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I have a scenario that will likely never happen, but I am curious in how it would work.

First, the preliminary, Combining Magical Effects:

The effects of different spells add together while the durations of those spells overlap. 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, or the most recent effect applies if the castings are equally potent and their durations overlap.

Now the setup:

Bob the 3nd-level Generic Wizard and Doug the 3rd-level Earth Wizard face off. Bob knows Doug's favorite tactic and readies a spell. Doug casts Earth Tremor under Bob. The spell say, "You cause a tremor in the ground within range. Each creature other than you in that area must make a Dexterity saving throw."

But Bob was ready and casts Earth Tremor as a 2nd-level spell for his reaction making it more potent (more damage) in the same area.

So what happens?

Doug's casting means Doug is not targeted by the tremor. But as a reaction Bob casts a more powerful version where Bob is not targeted by the spell. So does that overpower Doug's spell? And if so, does that mean Bob no longer has to make a Dexterity save and Doug does?

There may be other spells that do this but Earth Tremor was the first I found with wording stating that regardless of the target point, the caster is not affected.

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  • \$\begingroup\$ We've several questions regarding most potent effect for spells. Do any of these answer your question? rpg.stackexchange.com/questions/93074/…; rpg.stackexchange.com/questions/93114/…; rpg.stackexchange.com/questions/128318/… \$\endgroup\$ – Pyrotechnical Jul 20 at 23:22
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    \$\begingroup\$ You might want to use a non-instantaneous spell as an example. May I suggest charm person? \$\endgroup\$ – Ryan C. Thompson Jul 20 at 23:33
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    \$\begingroup\$ I see suggestions that this may be covered by other questions/answers, but I feel this is different as a second spell is being cast to negate another. Two fireballs cast at the same person, at least one of those is going to work. But in this case, it would be like casting Fireball to keep the other Fireball from going off. \$\endgroup\$ – MivaScott Jul 21 at 2:17
  • \$\begingroup\$ With Earth Tremor as the spell in example, there is nothing that differs this question from the Fireball question. If you simply read it as "are they affected by both fireballs?" rather than "do they take damage from both fireballs", the question is the same. The difference being that in fireball's case the effect is necessarily taking damage. \$\endgroup\$ – HellSaint Jul 21 at 3:07
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    \$\begingroup\$ @HellSaint Wouldn't that make there be no question the questions aren't duplicates? \$\endgroup\$ – Please stop being evil Jul 28 at 22:17
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It's an interesting question but Earth Tremor has a duration of 'Instantaneous' so, in this specific example, the two spells never overlap so Bob is targeted by the first spell and then his held spell kicks in and Doug is the target.

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.

Page 190 of the PHB also says

If the reaction interrupts another creature’s turn, that creature can continue its turn right after the reaction

which helps reinforce the fact that two instantaneous effects do not occur at the exact same time. Thank to Thomas Markov for pointing this out.

If this scenario happened agin with spells that do have a duration longer than instantaneous then I believe your interpretation would work as you explained. But, as you pointed out, this is so situational as to be very unlikely to ever actually happen.

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You might get lucky and pull it off... and get yourself booted from the table.

First, the easy bits:

  • The spells need durations that overlap, so they must be spells with durations. That means that if your adversary is casting an instantaneous spell, this tactic will not work at all.
  • You'll need to cast it using a spell slot at least as high level as your adversary is, and for that you're having to guess.

More importantly:

  • It still produces the same effect.
  • It must be the same spell, on the same target.

Your adversary is already casting this spell on Carol. If Carol is your ally, it's probably a harmful spell. If Carol is your enemy (your adversary's ally), it's probably a beneficial spell. Either way, you don't want them to have whatever your adversary is giving, but this method of suppressing their spell gives them exactly that and more.

If Carol is your party member, you'd better have a really good explanation why you're using harmful spells on her, and don't expect "I'm keeping him from doing the same to you" to fly.

Seriously, even if you can, don't.

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