How do you determine the 'most potent effect' for overlapping spells?

Question

Inspired by this question: Does the Spirit Guardians spell stack with multiple casters?

Using the spirit guardians example described in the linked question, suppose the 3 clerics had the following characteristics and cast the spell as follows:

• 5th level Cleric, DC 15, cast spell with a 3rd level slot (3d8 damage)
• 7th level Cleric, DC 14, cast spell with a 4th level slot (4d8 damage)
• 5th level Cleric/3rd level Wizard, DC 13, cast spell with a 4th level slot (4d8 damage)

The PHB states that for overlapping effects:

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.

When overlapping effects occur, how does one determine which one takes effect and thus which one to make the save against? Essentially is there any official clarification for what is 'the highest bonus'?

Answer 1

The highest effect applies. If there is more than one effect, the highest effect still applies to each individual effect. The rule doesn't differentiate between occurrences or individual spells.

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.

When spells overlap, you aren't saving against one spell. You're saving against them all simultaneously with a single roll. So in this instance, the highest DC would be 15, and the highest damage would be 4d8.

The rules clearly states they don't combine, only the most potent effect applies. So you can't add them together to get a spell save DC of 20+ and damage of 11d8, but you can replace their values completely by a better one.

This means the target makes it's save against the highest DC, in this case 15, and damage is rolled at 4d8.

Effectively, multiple instances of the same spell function as one spell modified for maximum benefit until their durations individually expire. Note that this conlcusion is derived specifically from the wording of the quoted text above. At no point does it say only the most potent spell applies. It specifically states the most potent effect does. If there's multiple effects, the most potent of those must apply by this rule.

Answer 2

There are no hard and fast rules for adjudicating between different criteria for "Most Potent Spell"

As outlined in the text, this will usually be simply the higher Spell Save DC, however this does not account for a higher level spell slot from a lower caster DC doing more damage. Unfortunately, there is no system in place for deciding this, but if you had to choose one, I would go with Spell Save DC as the deciding factor, as it is specifically called out in the relevant text, and seems to most directly correlate with spell potency.

Personally, I would err in favour of the Clerics, especially if they are players, and in the areas they overlap, take both the highest DC and highest damage available. This way they still get something for overlapping, but nothing too game-breaking or extreme as allowing multiple instances of the damage.

The Clerics have spent at least an extra level 4 slot for a paltry +5 damage on average, and are both using their concentration, which will in no way make or break your game, especially when you take into account what the other Cleric(s) could be doing with their concentration and level 4 spell slots instead.

In the end, this is still a ruling you must make though, as the game material does not offer detailed instructions when multiple criteria are present.

Answer 3

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

There is no difficulty here.

Resolve all of the spells completely including saving throws and damage - the most potent applies. That is, the one that does the most damage after accounting for saving throws.

Answer 4

Didn't notice this was about Spirit Guardians, which is an area effect. I first thought it was a spell that imparted a condition.

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 ...

The DM would have to decide between the most damage (4d8), or the highest save DC (DC 15). As the Rules as written aren't clear which is considered most potent effect. I'd argue, and likely rule, higher DC is more potent.

while their durations overlap.

If the save is on a person (something that imparts an ongoing condition or condition like effect, with a save at the end of each turn), you'd have to roll all three saves to see which are still in effect, and adjudicate which is currently the most potent effect again if one wears off. This isn't the case here as the effect is on an area not a person so the above section stands as the answer to the question.

Answer 5

An example from experience: Use Math, there should always be a clear winner.

My primary D&D group is composed of mostly Math degrees (or related). As such when we have questions on whos spell to use we simply run a (generally) quick calculation to determine whos is better to use. For example...

• The important factors are the DC (chance of success) and the damage (we will use average [with some notes for min/max]).
• Using your examples we have a DC15 @ 3D8 damage, DC14 @ 4D8 damage, and DC13 @ 3D8 damage.
• Assuming no modifiers for the defensive DC roll, chance to hit for those three abilities is 70%, 65% and 60%. (note DC15 effectively means roll a 14 or lower and do damage - thus (1-14/20) meaning 70% chance to fail)
• The average damage for those three rolls is 13.5, 18, and 13.5.

Some simple calculations

DC15@3D8 = 0.70 × 13.50 = 09.45 [Min: 0, Max: 18]

DC14@4D8 = 0.65 × 18.00 = 11.70 [Min: 0, Max: 24]

DC13@3D8 = 0.60 × 13.50 = 08.10 [Min: 0, Max: 18]

In this situation, the clear winner is DC14, followed by DC15, finally DC13.

The only thing I might argue for is that "potent" likely refers to stronger in terms of game mechanics (e.g. Higher Spell Level, higher DC, higher damage). However as far as I could find this is not clarified anywhere in RAW so it is up to the DM to decide.

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One concern that has been brought up is that doing calculations on the fly could be a challenging/time consuming concern for most groups. With that in mind this can be simplified somewhat by making note of some specific constraints:

1. You have to be comparing the same type of dice (e.g. a D4 will not compare to a D12).
2. You have to adjust based on the creatures bonus (e.g. if the save is +3, then use -3 when calculating the save).

Here is the quick reference table I made to help show this.

Link for both details and simple table: https://docs.google.com/spreadsheets/d/19VOp_rGHEQ4r7aDMlbW4mTnXiDK_YyZmtjUnWuCa870/edit?usp=sharing

This table depicts the number of DC steps up you would have to make in order to make a lower/higher number of dice viable.

    # Dice                                  
DC      1   2   3   4   5   6   7   8   9
    20  20  10  7   5   4   2   1   1   0
    19  19  10  6   5   4   2   1   1   0
    18  18  9   6   5   4   2   1   1   0
    17  17  9   6   4   3   2   1   1   0
    16  16  8   5   4   3   2   1   1   0
    15  15  8   5   4   3   2   1   1   0
    14  14  7   5   4   3   2   1   1   0
    13  13  7   4   3   3   1   1   1   0
    12  12  6   4   3   2   1   1   1   0
    11  11  6   4   3   2   1   1   1   0
    10  10  5   3   3   2   1   1   1   0
    9   9   5   3   2   2   1   1   1   0
    8   8   4   3   2   2   1   1   0   0
    7   7   4   2   2   1   1   1   0   0
    6   6   3   2   2   1   1   0   0   0
    5   5   3   2   1   1   1   0   0   0
    4   4   2   1   1   1   0   0   0   0
    3   3   2   1   1   1   0   0   0   0
    2   2   1   1   1   0   0   0   0   0
    1   1   1   0   0   0   0   0   0   0