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In D&D-5e, suppose my character casts moonbeam and two enemies start their turn inside the beam's radius. Would each creature's damage be calculated separately (2d10 for one and 2d10 for the other) or would their damage be calculated together with a single roll of 2d10?

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If you damage multiple creatures at once, roll once for all of them; otherwise, roll separately

The section on "Damage Rolls" states:

[...] If a spell or other effect deals damage to more than one target at the same time, roll the damage once for all of them...

The moonbeam spell certainly counts as "a spell or other effect" so all we have to know is if the spell is dealing damage to multiple targets at once.

If the moonbeam spell is damaging multiple creatures at once - perhaps multiple creatures were flung into it simultaneously or multiple creatures literally share a turn (such as through simulacrum or true polymorph) - you would roll damage once for all of them because of the rule on damage rolls.

There is one rather common time this might occur, which is with identical monsters. The section of the PHB on "Initiative" states:

[...] The DM makes one roll for an entire group of identical creatures, so each member of the group acts at the same time.

In this case the monsters perhaps actually share not just initiative, but also a single turn, and so the GM would roll damage once for all of them as they would be taking damage at the start of their collective turn. (There is debate on sharing initiative vs sharing turns and how this actually works out. Can monsters weave their actions in and out of one another or does one monster take their turn and then the next? These are discussed in questions such as: "Does a controlled mount share its rider's turn?", "Is initiative ordinal or numerical?", and "How do I handle initiative and turns for a group of monsters?", there doesn't seem to be much consensus, so ask your GM)

If creatures are taking damage at the same time, you roll damage for them together; if creature are taking damage at different times, you roll damage for each creature separately.


The moonbeam spell is confusing, here's some explanation of when it deals damage

The moonbeam spell states:

[...] 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 is rather confusing though what counts as "entering a spell's area" and the Sage Advice Compendium has a helpful question for this:

Q. Does moonbeam deal damage when you cast it? What about when its effect moves onto a creature?

A. The answer to both questions is no. Here’s some elaboration on that answer. Some spells and other game features create an area of effect that does something when a creature enters that area for the first time on a turn or when a creature starts its turn in that area. On the turn when you cast such a spell, you’re primarily setting up hurt for your foes on later turns. Moonbeam, for example, creates a beam of light that can damage a creature who enters the beam or who starts its turn in the beam. [list of spells with similar wording omitted]

Reading the description of any of those spells, you might wonder whether a creature is considered to be entering the spell’s area of effect if the area is created on the creature’s space. And if the area of effect can be moved—as the beam of moonbeam can—does moving it into a creature’s space count as the creature entering the area? Our design intent for such spells is this: a creature enters the area of effect when the creature passes into it. Creating the area of effect on the creature or moving it onto the creature doesn’t count. If the creature is still in the area at the start of its turn, it is subjected to the area’s effect.

Entering such an area of effect needn’t be voluntary, unless a spell says otherwise. You can, therefore, hurl a creature into the area with a spell like thunderwave. We consider that clever play, not an imbalance, so hurl away! Keep in mind, however, that a creature is subjected to such an area of effect only the first time it enters the area on a turn. You can’t move a creature in and out of it to damage it over and over again on the same turn.

In summary, a spell like moonbeam affects a creature when the creature passes into the spell’s area of effect and when the creature starts its turn there. You’re essentially creating a hazard on the battlefield.

As user @Peter Cooper Jr. points out there is also this question regarding moonbeam and how it deals damage: "Is the Moonbeam spell amazing, or are we doing it wrong?"

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