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10th-level Warlocks with a Great Old One patron gain the Thought Shield feature (PHB, p. 110):

You also have resistance to psychic damage, and whenever a creature deals psychic damage to you, that creature takes the same amount of damage that you do.

In this question, it is said that Thought Shield deals psychic damage back to the damager, who benefits from resistance to psychic damage if they have it.

Let's imagine Alice the Warlock, who has the Thought Shield feature from being a 10th-level Warlock with a Great Old One patron, casts synaptic static on Bob, who is also a 10th-level Warlock with a Great Old One patron.

Which of the following interpretations is correct?

  1. Does the damage keep going back and forth between them, and it is cut in half with each reflection? (Mathematically, I believe Bob would take 2/3 of the damage rolled, and Alice 1/3, in that scenario.)
  2. Or does the recursion stop after the first reflection?
  3. Or is the premise incorrect, and e.g. both characters take 1/2 the original damage roll? (I admit that would have been my ruling on a first reading -- Alice taking "the same damage" that Bob does.)
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    \$\begingroup\$ The question already contains a self-answer in the parenthetical at the end, so it seems like this isn't so much asking "what's the outcome?" as it is "is this really the outcome?". And if it is, it would help to know what the uncertainty is that makes this a question in the first place (what you think might happen if your self-answer was wrong). \$\endgroup\$ – sevenbrokenbricks May 27 at 21:08
  • \$\begingroup\$ @sevenbrokenbricks that is an excellent point, and I have updated the question with different interpretations (including the interpretation I would have expected before reading the linked question). \$\endgroup\$ – Louis Wasserman May 27 at 21:47
  • \$\begingroup\$ A note on the mathematics - if it bounced infinitely, halving each time, and you ignored rounding issues, then Bob would take 2/3 of the rolled damage, and Alice would take 1/3. It is a geometric series, and the damage is quartered each iteration (halved by each character). Ultimately irrelevant since this isn't how the rules work, however. \$\endgroup\$ – Sirv May 28 at 4:14
  • \$\begingroup\$ @Sirv yep, I did the wrong geometric series (1/8 instead of 1/4). \$\endgroup\$ – Louis Wasserman May 28 at 15:02
  • \$\begingroup\$ @Echox: All damage has a type. In the absence of explicit wording otherwise, any shared damage would be of the same type as the original. Also: don't answer in comments. \$\endgroup\$ – V2Blast May 29 at 9:35
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Alice and Bob each are affected once by the Thought Shield feature.

The Thought Shield feature is a game effect with a permanent duration (until the character/creature is dead) that grants the following effects as part of its permanent duration:

  1. Your thoughts can’t be read by telepathy or other means unless you allow it.
  2. Resistance to psychic damage.
  3. Whenever a creature deals psychic damage to you, that creature takes the same amount of damage that you do.

After the first bounce, the Thought Shield feature's same effect creates an overlap. The most potent instance of the effect will be applied. Apply the first instance of damage as it is the highest by default. Only one effect can be applied at any time, regardless of the number of ways that try to apply the effect.

Alice and Bob are affected once by the Thought Shield feature. Bob takes half of the damage of synaptic static, which is reflected towards Alice who takes half of the reflected damage, which then is reflected back to Bob who takes half of that damage. There are no futher bounces because the Combining Game Effects rules take place.

Consider the rules as described in Round Down (PHB 7) when you calculate your fractions.

DMG errata version 2.0 page 1:

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.

The basics for Damage Resistance and Vulnerability are found in the PHB (197).


Alternatively, it loops infinitely, even when the damage is reduced to 0 by the rules of resistance and rounding down.

The duration overlap here is the trigger. A different Thought Shield feature could damage either one and trigger a recursion. The effect overlap is the triggering of the recursion that is based on the duration of the feature's effects.

This game feature is not an instantaneous spell like a fireball. A fireball hits instantaneously, while the Thought Shield feature has a permanent duration. If you think that it is an instantaneous effect separate from its feature, then the infinite loop happens.

If the DM rules that the damage caused by the Thought Shield feature is not subject to feature but a new instance of an instantaneous effect of damage separate from its feature instead of an effect with a duration that grants you telepathy protection and resistance and also happens to causes damage, then the recursion loops even after the resistances have reduced the damage to 0.

Sage Advice Compendium V.2.3 (11):

Can damage be reduced to 0 by resistance or another form of damage reduction? There is no damage minimum in the rules, so it is possible to deal 0 damage with an attack, a spell, or another effect.

So if you follow this alternative ruling-way, Alice and Bob will forever be affected by Bob's foolish cast of synaptic static. Albeit the initial loops quickly reduce the looped damage to 0. Only death will break the loop because the corpse of the former creature has no Thought Shield feature.

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  • \$\begingroup\$ Once the damage diminishes to 0, in what sense does the loop continue? \$\endgroup\$ – Mark Wells May 29 at 14:07
  • \$\begingroup\$ I'm also not convinced that the "Combining Game Effects" rules apply here, since Alice and Bob are separate people. Alice's Thought Shield and Bob's Thought Shield are both affecting Alice, but one of them is affecting her by dealing damage to her, and the other one is affecting her by reflecting the damage back. \$\endgroup\$ – Mark Wells May 29 at 14:11
  • \$\begingroup\$ @MarkWells I have provided you with an alternative ruling, if you do not believe in permanent duration of features and separation of effects, that is fine - it is common. The loop is infinite because the rules say so RAW if you want to say that 0 damage is no effect at all feel free to do that at your table. \$\endgroup\$ – Akixkisu May 29 at 21:17
  • \$\begingroup\$ @MarkWells if you have any constructive feedback, then that is welcome. I do however not see how your comment does that - as it points out things that are adressed in my answer. If you want to know whether it has any consequence that there is a 0 damage loop, then ask a question. \$\endgroup\$ – Akixkisu May 29 at 21:37
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    \$\begingroup\$ @StarHawk why would it not? If you got an actual argument with solid back-up citation and reasoning, then I'm all ears. See the SA Compendium rules which you would have to argue against. \$\endgroup\$ – Akixkisu Jul 3 at 16:44
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Thought Shield (PHB p.110):

You also have resistance to psychic damage, and whenever a creature deals psychic damage to you, that creature takes the same amount of damage that you do.

So the rules as written will lead to this series of events, as an example:

  1. Alicio the GOO Warlock does 20 points of psychic damage to Bobarlo the GOO Warlock, both of whom have the Thought Shield class feature.

  2. Bobarlo applies resistance and takes 10 points of psychic damage.

  3. Bobarlo's Thought Shield triggers and deals 10 points of psychic damage to Alicio.

  4. Alicio applies resistance and takes 5 points of psychic damage.

  5. Alicio's Thought Shield triggers and deals 5 points of psychic damage to Bobarlo.

  6. Bobarlo applies resistance and takes 2 points of psychic damage.

  7. Bobarlo's Thought Shield triggers and deals 2 points of psychic damage to Alicio.

  8. Alicio applies resistance and takes 1 point of psychic damage.

  9. Alicio's Thought Shield triggers and deals 1 point of psychic damage to Bobarlo.

  10. Bobarlo applies resistance, who takes no damage

  11. Overall Alicio takes 6 psychic damage and Bobarlo takes 12 psychic damage.

There is no limitation on the class feature saying it can only trigger once a round, or any other clause, it triggers every time with no limit. There is no text even implying the damage takes place all at the same time, which means it can be a series of events, in a chronological order. There is nothing I can see to rule against the above scenario.

This RAW recursion is annoying given how little difference it makes for how much work and arithmetic is required. It would interrupt the flow of the game too much, in my opinion. It is very, very crunchy.

This is definitely an edge case of the rules, but if it became a "thing" at my table I would house rule that despite RAW, there was no recursion. That Thought Shield would trigger once for each Warlock and thus the series of events would stop at point 6. above. This would mean Alicio would take 5 points of damage and Bobarlo would take 12 points, almost no difference from the full recursion and less than half the "crunch".

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  • \$\begingroup\$ Why would it stop looping at 0? As "There is no damage minimum in the rules, so it is possible to deal 0 damage with an attack, a spell, or another effect." \$\endgroup\$ – Akixkisu May 29 at 10:16
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    \$\begingroup\$ @Akixkisu Because doing zero damage has no effect on anything. The point of the "no minimum damage" rule is that if you do the math and the damage on an attack comes out to zero, then it does zero damage, unlike some games where you're guaranteed at least 1 or something. \$\endgroup\$ – Mark Wells May 29 at 14:14
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Alice, the attacker, takes quarter damage. Bob, the target, takes half.

  1. Alice deals 8 psychic damage to Bob.
  2. Bob's damage resistance from thought shield reduces this by half. He takes 4.
  3. Alice has 4 damage dealt to her as "the same amount of damage". Alice's damage resistance from thought shield reduces this damage dealt by half. She takes 2.

JC's take on reflected damage concurs with this calculation.

A rationale for no recursion

The damage source for Alice is not Bob. Alice did the action and is the source of the damage. She takes 2 damage which satisfies the text of the thought shield feature.

whenever a creature deals psychic damage to you, that creature takes the same amount of damage that you do.

Substituting the caster's name for all the nouns makes a silly, but true statement: "Alice deals damage to Alice. Alice takes the same amount of damage that Alice does." When "a creature" and "you" are the same, the damage already satisfies the "creature takes the same amount of damage that you do".

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  • \$\begingroup\$ @Akixkisu does putting it in terms of damage dealt and damage taken make it more clear? \$\endgroup\$ – GcL May 28 at 15:20
  • \$\begingroup\$ Yes! Albeit I do not understand why that would stop the recursion. Kisumi does 16 psychic damage to herself - she has psychic resistance, and the damage is halved because Kisumi does 8 damage to herself, she does 8 damage to herself - she has psychic resistance, and the damage is halved etc. seems valid in this context unless there is a reason for that satisfying the condition prevents further recursion - why would it do that? \$\endgroup\$ – Akixkisu May 28 at 15:29
  • \$\begingroup\$ @Akixkisu When the instance of "a creature" and "you" are on and the same, the instance of the "damage to you" and "same amount of damage" are one and the same. At least by this line of reasoning. \$\endgroup\$ – GcL May 28 at 15:46
  • \$\begingroup\$ Yes, but why would that stop recursion? \$\endgroup\$ – Akixkisu May 28 at 15:48
  • \$\begingroup\$ Because that single instance of the damage satisfies the class feature. Alice takes two damage from Alice, which means she's already taken the same amount of damage as Alice (herself). It seemed like it stood to reason when there is only one creature being considered, there is only one instance of damage. \$\endgroup\$ – GcL May 28 at 15:50

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