When using spells that last several rounds (such as Create Bonfire or Flaming Sphere), should I roll the dice to determine damage just the first time (and keep doing the same damage every time someone fails the saving throw)? Or should I roll the dice again every time someone takes damage from the spell?
I thought I was supposed to roll every time, but another player said otherwise and the DM agreed with him.
As far as I can tell, there's no passage in the Player's Handbook that explicitly specifies that damage rolls for ongoing effects that trigger damage multiple times are, or are not, re-rolled for each instance of the damage. The closest it gets is when it specifies that for Area-of-Effect spells, or spells that otherwise deal damage to multiple creatures simultaneously, only a single damage roll is used for all creatures hit in a single instant (Player's Handbook, pg 196).
I think there are a few good reasons to support this ruling.
The biggest reason, in my opinion, is that "roll once" tends to turn the damage rolls into a kind of Goblin Dice Problem, where the fate of an entire battle can hinge on a single roll. If the initial damage is high, the spell wipes the entire enemy combatants (or the party!) in only a few rounds with little-to-no recourse. If the initial damage is low, then you've effectively wasted a spell slot (or whatever resource is fuelling the effect). That might be mitigated somewhat with spellcasters who [probably] have to concentrate on their ongoing damage effects (which makes it a non-guarantee that they'll maintain the same spell for the full duration anyways), but they often have ways to protect their concentration, and it doesn't solve the problem.
Another reason I think it's probably not good to use the same roll over and over is that most damage sources in the game are always rolled at the moment they deal the damage. It's inconsistent to have one (or a few) type of effect that instead rolls once and is applied multiple times, and having these exceptions carved out tends to result in players being unsure what they're supposed to do when asked how much damage a creature takes. It's far more acceptable if the spell itself is written that way, or if a Class Feature is overriding these rules.
The last reason I'll offer to support re-rolling the dice for each instance of damage is that it is more useful for role-playing/storytelling purposes. If, for example, the damage from a spell like Heat Metal happens to be high on a given turn, the DM could narrate how the monster tries to rip their armor off and scalds their hand in the process. If the damage is low, maybe there was a convenient bucket of water that the monster used to temporarily reduce the pain. I think this is probably more satisfying than having to narrate how the spell was botched and won't deal much damage.
The first paragraph of the section on damage rolls establishes the general rule for damage involving a dice roll:
Each weapon, spell, and harmful monster ability specifies the damage it deals. You roll the damage die or dice, add any modifiers, and apply the damage to your target. Magic weapons, special abilities, and other factors can grant a bonus to damage.
So unless otherwise noted, whenever you deal an amount of damage determined by a roll of the dice, you roll those dice to determine the amount of damage dealt. One notable exception is the rule for simultaneous damage, given a few paragraphs later:
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.
Obviously, this exception doesn't apply to damage dealt across multiple rounds. I'm not aware of any other major exceptions to the general rule, so unless something specifically says otherwise, always roll the damage dice each time damage is dealt.
Note that this interpretation has the desirable property that you never need to remember a damage roll once it has been resolved. Combat is already complicated enough without having to remember a damage roll from 2 turns ago. Once the damage is dealt and you have updated everyone's hit points, you can forget what the roll was and make room in your brain for keeping track of what's happening now.
"Until the spell ends, you can use a bonus action on each of your subsequent turns to cause this damage again."
I'm pretty sure "this damage" doesn't refer to "the last amount of damage you rolled" but instead refers to the "2d8 fire" whereas you would still roll again to determine the amount of damage, simply because it's still going to conform to the rule that states whenever damage is done, you roll.
The exact wording of the spell will tell you when it does damage and if it is the same damage or needs to be rerolled.
For example:
That said, most spells do call for rerolls and it wouldn’t make a big difference if you played all spells that way. I would be cautious about going the other way since refilling reduces the damage variance and lower variance benefits the PCs over the monsters (since the PCs use and are exposed to many more damaging spells than any given monster).
