How does the reaction timing work for Wrath of the Storm? Can it potentially prevent the damage from the triggering attack?

Question

With attacks of opportunity, the PHB is pretty clear that your reaction occurs as an interruption to your opponent's move, just before they move beyond your reach, then they resume their turn. Fair enough.

However, there are some abilities that happen during an opponent's attack - for example, the Shield spell, and a Tempest cleric's Wrath of the Storm ability. These both say they happen when you're "hit" by an attack - but the Shield spell raises your AC, so I assume that occurs before rolling damage, and makes it possible for you to be retroactively not hit by the attack. Is the same true for Wrath - that your reaction might KO your opponent, and you wouldn't have to worry about taking damage?

(I was planning to include Hellish Rebuke in this question but I see it specifically refers to "being damaged", so I assume that means it happens on your opponent's turn but definitely after getting hit, meaning you can only cast it if you're still conscious.)

Answer 1

In general, reactions interrupt whatever action they are reacting to, but only once that action has been resolved:

Reactions

Certain special abilities, spells, and situations allow you to take a special action called a reaction. A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else’s. The opportunity attack, described later in this chapter, is the most common type of reaction. When you take a reaction, you can’t take another one until the start of your next turn. If the reaction interrupts another creature’s turn, that creature can continue its turn right after the reaction.

In the case of Wrath of the Storm it happens when you choose to react to this:

When a creature within 5 feet of you that you can see hits you with an attack

This triggers your reaction, and the enemy's attack has not yet been resolved. Since the trigger happens before the damage (on the hit), it immediately interrupts the enemy's turn, including the damage resolution of the attack. This is in line with how reactions are described above. Thematically, think of it as the cleric exploding with electricity as an immediate response to being struck regardless of the damage the cleric takes in the process.

Note that, even if this kills the enemy, the damage from the hit that triggered wrath of the storm is still calculated.

The spell Shield is a reaction you take when you are hit by an attack. It has additional special properties in the spell description:

An invisible barrier of magical force appears and protects you. Until the start of your next turn, you have a +5 bonus to AC, including against the triggering attack, and you take no damage from magic missile.

So in this case, it is a specific exception to the rule that happens as a reaction to an attack that hits you, and it retroactively increases your AC against the triggering attack so that if the attack originally hit you, it might now miss.

Answer 2

Wrath of Storm does not protect from the damage of the triggering attack.

A reaction is an instant response to a trigger of some kind

This means that a reaction is preceded by the trigger chronologically, as a general rule. In the case of WoS, no further specific rules are given, so the general rule stands and the hit (and thus the damage) still occurs. For Shield, such a reading would render the AC bonus useless against the trigger. The text, however, reads:

+5 bonus to AC, including against the triggering attack

The specific rule here states that the spell has a retroactive effect on it's trigger. This is why it has this unique ability, one which Wrath of Storm would not.

You're also entirely right about Hellish Rebuke, the reference to damage makes it a nonissue.

Answer 3

Specific beats general.

The general rule is that reactions happen after the trigger is completed.

A reaction is an instant response to a trigger of some kind

The word "response" requires the thing being responded to to have happened first.

Shield is an exception as are attacks of opportunity as their specific rules make them happen before the trigger.

The Tempest cleric's wrath contains no such exception so you must still be conscious after the trigger.

It is worth noting that there are circumstances where you can be hit but take no damage: a creature with a negative strength bonus, immunity, grapple, shove etc. Some reaction triggers are when you are hit: these work even if you are not damaged, others are when you are damaged by an attack.