Does Potion of Maximum Power maximize all damage dice of a duration-based spell?

Question

The first time you cast a damage-dealing spell of 4th level or lower within 1 minute after drinking the potion, instead of rolling dice to determine the damage dealt, you can instead use the highest number possible for each die.

A fireball is simple and becomes an instant 48 damage.

But would a Moonbeam or Melf's Minute Meteors or Spirit Guardians cast at level 4 or less maximize their damage each round, or only for the first damage roll?

It says the first time you cast a spell you use the highest number possible for each die, not the first time you roll damage, so I'm leaning towards it maximizing the entire spell's damage but this seems like one of those things that's too good to be true.

Answer 1

Yes, delayed damage is enhanced

No matter if a spell does its damage instantly or after some time, that damage is still dealt by the spell.

Since the potion affects the damage dealt by the spell, and since delayed damage is still the spell's damage, then the potion should also affect this delayed damage.

The argument of "when you cast"...

It could be defended that the potion mentions that this change in damage calculation happens "When you cast a spell..." (or, more specifically, "The first time you cast a [...] spell").

It then seems valid to assert that this calculation modification only applies to rolls done while casting.

This Q&A discusses in more details the exact timing of "when" a spell is cast. The global consensus seems to be that you are only casting a spell during its casting time, and holding concentration does not count as casting.

As such, damage rolls done during the next turns shouldn't be affected by the potion... Right?

... contradicts itself.

There is a flaw with this reasoning, however. Even spells that only deal damage instantly do not roll their damage during casting, but rather after the casting is complete and the spell resolves.

If we apply our previous logic to this situation, the original damage is not rolled during casting. As such, the potion may not apply its effect to a spell's instant damage.

This would mean that the potion can never apply its effect to a spell's damage, which defeats the point of the potion existing itself. There is a contradiction here.

With this, I conclude that the potion should enhance the delayed damage of a spell, even though that damage isn't done on the same turn as the turn when the spell is cast originally.

Answer 2

Yes, it will apply to the entire duration of the spell.

Your analysis seems correct, since the potion specifies that the next single spell you cast is maximized. This is supported by the fact that you are still casting the spell.

Unless I'm not thinking of something, there are few if any spells that deal damage over multiple rounds that don't require concentration. If you are concentrating on a spell, you are still actively casting it, so it is still a part of the same single casting. If there are any spells that don't require concentration, but still do damage over multiple rounds, I would think that it still applies the same logic since it is the same spell, but I would understand if a DM would rule differently, saying it is no longer the time of the casting.

Answer 3

I would agree with TheLittlePeace, but not just because the technicality of the wording, but additionally due to the balance of the overall damage dealt. Spells that have an extended duration tend to do slightly more damage (tend), because they are dealt over a period of time so there is opportunity to avoid it. In this case, the damage over time aspect would remain and the single casting instance would make it reasonably balanced. So for both reasons I would say yes.