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Unlike most spells, prismatic spray (and prismatic wall) can do a number of different damage types depending on dice rolls. But how many different damage rolls would you actually make? Do you just roll 10d6 once, and apply that same damage to all damaging rays? Or do you roll 10d6 separately for each damaging ray that hits?

If the latter, how does the damage roll or rolls interact with the sorcerer's Empowered Spell metamagic option or the College of Lore bard's Cutting Words feature (when used to reduce damage)?

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  • \$\begingroup\$ What do you mean by "what about metamagic like Empowered Spell or the Cutting Words ability to reduce damage?"? What are you actually asking in that sentence? \$\endgroup\$ – V2Blast Dec 22 '18 at 2:25
  • \$\begingroup\$ Empowered metamagic allows you to reroll a number of dice up to your charisma modifier. But if you roll for every single ray that is not many rerolls for the number of total dice - especially if you roll damage before rolling to see what hits. Cutting words only works on a single damage roll either any if you made your ray damage rolls before rolling to see what hits who then that ability could be next to useless for those spells. The prismatic spells seem designed to make both of those abilities less useful depending on how you handle all the rolls. \$\endgroup\$ – Allan Mills Dec 22 '18 at 4:25
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You should roll damage once for each beam.

Chapter 9: Combat in the Player's Handbook (and the Combat section of the Basic Rules) has the following rule under the heading "Damage Rolls" (emphasis from original source):

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.

It uses the examples of fireball and flame strike, which are simple since they produce a single effect which hits multiple targets for the same amount and type of damage. In other cases like eldritch blast, magic missile or scorching ray which produce more than one ray, beam or projectile, each one deals the same amount and type of damage. In those cases each ray, beam or projectile only hits one target, even if more than one of them hit the same target.

Prismatic spray is an unusual case, since each beam produced can potentially hit multiple targets, and each beam does not do the same amount and type of damage - only the same amount (for those beams which have a damaging effect). However the multiple targets hit by a beam would take damage "at the same time" for the purposes of the rules (i.e. in the same instant/action the beams are cast), and the same amount and type of damage too, so it seems reasonable to roll damage once for each beam - i.e. each target hit by the same beam uses the beam's single damage roll. (For ease at the table, I'd have all the affected targets roll to see which beam hits them before rolling any damage.)

Prismatic wall is more straightforward - each layer of the wall is a separate effect, so the damage is rolled separately for each layer. This happens on each creature's turn, when they try to pass through the wall, so the damage would be rolled separately for each creature affected.

To answer your last question: both the sorcerer's Empowered Spell metamagic option and Lore bard's Cutting Words feature work as normal, though the former is a little ambiguous. Empowered Spell says:

When you roll damage for a spell, you can spend 1 sorcery point to reroll a number of the damage dice up to your Charisma modifier (minimum one).

This is the exception to the normal rule that you have to spend the points and use a metamagic option when you cast a spell, but it doesn't address whether it applies to a single or multiple damage rolls. I would say you could split the re-rolls across multiple beams, but it's not going to have a big impact on more than one, since at most you could re-roll five of the dice, and so it's true that it won't be as effective for prismatic spray as for a spell with a single damaging effect.

Cutting Words isn't going to make a big difference to the damage taken from prismatic spray unless the caster rolls really badly, but that's true of its effect on the damage rolls of most high-level spells. That fits the flavour of the feature - it makes sense a bard's distracting words would reduce the damage from a sword thrust better than they would a magical explosion.

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  • \$\begingroup\$ @Medix2 hmmm...yes. Not so clear as I thought, perhaps. Though I note that ruling becomes very confusing when concentrating fire or splitting damage unevenly; and the spell does say “each dart deals 1d4+1 force damage”... \$\endgroup\$ – Guybrush McKenzie Dec 24 '18 at 4:56
  • \$\begingroup\$ @McKenzie fireball also says "Each creature takes 8d6 fire damage", yet each takes the same amount, not different ones. \$\endgroup\$ – Medix2 Dec 24 '18 at 6:08
  • \$\begingroup\$ @Medix2 ah - but that’s “each target”, and the rule for rolling damage covers multiple targets taking the same damage from a single effect, which fireball clearly is. \$\endgroup\$ – Guybrush McKenzie Dec 24 '18 at 7:23
  • \$\begingroup\$ @McKenzie "If a spell or other effect deals damage to more than one target at the same time" this (and magic missile) is indeed a single spell hitting more than one target at the same time. \$\endgroup\$ – Medix2 Dec 24 '18 at 14:23
  • \$\begingroup\$ @Medix2 okay. I’ll change the examples in my answer and clarify my position some more. Though it sounds like you disagree with my answer, perhaps you should post your own with your different take? \$\endgroup\$ – Guybrush McKenzie Dec 24 '18 at 21:35

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