as far as i know there is a rule for that in the PHB up to the dm if it reduce the size that it can fit in the space or it is not possible to become a Creature that cant fit in the room/ or breack some walls etc. 

So if an enemy caster used this spell on you to transform you into a cockroach, your best move would be to try getting stepped on or crushed in order to transform back into your natural form? So you should probably never use this to transform an enemy into anything because its easy to just kill yourself and revert back?

How does challenge rating translate to character lvl if one casts polymorph on themself?

Would you be able to use this on an incapped ally with zero hit points and temporarily get them back into combat (or delay death) by giving them an animal form? 

everyone can cast polymorph on themselve

Casting time for Polymorph is 1 Action.  Let's say you cast it on yourself.  Does the process of reverting to your true form also take an action?  Not talking about being forced to revert through loss of concentration, but actively deciding to revert back to true form.

This is my favorite comment in DnDBeyond so far. 

Well-done, great imagery.

My player transformed into a dinosaur recently but we have not clarified that he has seen one in his backstory, nor has he seen one in session. Can one transform based off of descriptions in books or do they physically have to have seen what they're transforming into?

This spell is far too overpowered for a fourth  level spell. As written it resembles the 8th level spell from 2nd edition. I think the duration should  be limited to  1 to  2 rounds, or  the  save modified for higher  level characters or monsters  to give them  Advantage  on the save. As a DM, this spell can quickly ruin a carefully powered encounter, destroying the challenge, ( and fun) of the adventure.

The answer is yes. The Legendary Resistance can only work 3/day, but when your druid is an absolute mad lady and summons 34 Sprites that can all cast polymorph at once, they can burn through those pretty quick, even with a low save DC. This did happen in a campaign I played in and no, we couldn't stop laughing when the dragon got turned into a rat.

Im thinking this spell is gonna be my new best friend when playtesting the one dnd druid

Seems lot better than the new playtest wildshape

Quote from Madzapple >>

My player transformed into a dinosaur recently but we have not clarified that he has seen one in his backstory, nor has he seen one in session. Can one transform based off of descriptions in books or do they physically have to have seen what they're transforming into?

 Theres no mention of having to have seen the creature so yh they can. The druid wild shape specifically stated that they had to had seen the creature 1st

If I were to polymorph into a creature, say a T-Rex, and then summon an avatar of death would the avatar use my original HP dice or the T-Rex’s HP dice to determine its HP.

I’m wondering because my level 8 bard has 60 hp, so the avatar would have around 30hp. A T-Rex’s chomp does an average of 33 damage, potentially one-shoting the avatar.

In the campaign I'm in, were pvping in a stadium right now. Could I possibly use this spell to turn someone into a cat, and put them in a cage?

I have someone who is wanting to use components for a spell and the component they need is fleece. If they use polymorph and turn into a sheep and have themselves sheered, do they keep the fleece? Also, will it affect them as their normal self like will they be bald? And will the location of baldness change depending on where they were sheered?

Quote from Wegan97 >>

I have someone who is wanting to use components for a spell and the component they need is fleece. If they use polymorph and turn into a sheep and have themselves sheered, do they keep the fleece? Also, will it affect them as their normal self like will they be bald? And will the location of baldness change depending on where they were sheered?

You've just completely hit RAW bedrock right there, if we're being honest.

That player is trying to cast minor illusion, right? The fleece doesn't have a price specified, and it isn't consumed. Thus, you may use a spellcasting focus in place of it, and avoid the question entirely. If you really must know, I'd rule that as the separated wool remains where it is after the spell ends, and the player doesn't lose any hair, just to keep things simple.

I really think this should let you change into things other than beasts.

Does this spell affect undead?

I could have sworn it doesn't,  but maybe I'm confusing editions.  🤔

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