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?
- 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
- Or does the recursion stop after the first reflection?
- 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.)
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:
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:
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):
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.