player characters are keenly aware of this, so it seems only fair that the NPCs should be aware of it too.

The best response to this is in encounter design. Either (1) don't give the player characters a battle where there's only one important foe, or (2) if you must do that, give the foe some points of legendary resistance, so it doesn't go down so hard when hit with save-or-lose effects.