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Does a character need to know a creature to use Polymorph?

If yes, is a little information found in an old book enough, or does it have to be an encounter with said creature? If no, does the character know everything about all creatures he is able to polymorph into, including Resistances and abilities?

Is it something in the middle like: Hey I learned Polymorph, now I know all the beasts out there, but I only know their shape and size and nothing about abilities etc...

Thanks for your help

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up vote 5 down vote accepted

The description of the spell says:

The new form can be any beast whose challenge rating is equal to or less than the target’s.

Beast is a defined type of creature within the game, as per page 6 of the Monster Manual:

Beasts are nonhumanoid creatures that are a natural part of the fantasy ecology. Some of them have magical powers, but most are unintelligent and lack any society or language. Beasts include all varieties of ordinary animals, dinosaurs, and giant versions of animals.

A reasonable interpretation is that the PC needs to have knowledge of the beast.
But how much knowledge? It seems reasonable that the caster can turn the target into a frog, even if the caster is not a frog expert. The caster may not even know much about frogs at all, except maybe that they hop. To my mind, it seems reasonable that the magic fills in the gaps. That if frogs say ribbet, then the transformed creature probably says ribbet, too, even if the caster doesn't know that frogs say ribbet.

Regarding your question about a little information in an old book, that is an interesting question. What if the old book is wrong? Old books are often filled with creatures which never actually existed. What if it is a new book? What if the book is fiction? What if the book is fiction and the caster wrote it? What if the book is fiction and the caster wrote it just now?

In the end, it is up to the players, particularly the GM to determine whether what the caster intends fits within the definition of "any beast".

As a GM, I would tell the player, that's an interesting idea, I suppose you could give it a try if you wanted. I would let the player use any source of in-game information, including the character's own imagination. I would also expect the character to be able to hold the concept of the target creature in its mind and that the player be able to state the target fairly succinctly. After all, it has a casting time of one action. If the intended results were out of balance with the power of the spell, I would modify those results. "I transform the big bad wolf into a cute kittycat", might work pretty well, at least the kittycat part. "I transform the big bad wolf into a piggy" would probably work. "I transform the big bad wolf into a flying piggy" might even work. Of course, if it were a chaotic evil wolf inclined to eat first and ask questions later, then the kitten, piggy, and flying piggy, would all be chaotic evil inclined to eat first and ask questions later, too. Having a face full of avian wild boar might not have been what the caster intended. The more absurd the caster's intention, the more absurd the results would be. That seems like rules-as-fun.

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Nice thoughts. One point of order, the spell description does say "chosen beast," so I think it's fair to say that RAW support the form being chosen, not random (in line with previous editions). – timster Feb 27 at 15:29
Excellent point. I will amend my answer. – Jack Feb 27 at 15:33

The spell description describes taking on the statistics of the "chosen beast." That would seem to imply that:

  1. the caster merely has to identify the creature, and that
  2. the target form is a sort of "multiple choice" of existing creatures.

You might think of the new form being instantiated based upon a "platonic ideal."

Allowing flying piggies definitely seems like more fun though. Lots of opportunities for things to go slightly haywire there.

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The multiple choice of existing creatures bothers me a bit. Because how does the caster or the magic know what the existing creatures are? Creatures like existing beasts seem pretty reasonable as beasts. Sus scrofa domesticus avians seems like a beast to me. – Jack Feb 27 at 15:54
Multiple choice was not exactly the best phrasing. The caster would have to be able to identify the beast. Just like choosing what you car you want: you can't choose something that doesn't exist, and you can't choose something if you've never heard of it. I'll add that I think it's reasonable that any spell caster who learns this spell would want to spend a few evenings perusing a bestiary for ideas. (Whatever is in said bestiary is up to the DM, of course.) – timster Feb 27 at 16:12
You have a good point. I guess what does "identify" mean exactly? What bestiary? How do we know it is accurate? I think how the beast gets chosen is well within player/GM interpretation. The RAW doesn't say anything regarding the possible beasts available to the caster. Unlike the druid transformation, it doesn't even say the caster has to have seen the target beast. – Jack Feb 27 at 16:41
Here's how I do it: "I guess what does 'identify' mean exactly?" - Naming it. "What bestiary?" - The Monster Manual, but not other books, plus other reasonable choices, like blood hound. "How do we know it is accurate?" We don't. The DM can spring surprises on you, but they ought to be careful not to ruin the spell that way. One other house rule of mine is if the caster has minimal knowledge of the creature, then the creature might look strange to observers very familiar with it, especially members of that species. (That's just straight out of DS 9.) – timster Feb 27 at 17:48

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