What is the CR of a Metallic Dragon after it uses Change Shape?

Question

The older Metallic Dragons have the Change Shape ability:

Change Shape. The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (the dragon's choice).

In a new form, the dragon retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.

The dragon retains many things, but not explicitly its Challenge Rating (CR). What happens to a metallic dragon's CR when it uses Change Shape?

1. The CR remains the same (ex: CR 20 for Ancient Brass Dragon)

2. The CR is that of the assumed humanoid or beast (ex: CR 0 for Commoner)

I am honestly not quite sure; it could go either way.

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The reason I'm asking is that in the module "Trust and Understanding", there is something that blocks creatures below a certain CR threshold from passing through. The effect reads:

Any creature that is of CR 16 (for characters, level 16) or less that enters this space must succeed on a DC 28 Wisdom saving throw or be subject to a permanent geas that delivers the command "Return the way you came."

If a Metallic Dragon that is normally over this threshold used Change Shape to polymorph into a beast/humanoid under the threshold tries to pass, what would happen then?

Answer 1

Treat the Dragon's CR as 20

In the specific example you give at face value the current CR seems appropriate, but I would argue given how the spell works and its effects that it was specifically designed to deter low level players and weaker creatures from entering. While the shape-changed dragon has some of the stats of a lower CR creature, it retains some of its own stats, abilities, and actions. Ultimately its a question of whether you believe the magic in D&D is essentially aware of the narrative, a CR20 dragon is an incredibly powerful entity even if temporarily within a "weak" body.

On a DM meta level just always treat the CR of a creature as its true-form, the mathematical threat to the party that CR representsif a fight occurs is still the same.

Answer 2

Technically the CR would change depending on the form chosen, but it probably won't matter in your situation, because the dragon could just change back and become CR20 or use the legendary resistances to overcome the magical barrier.

To use your example, if an Ancient Brass Dragon turned into a commoner, the dragon-commoner would still have 17 hit dice and access to its lair/legendary actions. Even accounting for changes in STR and DEX, the dragon/commoner form would be far more deadly than a typical commoner. But in order to calculate what the new CR would be, you would need to recompute how the dragon's damage output changes, make note of the AC of the new form, and compare all of that to the tables in the Dungeon Master's Guide to determine the CR as if it were a new homebrew creature.

However, this change in CR is only applicable while in its non-dragon form, and it can change shape back into its true form at any time. Depending on how often the dragon shifts form in your game, it may be too much effort to keep track of in your game. What I have done with some success in my games is calculate the CR of the dragon's humanoid form in advance and swap to the original form's CR as needed. But in my mind, I'm always estimating the CR for the encounter as though it was the original form's CR, mostly because the original form could come out at a moment's notice and I try to minimize the potential for a total party kill.

Edit: My reasoning in favor of changing the CR is because when the dragon changes shape, its damage output also changes. In the case of the Ancient Brass Dragon to Commoner example, the dragon would go from dealing an average of 117 damage per round (from multiattack, breath weapon, tail legendary, and wing legendary) to dealing 2 DPR from the Commoner's club. According to the DMG, the CR of a creature is calculated as the average of its defensive CR and offensive CR. The change in damage output would result in in going from a defensive CR of 18ish to the commoner's 1/8 CR. It is not applicable in most cases, but if you have a dragon that spends considerable time as a human so as to not blow their cover (as I do in one campaign), it is an edge case worth considering.

Answer 3

Rules as Written, the creature's CR becomes 0.

The ability text says that the dragon's statistics are replaced by the statistics of the new form, and many things are marked as exceptions but CR is not marked as an exception. Following the rules literally, you would have to say that the creature's CR was 0.

In normal play, this should never come up.

CR is normally used for estimating the difficulty of combat, or for awarding experience after combat is over. But the dragon isn't going to shapeshift into a commoner in order to have a combat. If somebody is fighting the dragon, then the dragon is going to use its true form, or some other form that it thinks will be more effective than its true form. So, whenever the dragon's CR matters, it should be using its real CR.

We can imagine a DM who tries to exploit this CR-changing mechanic:

Just before you kill the dragon, it changes its shape. The dragon, now a commoner, laughs weakly at you. "I wasn't able to kill you", it says, "but at least I can deny you the experience points for defeating me..."

This might be funny, but a DM should never actually do this.

In your specific case, the dragon can easily enter the barrier.

You've told us that your specific problem involves a magical barrier that does this:

Any creature that is of CR 16 (for characters, level 16) or less that enters this space must succeed on a DC 28 Wisdom saving throw or be subject to a permanent geas that delivers the command "Return the way you came."

The dragon could enter this barrier by briefly shapechanging back into its true form, crossing the barrier, and then shapechanging back into a commoner. Or, even more simply, it could use this ability:

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

and simply choose to succeed at the saving throw.

Answer 4

The important bit about shape change is that is says it polymorphs, which contains the following (emphasis mine):

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). The target’s game statistics, including mental ability scores, are replaced by the statistics of the chosen beast. It retains its alignment and personality.

The dragon turns into a CR0 commoner and is CR0 for all intents and purposes until it reverts back into a dragon, even though the shape change ability modifies some of the restrictions imposed by polymorph.