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After researching into how spell effects stack, I find some ambiguity regarding certain spells that have multiple possible effects.

Notably, this answer regarding stacking spell effects contains updated information from the DMG errata:

Combining Game Effects (p. 252). This is a new subsection at the end of the “Combat” section:

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. See the related rule in the ‘Combining Magical Effects’ section of chapter 10 in the Player’s Handbook.

There are, however, no references in deciding how to determine what "the most potent one" may be when not using raw numbers (such as with paladin auras).

This is also applicable to spells like contagion, which inflicts a "natural disease" (and you can be afflicted by multiple natural diseases).

If both effects of blindness/deafness cannot influence a character at the same time, how do you determine which one takes effect (assuming both casts are at the same spell level)?

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Effects do not stack

Your citation of the relevant rules is spot on. The effects will not stack.

Most recent replaces previous

Relevant Sage Advice interview with Jeremy Crawford (~25:30) regarding person under the effect of a spell (true polymorph) and that spell being superseded by a subsequent casting of the same spell.

"The next one on the stack replaces the previous one on the stack unless the previous one ... is more powerful."

As pointed out by Rubiksmoose, the most recent errata for the Player's Handbook clarifies:

Combining Magical Effects (p. 205). In the first paragraph, the following sentence has been added to the first paragraph: “Or the most recent effect applies if the castings are equally potent and their durations overlap.”

The "most potent"

Lacking a more specific game term, go with the vernacular meaning of potent:

Having great power, influence, or effect.

More potent would be having greater power or effect. A higher level spell, a higher spell casting DC, or sorcery boosted effect would add to the potency of one spell over another.

Simultaneous Effects

In the event two effects happen at the same time on a target's turn, the guidance in Xanthar's Guide to Everything is the player or DM in charge of the target chooses the order of the effects.

For example, two casters held blindness/deafness on the trigger than when a fighter moves forward they cast both it on the fighter. In this case, the fighter is effected simultaneously on the fighter's turn. The person at the table controlling the fighter chooses which spell effects them first.

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  • \$\begingroup\$ I don't feel this addresses the question. The quote states not that the "equal or lesser potency" won't take effect, but that only the more potent version of the two will, indicating that timing has no preference on which one takes place. The second bullet indicates how "potency" might be determined (but no reference to how that applies to Blindness/Deafness"), and the third bullet refers to simultaneous effects, which is also dependent on timing (which isn't relevant to which one is applied). A DM could decide that timing matters, but that isn't expressed in your answer. \$\endgroup\$ – Daniel Zastoupil Nov 15 '18 at 18:45
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    \$\begingroup\$ @DanielZastoupil Finally found the answer in a SA interview regarding polymorph \$\endgroup\$ – GcL Nov 15 '18 at 18:57
  • \$\begingroup\$ That'll do it! Nice work! \$\endgroup\$ – Daniel Zastoupil Nov 15 '18 at 18:59
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    \$\begingroup\$ BTW you SA quote is now official in the most recent PHB errata \$\endgroup\$ – Rubiksmoose Nov 16 '18 at 20:39

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