How does resistance/vulnerability/immunity interact with carryover damage after reducing a Polymorphed (or Wild Shaped) form to 0 HP?

Question

A caster casts polymorph on another creature. Let's say the polymorphed creature has 10 HP in its new form, but takes 30 piercing damage and its current form is reduced to 0 HP. This causes it to revert back to its original form, with 20 more piercing damage that would carry over. However, its original form is resistant to piercing damage.

How much damage would the new form actually take? Would its original form's resistance to the damage type apply to the carryover damage?

The same question can be extended to the original form having immunity or vulnerability, as the answer would ostensibly use the same logic.

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The druid's Wild Shape ability also works similarly to polymorph in this regard (if you reduce the new form to 0 HP, then any remaining damage carries over to its original form), so I suspect the answer would be similar for a similar question about Wild Shape.

Answer 1

Any damage carried over will be affected by the new form's traits

This is because resistances, etc. apply whether the damage is dealt or applied in some other way.

If a creature or an object has resistance to a damage type, damage of that type is halved against it. If a creature or an object has vulnerability to a damage type, damage of that type is doubled against it.

To clarify, it specifies that damage is halved without any mention of how the creature receives it. I think you would be hard pressed to say that the carried over damage is not "against it." Also, the damage itself is changed, not just the HP lost, which leads to the entire set of damage being modified before carrying over to the other form.

Here are a few cases to consider (Altered Form has 10 HP, takes 40 damage):

1. Altered Form has resistance, Original Form does not

• Altered Form modifies the damage to 20 causing 10 to carry over.

2. Original Form has resistance, Altered Form does not

• Altered Form takes full damage causing 30 to carry over which is modified to 15.

3. Original Form and Altered Form have resistance

• Altered Form modifies the damage to 20 causing 10 to carry over which is modified to 5.

The stacking of resistances in the 3rd case is due to the wording of the feature. While normally multiple instances of resistance against the same damage type do not stack, the creature only has one instance of resistance at a given time. A plain reading of the polymorph spell gives us that the damage is "carried over" which means "transferred or resulting from a previous situation or context." An answer on a similar feature has clarified that transferred damage is affected by type-based damage modifications. If we take the second portion of the definition of carry over, the word "resulting" give us an entirely new instance of damage as a "result" is a separate consequence than the cause in plain reading.

Sage Advice

This answer is further reinfoced by the official rules clarification document Sage Advice Compendium as of October 2020:

If a creature under the effects of polymorph takesenough fire damage to revert to its true form and that form has fire resistance, does the true form take the full remaining damage or only half due to resistance? When the creature reverts to its true form, any leftover damage is subject to that form’s damage resistances, if any.

Answer 2

Altered form absorbs damage until it hits 0 hp. Remainder is applied to original form.

• Altered form with 10 hp.
• Damage to be applied 40.

Results

Resistant Altered Form

Altered form is resistant. It takes 20 of 40 damage before being reduced to 0 hp. 20 damage remains to apply to original form.

Non-resistant Non-vulnerable (normal) Altered Form.

Altered form is not resistant nor vulnerable. It takes 10 of 40 before being reduced to 0 hp. 30 damage remains to apply to original form.

Vulnerable Altered Form

Altered form is vulnerable. It takes 5 of original 40 damage before being reduced to 0 hp. 35 damage remains to apply to original form.

Altered form eats some of the damage affected by resistance/vulnerability. Original form eats the rest affected by the resistance/vulnerability.

Rationale

The rules on damage resistance and vulnerability state:

If a creature or an object has resistance to a damage type, damage of that type is halved against it.

The description of the polymorph spell says:

If it reverts as a result of dropping to 0 hit points, any excess damage carries over to its normal form.

The damage is applied to the target. The target halves that damage against it. e.g. 20 damage applied 10 taken. The excess damage is the remaining balance.

Answer 3

Your normal form would modify the damage assuming that instance of damage has not already been modified the same way.

Resistance/Vulnerability can only be applied to a given instance of damage once.

This claim is ironically supported by a response by Jeremy Crawford misunderstanding a question on Twitter about Warding Bond but it does reword PHB p197 to make a touch more sense.

Resistance is applied only once to any instance of damage. See the Player's Handbook, page 197.

This is a blanket statement that is clarifying a misunderstanding that some seem to have. Warding Bond, and its ilk, break one instance of damage into two whereas the OP's question would be a single instance of damage affecting the same creature (unless you think that polymorphing makes you a separate creature, which it doesn't).

PHB 197

Multiple instances of resistance or vulnerability that affect the same damage type count as only one instance. For example, if a creature has resistance to fire damage as well as resistance to all nonmagical damage, the damage of a nonmagical fire is reduced by half against the creature, not reduced by three-quarters.

If you apply resistance after it has already been applied you are essentially instigating that last line of trying to reduce it by three-quarters or lower since it will be even less by the time it gets to your standard form.

The Difference in the linked questions. To help alleviate the conflation between the other questions linked as they don't work quite the same way I offer the following:

In the cases of Shield Guardian/Warding Bond effects the benefactor of the effect (the wearer of the amulet in this case) is hit with an arrow for 40 damage. The wearer never takes that damage as it is split prior to application so that 20 affects him and 20 affects the construct then resistances and immunities are applied. This effectively makes them a separate instance of damage for each creature involved and thus each can benefit independently from resistances/immunities/vulnerabilities.

Damage A is rolled and immediately split into Damage A1 and A2 before application.

The issue at hand. This issue is not splitting the damage between two creatures before application it is actually applying a single instance of damage to a single creature. Resistance would be applied once and only once to that instance of damage and thus in your example:

1. Assumed form doesn't have resistance and standard form does which is your main question... You take 40 get reduced to 0 then the remaining 30 has resistance applied so that you take 15 as no resistances have been applied to that instance of damage, yet.
2. Assumed form has resistance... incoming damage has resistance applied and is halved. So you take 20, this reduces your assumed form to 0 and the remaining 10 is applied to your standard form.
3. Assumed and Standard form have resistance... incoming instance of damage is halved by application of resistance. You take 20 damage which reduces you to 0 you fall out of your assumed form and since this instance of damage has already been affected by resistance it cannot further be affected by it therefore you take the remaining 10 to your standard form.
4. Assumed form has vulnerability and standard form has resistance... incoming damage is doubled to 80, you take 10 and drop to 0 the remaining 70 damage is reduced to 35.
5. Assumed and Standard form have vulnerability... incoming damage is doubled to 80, you take 10 and drop to 0 the remaining 70 damage is take by your standard form, because that instance of damage was already modified by vulnerability. (i.e. it can't be subject to vulnerability twice before application to the creature)