When a creature is hit with more than one fireball simultaneously, do they take damage from all of them?

Question

Say a creature walks through a door and triggers two spell glyphs from two glyph of warding spells that are programmed to cast a fireball at the creature that walks through the door. Both glyphs are triggered by the same action/trigger.

My intuition and off-the-cuff ruling would have been that the creature rolls for both fireballs and takes damage and effects as if they had been hit by two separate spells.

However, the PHB says in Chapter 10 under "Combining Magical Effects" (here in the basic rules):

The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect - such as the highest bonus - from those castings applies while their durations overlap. (PHB p. 205)

Does this imply that the creature would only effectively get hit by one fireball? Does this section even apply here to spells of instantaneous duration?

In short, does the creature take damage from one or two fireballs when hit by two simultaneously?

A more extreme example of this scenario (and the inspiration for this question) can be found in: Can I create a superbomb with Glyph of Warding and a book?

Answer 1

They take damage from both fireballs

The key for me is the word "effects" from your quote:

The effects of the same spell cast multiple times...

I would argue that the damage dealt from the fireball is not an "effect" in the context of what your quote is talking about. This spell simply deals damage and has no additional "effects".

Furthermore (as pointed out by @PJRZ), the section you quote from begins with this sentence:

The effects of different spells add together while the durations of those spells overlap. (PHB, pg. 205)

This suggests that spells that simply have an instantaneous effect do not come under this rule, this they effectively have no durations. This also confirms that the context I mention above is for ongoing effects of a spell, where considering overlapping bonuses or additional effects makes more sense.

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Instantaneous isn't a real duration, it's just a category of duration for the purposes of that property of a spell

Instantaneous means it has no duration; as for the duration of a spell being listed as instantaneous, I understand duration in this context to simply mean the name of a category, a property of the spell, but it isn't actually a "duration" unless a number is given (or "Until dispelled"); i.e. until a period of time passes to make "duration" meaningful.

From PHB, pg. 203:

Duration

A spell's duration is the length of time the spell persists. A duration can be expressed in rounds, minutes, hours or even years. Some spells specify that their effects last until the spells are dispelled or destroyed.

Instantaneous

Many spells are instantaneous. The spell harms, heals, creates or alters a creature or an object in a way that can't be dispelled, because the magic only exists for an instant.

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Further, Xanathar's Guide to Everything (pg. 77) has this to say about Simultaneous Effects (thanks to @V2Blast for suggesting this):

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.

This also suggests that both fireballs should be resolved separately, making separate saving throws and damage rolls as though they did not happen simultaneously, but rather one after the other as though they happened a fraction of time apart.

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Finally, the Essentials Kit includes some of these rules, but with a slightly different wording (thanks @Medix2). In the Essentials Kit Rulebook, on p. 3, under the section "Effects with the Same Name Don't Stack":

But when two or more effects have the same proper name, only one of them applies while the durations of the effects overlap (a duration is a time span of 1 round or more).

Durations is mentioned again, as in the PHB quote I included earlier in this answer (from p. 205), but in this case, what counts as a duration is made explicit; "a duration is a time span of 1 round or more". This also points to Instantaneous not being a real duration, further suggesting that both fireballs would deal damage.

Answer 2

Two Saves

Your intuition is correct. Although the described effects occur on the same turn and from the same trigger, they are two separate and unique effects that need to be adjudicated separately. This is most readily construed by virtue of Fireball's Instantaneous duration and there are an infinite number of Instantaneous durations that occur on any one turn.

This could be construed similarly to stepping on a pressure plate that triggers two separate traps, perhaps poison darts and a swinging blade. Although the two effects are happening because of the same trigger, they are separate and unique.

Your specific example describes instantaneous effects, but it may be helpful to your understanding to view these other questions to compare to effects that have long-term effects as they relate directly to your rules citation.

How do you determine the 'most potent effect' for overlapping spells?

Does the Spirit Guardians spell stack with multiple casters?

Answer 3

As a GM, I would look first to what makes sense, and second to the rules.

What makes sense is that if you are already on fire, a second one doesn't make all that difference. Fires don't "add up" in a linear way, otherwise you could just hold enough candles together to get fusion started.

What also makes sense is that two is more than one and the additional effort spent should be rewarded.

I would rule like this:

Roll damage and save for both fireballs, apply only the higher damage

This also solves the exploitable nature of the glyphs mentioned in other comments: No matter how many glyphs you add together, at best you still only get one maximum-damage fireball.

Answer 4

This answer will provide an alternative interpretation of the rules in contrast to this other answer.

One Save

"Instantaneous" is a valid duration

Given that it is listed under the "Duration" entry, "instantaneous" certainly is a duration, albeit one so small that it cannot be described in the usual rules framework (e.g. rounds). This also matches the usual usage of "an instant", as a very short duration.

Since both spells trigger at precisely the same instant, the fireballs overlap for exactly the same short amount of time.

Damage is a valid effect of a spell

The term "effect" is, as far as I know, not defined in the rules. However, area spells (such as fireball) have an Area of Effect, which would be pointless, if they didn't produce any effects.

In common language, an explosion (and the damage it inflicts) should certainly also count as "an effect" of some kind.

Summed up: The two fireballs produce a certain effect (fire damage) during precisely the same duration (however small), and thus they overlap and only the strongest effect (highest damage) applies.

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Personal ruling

I would never allow glyphs of warding to stack like that, simply because it is so easily abusable: Why would any self-respecting BBEG and dungeon-architect ever spend 10.000gp on fiendish traps, when they could simply cover the entrance with 50 Glyphs of Warding instead for the same price, guaranteeing instant death for anyone foolish enough to break in? They would still have to be found and disarmed individually, providing an almost certain failure for any greedy rogues, and even with evasion, they would not make all 50 of those saves.

This does not even consider other uses, like being able to instantly heal and buff themselves while in their inner sanctum, without even taking an action to do so.