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If two moonbeam spells are placed on different parts of a size Large (or larger creature), does it take damage from both beams on its turn?

Or is this subject to the rule on combining magical effects that "the effects of two of the same spell do not combine, the most potent effect applies while their durations overlap"?

For reference the text says:

A silvery beam of pale light shines down in a 5-foot-radius, 40-foot-high cylinder centered on a point within range. Until the spell ends, dim light fills the cylinder.

When a creature enters the spell’s area for the first time on a turn or starts its turn there, it is engulfed in ghostly flames that cause searing pain, and it must make a Constitution saving throw. It takes 2d10 radiant damage on a failed save, or half as much damage on a successful one.

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The creature takes damage from both spells.

Moonbeam is cast on an area, not a creature. It then causes damage to creatures within that area. The effect of said damage is instantaneous, and so the rules for overlapping spells doesn't apply.

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – V2Blast Jan 28 at 9:15
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    \$\begingroup\$ I believe this is the correct interpretation, though only in the case where the two moonbeams are on different spaces. A space where the moonbeam overlaps with another moonbeam would only trigger once. \$\endgroup\$ – Ahndwoo Jan 28 at 12:52
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This is combining the same spell multiple times.

The rule you're referencing is found in chapter 10 of the PHB, Spellcasting, in the subsection Combining Magical Effects. [emphasis mine]

The effects of different spells add together while the durations of those spells overlap. 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, or the most recent effect applies if the castings are equally potent and their durations overlap.

For example, if two clerics cast bless on the same target, that character gains the spell's benefit only once; he or she doesn't get to roll two bonus dice.

Or in our case, a creature under the effect of moonbeam would be affected by the more powerful of the two.

The relevant portion of moonbeam says this:

When a creature enters the spell's area for the first time on a turn or starts its turn there, it is engulfed in ghostly flames that cause searing pain, and it must make a Constitution saving throw. It takes 2d10 radiant damage on a failed save, or half as much damage on a successful one.

Clearly, the creature itself is the target of the spell, and not some portion of the creature. Moonbeam targets a location to set the spell's area of effect, as well as each creature it harms.

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – V2Blast Jan 28 at 9:25
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    \$\begingroup\$ Does this mean a (medium) creature walking through four Moonbeam spells on its turn only takes damage from the first one? Making this comparison would help make this answer more clear (and if the answer is "no", explaining why it's different from the question's situation would be very helpful). \$\endgroup\$ – MartianInvader Jan 28 at 17:28
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    \$\begingroup\$ @MartianInvader from my perspective they're different because they're not happening at the same time. A large creature is starting its turn in 2 moonbeams falls under Combining Magical Effects (They're simultaneous) while walking through 4 of them is just consecutive effects \$\endgroup\$ – Glenn Driver Jan 28 at 19:59
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    \$\begingroup\$ This interpretation leads to weirdness: if they only partially overlap, entering through a non-overlap and moving into an overlap leads to more damage (hit by both) than entering in the overlap and moving to non-overlap (hit by only one) \$\endgroup\$ – TemporalWolf Jan 29 at 20:17
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The creature takes damage from both

Everybody is quoting the DMG on spell effects, but that is completely irrelevant here because nobody is trying to overlap moonbeams over each-other.

If the question was 'can I cast two Moonbeams ontop of each-other and have a creature take damage twice?', the answer is, indeed, no, because you can't overlap spell effects, so that 'tile' will only be affected by the strongest of the two Moonbeams.

However, we're not overlapping Moonbeams, we have two separate instances of Moonbeam side-by-side, and by virtue of being big, the creature happens to be in both of them at the same time.

Ask yourself this:

If a medium-sized creature walked through both Moonbeams to pick up items placed in the center of each, would they take damage once or twice? I think everybody would agree that they'd take damage twice, because they passed through two separate moonbeams.

The large creature has the exact same problem, except he's going through both of them at the same time because of his size.

The moment you start ruling that stepping into two moonbeams at the same time counts as 'the same effect', you're giving large creatures the possibility of cheesing out less damage by simply positioning themselves advantageously.

If I stand here, I'm inside two moonbeams at the same time, I'll only take damage once, and I can then still decide which way to go without having to take the damage again, unlike my smaller friends who will have to decide which Moonbeam they want to stand in, and they might take damage from the other Moonbeam as well if they decide they need to go the other way after all.

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    \$\begingroup\$ The rule about combining magical effects doesn't care if the spells overlap in space, only in time. \$\endgroup\$ – Ryan C. Thompson Jan 27 at 14:59
  • \$\begingroup\$ This answer is predicated on logic, and while you can apply logic to some D&D rules, you can't apply it to all of them \$\endgroup\$ – Glenn Driver Jan 27 at 14:59
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    \$\begingroup\$ @illustro Literally the first sentence: "The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however." \$\endgroup\$ – Ryan C. Thompson Jan 27 at 15:28
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    \$\begingroup\$ I don't quite understand your argument that the larger creature gets benefits others don't. The inverse is the case should there be any gap between two moonbeam spells, where only smaller creature can get through unscathed \$\endgroup\$ – Medix2 Jan 27 at 16:01
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    \$\begingroup\$ @Medix2 You've misunderstood that Theik is saying. This answer is considering whether the 2 adjacent non-overlapping moonbeam areas are effectively one big area for a large creature, such that it could walk out of one area and into another and avoid the effect of entering the 2nd area. (For the record, I'm not sure where I come down on this specific issue, but I maintain that in general, whether spells overlap in space is irrelevant to whether their effects stack.) \$\endgroup\$ – Ryan C. Thompson Jan 27 at 16:44
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Size doesn't matter; you cannot stack the same spell on the same target within its duration.

Let's compare PHB ("Combining Magical Effects"):

The effects of different spells add together while the durations of those spells overlap. 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, or the most recent effect applies if the castings are equally potent and their durations overlap.

to the DMG ("Combining Game Effects"):

Different game features can affect a target at the same time. 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.

The PHB version is fairly clear, but the DMG Version is extremely clear and even gives an example that works in this case: ongoing damage doesn't increase if the effect is used twice. The descriptions don't say the spells need to affect the same area, but only that they affect the same target. Nothing here prevents two wizards from casting magic missile (as an example) on their turns and having them both deal damage.

Let's consider the implications of this. Could you cast 4 Incendiary Clouds at 4 corners of an Ancient Dragon and force 4 saves and multiple damages? No, you cannot. The designers have considered this and prevented it via the above rule.

Two castings of the same spell within their durations will not multiply damage, saving throws, or anything else.

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  • \$\begingroup\$ Your last statement doesn't seem to work when applied to other situations though. An example I've brought up before is Melf's Minute Meteors. That is a spell, with a duration, with certain conditions that cause damage to be dealt within that duration. By your ruling, if two wizards cast MMM at the same target, only the first would deal damage. Or, if the second was up-casting the spell, then only the second would deal damage, which creates the strange situation of un-doing the damage done by the first caster before the second one does damage. \$\endgroup\$ – AgentPaper Jan 29 at 0:57
  • \$\begingroup\$ @AgenrPaper. The attack of Melf's doesn't have a duration, just the meteors themselves. I'm sure we can find exceptions, but it's clear the designers do not intend for people to overlap spells on a target to give multiple damages (cloudkill, fire wall, etc.) \$\endgroup\$ – Tiger Guy Jan 29 at 2:25
  • \$\begingroup\$ The attack of Melf's is the meteors though. Once per turn, under certain circumstances, creatures might need to make a saving throw in a certain area based on whether they make a saving throw or not. The only real difference is how the area is determined and the flavor of the spell. \$\endgroup\$ – AgentPaper Jan 29 at 4:33
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Spatial overlap is irrelevant for combining effects

In the DMG section on combining game effects1:

Different game features can affect a target at the same time. 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. Game features include spells, class features, feats, racial traits, monster abilities, and magic items.

This section says nothing about whether or not 2 effects are spatially overlapped. The rule applies regardless of whether two moonbeam spells cover overlapping areas or not.

Creature size is irrelevant for combining effects

If a creature is larger than medium, its space includes multiple 5-foot squares. This creates the opportunity for a single creature to be simultaneously subjected to multiple non-overlapping areas of effect, which is a bit non-intuitive. However, nothing about the rules really changes in this situation. If a creature is subjected to two effects, then those effects combine according to the above rule, as normal, even if their areas don't overlap. In particular, if the two effects are ongoing and have the same name, the creature only suffers the effects of one of them.

A creature can only take damage from moonbeam once per turn

Instead of 2 non-overlapping moonbeam spells, let's set up a line of 10 moonbeams. Then let's cast haste on a giant eagle and send it flying through the entire line. How much damage does it take? Just 2d10 damage, halved on successful save. This is because the damage from moonbeam can only trigger once per turn (emphasis added):

When a creature enters the spell’s area for the first time on a turn or starts its turn there, it is engulfed in ghostly flames that cause searing pain, and it must make a Constitution saving throw. It takes 2d10 radiant damage on a failed save, or half as much damage on a successful one.

It doesn't matter that our eagle is entering multiple moonbeam spells. The eagle takes the damage the first time it enters any moonbeam spell on its turn, and no effects occur on any subsequent entries into any moonbeam spell for the remainder of that turn, because all of these spells' damage-dealing effects are ongoing effects from spells of the same name with overlapping durations, so they don't combine. Note that this is a logical outcome, because the exact same thing would happen if the eagle flew back and forth through a single moonbeam 10 times. Also note that the result is the same whether or not any of the 10 spells have overlapping areas.

Yes, there are weird edge cases, but that's OK

Instead of having 10 moonbeam spells going at once, let's have 10 druids each ready a moonbeam spell to release right in front of the eagle's flight path. Each one releases the spell right before the eagle reaches their targeted point, then drops their concentration as soon as the eagle takes damage from their spell, before the next druid releases their spell a split second later. Now, none of the spells' durations overlap, which means the above rule about combining effects with the same name technically no longer applies, and the eagle suffers the effects of entering a moonbeam spell 10 times in a single turn. Whoops.

Of course, it is utterly illogical and rather unsatisfying that this situation should produce a different result than an eagle flying through 10 ongoing moonbeams. But I'd argue that this isn't a problem in practice, because this illogical situation is never going to occur in actual play. It's probably never going to occur even with just 2 moonbeam spells. This would require 2 druids on the battlefield, one with an active moonbeam spell and one with a readied moonbeam spell, and then a creature would, in the span of one turn, need to run through one moonbeam spell, break the druid's concentration on that spell, trigger the other druid's readied action, and then intentionally enter the 2nd moonbeam spell's area.

Remember this important quote from the DMG's introduction:

The rules don’t account for every possible situation that might arise during a typical D&D session.

If such an exceptional case as the one described above actually arises in your game, the rules expect the DM to make an appropriate ruling. In this case, it would be reasonable to rule that the eagle only suffers the effects of the first moonbeam spell, just like in the prior example of 10 ongoing moonbeams.

Another edge case occurs when the moonbeam spells are each cast at different levels and/or by casters with different spell save DCs. Once again, the rules don't explain what to do in this edge case, and it's the DM's job to fill in the blanks. If the first moonbeam spell has a DC of 17 and deals 2d10 damage while the 2nd one has a DC of 15 and deals 3d10 damage, what happens when the eagle flies through both? I dunno. Ask your DM.


1Thanks to user Scott Dunnington for pointing out that the DMG's version of the rule for combining effects specifically calls out ongoing damage effects as affected by the rule, even though each individual instance of the damage is instantaneous.

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  • \$\begingroup\$ You are welcome. Your explanations in comments led to me refining my answer. \$\endgroup\$ – Tiger Guy Jan 27 at 18:32
  • \$\begingroup\$ Let us continue this discussion in chat. \$\endgroup\$ – Ryan C. Thompson Jan 27 at 19:39
  • \$\begingroup\$ Moonbeam is a damage over time so that is why the number of casts isn't important, however the highest level spell should still apply. I wonder why the spells don't have any wording to reflect that they are DOTs and can only take effect once per turn? \$\endgroup\$ – Sean Culligan Jan 28 at 1:05
  • \$\begingroup\$ What's the difference between standing in a single Cloudkill spell area for a single round as oppossed to traversing two or three cloudkill spell areas in a single round? Its effectivrly just a bigger poison cloud. Moonbeam is no different. The damage is from "ghostly flames" and is effectively a damage over time effect for as long as you stand in it. It makes no difference how many you cross, you're in a moonbeam. \$\endgroup\$ – Sean Culligan Jan 30 at 5:20
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The creature takes damage from both effects

(based on an optional rule)

However the area is only lit by dim light by one of the beams. This is because moon beam has two parts:

Until the spell ends, dim light fills the cylinder.

This is a continuous effect and the duration of that effect overlaps for both moonbeams, therefore the area is only lit by dim light once.

The second part of moonbeam says:

When a creature enters the spell’s area for the first time on a turn or starts its turn there, it is engulfed in ghostly flames that cause searing pain, and it must make a Constitution saving throw. It takes 2d10 radiant damage on a failed save, or half as much damage on a successful one.

This is an instantaneous effect. This effect triggers for both moonbeams at the start of the turn of the creature or when it enters the area. Now the question is, do they get resolved at the same time or at different times? If they would be resolved at the same time, the durations would match and only one would apply.

To resolve this question, we turn to Xanathars Guide to Everything. Xanathars guide on page 77 states (this is an optional rule, as most rules in Xanathars are mere suggestions):

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.

If the two effects happen in some order, that means they happen after one and another. Then the duration clearly no longer overlaps, so both spells do damage to the creature.

(This has also been referenced by this highly relevant answer: https://rpg.stackexchange.com/a/128320/51849)

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  • \$\begingroup\$ duration definitely overlaps, since the spell has a duration. \$\endgroup\$ – Tiger Guy Jan 27 at 16:42
  • \$\begingroup\$ I think if you're going to use the optional rule from Xanathar's instead of the non-optional rule from the basic rules, the headline of your answer should read something like "Just one, unless you're using the optional rule from Xanathar's." Or, at least "Both, according to the optional rule from Xanathar's." \$\endgroup\$ – Rykara Jan 27 at 16:45
  • \$\begingroup\$ @Rykara I wouldn't go as far as to tell that it is just one without the optional rule. I believe it would still be both, however the optional rule just makes it more clearly arguable \$\endgroup\$ – findusl Jan 27 at 16:48
  • \$\begingroup\$ Moonbeam isn't an instant spell, it has a duration. The order would be "you start taking fire damage from moonbeam" followed by "you start taking fire damage from moonbeam". The only relevance is which of the two moonbeams is more powerful. \$\endgroup\$ – Sean Culligan Jan 28 at 5:06
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    \$\begingroup\$ @SeanCulligan there is no "you start taking fire damage" in the spell. It is "you take damage". You act as if you think you have the answer already even though you asked a question about it? \$\endgroup\$ – findusl Jan 28 at 8:17

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