If the same attack that causes a druid to revert from Wild Shape also reduces their maximum HP, what happens?

Question

I am aware of Jeremy Crawford's ruling that maximum HP reduction applied to a Wild Shape form does not apply to the druid's true form when Wild Shape reverts. However, this doesn't answer the question of what happens when the attack that causes the maximum HP reduction is also the attack that causes the druid to revert.

This is certainly a related question to the appropriate order of damage and HP reduction effects, with the added wrinkle of resolving when the druid reverts. Assuming someone can cite the answer to the linked question (which was made more by reductio ad absurdum than a cited source), I can see four likely scenarios:

1. The wild shape takes damage and reverts, following the full max HP reduction is applied to the druid. (This is, to me, most logical scenario, and results from the order damage-revert-max HP reduction.)

2. The wild shape takes damage and absorbs as much of the max HP reduction as possible, applying the remaining damage and max HP reduction to the druid. (This is hard to sell, as the max HP reduction is applied at a time when no rule says it should be, but based on Crawford's tweet above seems to be the rule as intended.)

3. The wild shape takes damage, and max HP reduction is applied, after that, the form reverts and additional damage is applied to the druid. The druid's max HP is not affected. (This is the most lenient case, but seems more likely to be correct than case 2 if the correct order of application is damage-max HP reduction-revert.)

4. The druid dies before it reverts because its wild shape form was reduced to 0 maximum hit points. (This is the harshest case and results from the same order, damage-max HP reduction-revert. This seems more likely again than the previous two, as "The target dies if this effect reduces its hit point maximum to 0," is not like Disintegrate's check after the spell's damage has been dealt.)

When the same attack that causes the druid to revert from Wild Shape also reduces their max HP, what happens?

Answer 1

Simultaneous Effects (XGtE 77):

Most effects in the game happen in succession, following an order set by the rules or the DM. In rare cases, effects can happen at the same time, especially at the start or end of a creature's turn. If two or more things happen at the same time on a character or monster's turn, the person at the game table-whether player or DM-who controls that creature decides the order in which those things happen. For example, if two effects occur at the end of a player character's turn, the player decides which of the two effects happens first.

The controller of the creature decides the order of the effects on their turn and thus which of the options is correct.

So if this happens on the druid's turn, the druid chooses their option, otherwise, the DM or the other controller decides the order.

Option 2 is not correct as the druid and the beast don't share an HP pool.

Compare to the SA Compendium V2.3 ruling on Tough (page 8), so either the druid's or the beast's hp are reduced, it is a binary choice. (I use this wording as the PHB 67 "you assume the beast’s hit points and Hit Dice." and "Your game statistics are replaced by the statistics of the beast" leave room for misinterpretation)

Does the Tough feat have an effect for a druid while in beast form? The intent is no. The Tough feat affects a druid’s hit points, which are replaced by the beast’s hit points while using Wild Shape.

The mere maximum HP reduction cannot result in excess maximum HP reduction unless the hp reduction is bigger than the total amount of maximum HP of the Beast, (a creature with a maximum of 0 HP can live). All effects that could reduce your maximum HP below 0 would kill the beast and thus the druid. Note that the PHB Errata (page 1) has clarified that the minimum HP maximum that you gain per level is 1.

PHB 66:

You automatically revert if you fall unconscious, drop to 0 hit points, or die.

Answer 2

This is quite clearly ambiguous.

Wild Shape "revert at 0 HP" rules contain:

if you revert as a result of dropping to 0 hit points, any excess damage carries over to your normal form.

So the damage "carries over" to your normal form. It isn't new damage, it is damage that is applied to the Druid from the original source.

The Wraith says:

...its hit point maximum is reduced by an amount equal to the damage taken.

So, damage is dealt.

During damage being dealt, the Druid's Wild Shape reverts, and the excess damage is applied to the Druid's maximum HP.

The Druid-as-Druid has only taken the excess damage. It has not taken the damage to the Wild Shape form.

Having taken the damage and being reduced to 0 HP, there are two things that need to happen. The Wild Shape form needs to make a Constitution save and it needs to revert to being a Druid.

Which happens first is determined by the person controlling the creature who is acting:

Most effects in the game happen in succession, following an order set by the rules or the DM. In rare cases, effects can happen at the same time, especially at the start or end of a creature's turn. If two or more things happen at the same time on a character or monster's turn, the person at the game table — whether player or DM — who controls that creature decides the order in which those things happen. For example, if two effects occur at the end of a player character's turn, the player decides which of the two effects happens first.

If this happened on the Wraith's turn, the Wraith might force a saving throw before the revert. The failure might result in the creature's max HP being reduced to 0, and death.

If max HP wasn't reduced to 0, then the Wild Shape form reverts. The Druid takes the excess damage; that excess damage is damage caused by the Wraith's attack. It reduces the Druid's maximum HP by the amount of damage taken unless the druid succeeds in a DC 14 Con saving throw.

It is unclear here if the success/failure of the Wild Shape on the saving throw applies to the Druid. I'd be biased towards yes.

Or maybe the Wraith will let the revert happen first. In that case, the excess damage is applied to the Druid. Then the Druid has to make a DC 14 saving throw, or have the Druid's max HP reduced by the damage taken - in this case, only the excess damage.

Only the excess damage is applied to the Druid's HP pool, so only that excess damage should reduce the Druid's max HP. But, that damage is dealt to the Druid, so if the Con save failed, it must reduce the max HP of the Druid.