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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?

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    \$\begingroup\$ Related: rpg.stackexchange.com/questions/128273/… \$\endgroup\$ – Pyrotechnical Jul 27 '18 at 13:46
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    \$\begingroup\$ Related: Can any actions happen at the same time/instant? (your example of using glyph of warding makes it very different in my opinion since this one is about readied actions) \$\endgroup\$ – Sdjz Jul 27 '18 at 14:29
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    \$\begingroup\$ @Tom Please do not answer in the comments. They are only for seeking clarficiations or improvements to the question. Please do put your answer in an actual answer below if you want to keep it as such comments will get deleted. See here for more information. Thanks! \$\endgroup\$ – Rubiksmoose Jul 29 '18 at 17:29
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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.


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.


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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    \$\begingroup\$ Instantaneous is a duration of "existing or measured in a particular instant", and an instant is the most precise moment of time. Given there are infinite precise moments between 10:18:56 AM and 10:18:57 AM, and that you can always get more specific with your measurement of time, I would say it is perfectly reasonable to say "instantaneous" cannot exist with another "instantaneous" \$\endgroup\$ – Blake Steel Jul 27 '18 at 14:21
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    \$\begingroup\$ You might also want to reference the Xanathar's guidelines on "Simultaneous Effects" on p. 77: "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." \$\endgroup\$ – V2Blast Jul 27 '18 at 18:18
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    \$\begingroup\$ The rules explicilty spell out damage as an example of an "effect." In the DMG p. 252 post errata, "But when two or more game features have the same name, only the effects of one of them —the most potent one—apply while the durations of the effects overlap. For example, if a target is ignited by a fire elemental’s Fire Form trait, the ongoing fire damage doesn’t increase if the burning target is subjected to that trait again." \$\endgroup\$ – Gandalfmeansme May 17 at 21:34
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    \$\begingroup\$ @Gandalfmeansme notably that is ongoing damage though which makes it a bit different I think. \$\endgroup\$ – Rubiksmoose May 18 at 2:01
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    \$\begingroup\$ @Rubiksmoose Absolutely. But the first point this answer makes is "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'." I definitely agree that the "duration" part is a good point: it's the first point I'm hoping gets revised. \$\endgroup\$ – Gandalfmeansme May 18 at 3:02
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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?

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  • \$\begingroup\$ The durations of those two Bless spells would overlap, thus they are occurring in the same period of time. That is not the case with the fireballs. \$\endgroup\$ – Pyrotechnical Jul 27 '18 at 14:15
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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.

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  • \$\begingroup\$ As an aside, it actually doesn't solve the exploit since you can just add a time delay into triggers so that the fireballs go off one after another. Eg "After creature walks through door and 6 seconds has passed...." or something like that. \$\endgroup\$ – Rubiksmoose Jul 30 '18 at 17:05
  • \$\begingroup\$ @Rubiksmoose - but this gives the victim time to react and do something (get out of the way, or whatever) \$\endgroup\$ – Tom Jul 30 '18 at 17:56
  • \$\begingroup\$ As long as they are still within targeting range it really wouldn't matter what they did. It might even be more surprising the second time. Regardless, all I'm saying is that there are certainly still ways to exploit stacking glyphs even with your ruling. \$\endgroup\$ – Rubiksmoose Jul 30 '18 at 18:02
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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.


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.

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    \$\begingroup\$ Why? Because if that were the case, the entrance will simply be bypassed. \$\endgroup\$ – March Ho Jul 28 '18 at 20:19
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    \$\begingroup\$ If you put 50 glyphs and a few of them are detected, the PC's will set it off with a Summon Monster, a pet, an NPC, a spare body lying around, or myriad other techniques. There are other problems with the assumption that this is the best approach, such as simply teleporting through that portion of the area and bypassing everything, finding a different entrance, etc. \$\endgroup\$ – blurry Jul 29 '18 at 17:14
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    \$\begingroup\$ While true, that is also true of most other, considerably more expensive and less lethal traps. Also consider that the trigger can be "When someone says CorrectHorseBatteryStaple", providing the BBEG (or even the PCs, if they have time to fortify a position) with 100d8 fire damage that only takes a free action to activate. \$\endgroup\$ – Surpriser Jul 29 '18 at 17:23

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