What happens when the Aid spell ends on a creature while its hit point maximum has been changed to another value?

Question

A number of effects in D&D 5e can change a creature's hit point maximum. Some issues in the specifics of how some features are written can arise when these effects coexist or otherwise interact.

All emphasis mine. From the description of the druid's Wild Shape feature (PHB p. 64):

• Your game statistics are replaced by the statistics of the beast, but you retain [some non–hit-point-related statistics].
• When you transform, you assume the beast's hit points and Hit Dice. When you revert to your normal form, you return to the number of hit points you had before you transform.

An important note is that the latter bullet point refers to current hit points, not hit point maximum, as per PHB p. 196:

A creature's current hit points (usually just called hit points)...

Similarly, from the polymorph spell description (PHB p. 226) and the Creature into Creature section of the true polymorph spell description (PHB p. 283):

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

Finally, from the description of the spell aid (PHB p. 211):

Each target's hit point maximum and current hit points increase by 5 for the duration.

It appears unclear how the change to a creature's hit point maximum via the aid spell interacts with the change caused by the other listed features—especially when this happens multiple times. This raises an issue of interpretation which comprises two sub-questions.

Part 1: aid → transformation → aid ends

If aid is cast on a creature who then is affected by an effect such as Wild Shape which changes its hit point maximum in this way, what happens when the aid spell ends before the transformation effect?

0. The (transformed) creature's hit point maximum decreases by 5 (to 5 less than the hit point maximum of the target form without the effect of the aid spell)
1. The (transformed) creature's hit point maximum does not change
2. The (transformed) creature's hit point maximum decreases by 5 (to the hit point maximum of the target form without the effect of the aid spell)

Part 2: above transformation ends

Continuing from the previous situation, when the creature reverts to its previous/original form (after aid has already ended), what is its hit point maximum?

3. The creature's hit point maximum is the hit point maximum it had before it transformed
4. The creature's hit point maximum is the hit point maximum it had before the effect of the aid spell

It is likely that the answer to one of these questions will determine the other according to the accompanying reasoning, but it seems best to present both questions to allow for different approaches to the problem and a larger set of relevant rules to answer the situation as a whole.

Answer criteria

An accepted answer will attempt to interpret the rules in plain English while putting no weight on separate considerations such as lore, flavour, or game balance and provide its reasoning as such. This is because the answer would otherwise be highly opinion-based (in the case of lore or flavour) or, at least for the latter sub-question, trivial to answer. Please also note if (and why) the answers would be different for the listed transformation effects.

Answer 1

Aid affects the transformed and non-transformed the same

Since the transformed target is still a valid target for the effects of the aid spell, it is still subject to the effects of the spell. Neither Wild Shape nor polymorph dispel the effect of aid.

Example druid

• Alice is a druid with a hit point maximum of 10.
• Alice can use Wild Shape to transform into a bear with a max hp of 20.

Example sequence

1. Alice is the target of the aid spell, and now has a max hp of 15.
2. Alice uses Wild Shape to transform into a bear, and now has a max hp of 25.
3. The aid spell ends on Alice-bear and now the max hp is 20.
4. Wild Shape ends and now Alice has a max hp of 10.

The druid's beast form has a different hit point maximum which replaces the druid's as part of the "game statistics are replaced by the statistics of the beast" effect of Wild Shape. The beast form is still under the effect of aid, which affects the hit point maximum.

In the middle of this sequence, the effect of aid ends. The target no longer has a bonus to their hit point max. The beast form druid is back to the hit point maximum specified by their game statistics.

When Wild Shape ends, the druid's reverts back to their normal form. Without the effect of aid, the hit point maximum of their normal form is 10. In short, aid doesn't permanently change the hit point maximum of a target. The effect only lasts for the duration of the spell.

Sage Advice concurs

The Sage Advice Compendium directly addresses the question of spell effects remaining after polymorph:

Can a creature under the effects of polymorph have other spell effects on them, or are those game statistics also replaced by the those of the beast form? Polymorph replaces only the target’s character sheet or stat block with the stat block of the chosen form. Other effects, such as other spells, still exist.

Answer 2

This is mostly the same text for another question

For our example, Alice is a 2nd level Druid

Alice's hit points and hit point maximum change, but that's it

Alice is targeted with aid and goes from 10 hp to 15 hp from the spell. After being hit, she now has 10 hp again.

She then Wild Shapes into a wolf... and has 11 hp, just like a normal wolf.

Why? Because Alice is still the same target. Wild Shape doesn't change Alice into a new target; Alice is still Alice, just in a different form. Alice's hit point maximum was raised by 5 along with her hp. She then "assume[s] the shape of a beast".

The aid spell is still in effect on her. Her maximum did not decrease. And as the description goes: "...Your game statistics are replaced by the statistics of the beast...".

Sage Advice concurs

The Sage Advice Compendium directly addresses the question of spell effects remaining after polymorph:

Can a creature under the effects of polymorph have other spell effects on them, or are those game statistics also replaced by the those of the beast form? Polymorph replaces only the target’s character sheet or stat block with the stat block of the chosen form. Other effects, such as other spells, still exist.

Hit points are part of the stat block; therefore replaced

Alice's hit point maximum statistic is 15; not 10 + 5.

It is replaced with the wolf's hit point maximum statistic, which is 11 (or 2d8+2).

Aid still exists, as do other spells. But aid effects the stat block which is replaced. Invisibility, pass without trace, guidance can all continue to work also; but they are not controlled by the stat block.

Let's look at it from another perspective

We will go on the premise that the aid spell is still active and follows the form.

Alice was hit again and has only 5 hp left. To escape, Alice Wild Shape's into a wolf and the aid benefits follow; that means Alice would lose her benefits. So Alice is now at max hp 10 again, but she would also lose the 5 hit points that were granted by aid. If that's the case, she now has 0 hp out of max 10, knocking her unconscious.

She has become trapped in true form until she receives some form of healing.

The real kicker is what happens when aid ends prior to the transformation ending

Alice, the target, has her hit point maximum reduced by 5. It goes back to 10 regardless of transformation.

Since the spell says "current hit points", 5 hit points are removed from whatever Alice is transformed into. "Current hit points" get added to, "current hit points" gets subtracted from when the aid ends.

It seems odd that one bullet point effects Alice while the other effects the transformed beast, but the spell is clear in saying "current hit points" but does not say "current hit point maximum" so we can only deduct from the hit point maximum value at the time the increase was made; Alice's.

Let's look at it from another perspective

We will go on the premise that when aid ends, and the max hit points is removed from Alice's true form, when the 5 hp is removed it deducts from her true form also.

Once again, Alice only had 5 hit points left when she Wild Shaped into the wolf. The wolf has not been hit and has 11 hp. When aid ends, Alice's true form loses the 5 hit points and she becomes...? Unconscious with 0 hp? Still awake because the wolf has hit points? When does she start making Death Saving Throws?

Alice cannot be both unconscious and awake at the same time. It just makes more sense (and easier) to subtract the 5 hp from her current form (like the spell says) then trying to figure out how a druid with 0 hp is still awake and maintaining a new form.

Answer 3

The description of the aid spell says:

[...] Each target's hit point maximum and current hit points increase by 5 for the duration.

Whatever your hit point maximum and current hit points are, they are increased by 5.

So the correct answers to your two situations are (2) and (3).