Does fire shield damage trigger if cloud rune is used

Question

If a wizard cast fire shield and is later hit by a melee attack, then a fighter uses cloud rune to switch targets. Would the fire shield still do damage?

Fire Shield Whenever a creature within 5 feet of you hits you with a melee attack, the shield erupts with flame. The attacker takes 2d8 fire damage from a warm shield, or 2d8 cold damage from a cold shield.

Cloud Rune When you or a creature you can see within 30 feet of you is hit by an attack roll, you can use your reaction to invoke the rune and choose a different creature within 30 feet of you, other than the attacker. The chosen creature becomes the target of the attack, using the same roll. This magic can transfer the attack's effects regardless of the attack's range.

Answer 1

The features have the exact same trigger; ask the GM how it plays out

The fire shield spell's effect is activated:

[...] Whenever a creature within 5 feet of you hits you with a melee attack [...]

Meanwhile, the Cloud Rune is activated:

[...] When you or a creature you can see within 30 feet of you is hit by an attack roll [...]

These are, effectively, the exact same trigger. Thus, what happens with them, what order they are triggered in, and how that is resolved, is left to the GM. The two options, at least to me, are as follows:

1. The fire shield spell activates, dealing damage, and then Cloud Rune can be used; changing the target.

2. Cloud Rune activates, changing the target of the attack, thus making you no longer hit by the attack, meaning fire shield can't trigger and deal damage.

Which of these actually occurs is ambiguous, and both are a bit odd to begin with, but it'll be up to GM to adjudicate the situation.

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There are technically optional rules for this in Xanathar's Guide to Everything

[...] Most effects in the game happen in succession, following an order set by the rules or the DM. In rare cases, effects can happen at the same time, especially at the start or end of a creature's turn. If two or more things happen at the same time on a character or monster's turn, the person at the game table—whether player or DM—who controls that creature decides the order in which those things happen. For example, if two effects occur at the end of a player character's turn, the player decides which of the two effects happens first. [...]

But these rules are not particularly helpful when it comes to exceptional situations like this where one of the triggered events removes what would trigger the other event. We have some questions on somewhat similar situations with various different answers:

• Does blocking an attack with the Shield spell still trigger Armor of Agathys?
• Can a Precision Attack be added to an attack roll after an enemy uses Shield as a reaction to that attack roll?
• How does the Open Hand monk's ability to prevent a target hit by Flurry of Blows from taking reactions work?
• Shield Spell and triggering effects of damage not taken
• Are getting hit and taking damage simultaneous events?

Though these are about the shield spell which... is a a particularly odd spell that may or may not involve time travel (we have a lot of questions on this spell), but they are similar to the case you've described here.

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Now, the features are not entirely identical in their triggers. One is when hit by an attack the other is when hit by an attack roll. I, however, see no meaningful difference between these as they both require you to be hit. I also so not know any other features that use the phrase "hit by an attack roll" (or an equivalent phrasing) so I can't imagine this one feature was where the writers suddenly decided they needed a new trigger type.

Answer 2

No

You cannot have your cake and eat it.

For simplicity, I'm assuming that the 'attack roll' from Cloud Rune is sufficient to hit any creature, and thus treating 'attack roll' as synonymous with 'attack'. I will use 'wizard' to signify the Fire Shield caster, and 'goblin' to signify the new target from Cloud Rune

1. The attack has only one target

The chosen creature becomes the target of the attack

'Becomes the target' denotes substitution, rather than addition; there is one target - 'the target' - and the thing which changes is the identity of the target, rather than the number of targets.

The wizard casting Fire Shield can be the target of the attack, or the goblin targeted by Cloud Rune can be the target. They cannot both be 'the target' of the attack. This should be obvious, because the whole point of Cloud Rune is that the original target is spared the effects of the attack.

2. Cloud Rune transfers all the effects of the attack

The chosen creature becomes the target of the attack, using the same roll. This magic can transfer the attack's effects...

There is no case here for a division between 'being hit by an attack' and 'taking damage from an attack'. Cloud Rune does not transfer attack damage; it transfers the entire attack and all of its effects.

Only one target creature is hit by the attack, and that creature suffers the attack's effects. No other creatures:

• Can be considered the target of the attack
• Are effected by the attacks effects

3. One of the 'attack's effects' is the opportunity to trigger certain reactions

Imagine our goblin also has a reaction which is triggered when she's hit by an attack - the Shield spell, for instance. Because Cloud Rune has been used to make her the target of the attack, she may use her reaction to cast Shield. As the attack's target, that's her sole prerogative. The wizard cannot use a reaction to cast Fire Shield, because the wizard was not targeted by an attack; There is one attack, and the goblin is its only target.

To put it in concrete terms: If you're going to deal damage with Fire Shield, you must take the damage* from the triggering attack. These are both consequences of being the attack's target, and you can't have one without the other.

Guiding principle: Reactions must result in consistent realities

Many reactions appear to tamper with reality. Shield, for example, cancels its own trigger. This is fine, because the purpose of the spell is to substitute the triggering reality for the desired one. In one reality, the triggering attack hits. In another reality, the triggering attack misses. Both realities are coherent within themselves.

In this questioner's example, a paradoxical, inconsistent reality is being created. The wizard is hit by an attack and triggers Fire Shield. Cloud Rune is cast, making the goblin the target of the attack. The wizard is no longer the target of the attack, so the wizard cannot possibly have cast Fire Shield in response to being that attack's target. You cannot have your cake and eat it.

* Except in the case of exceptions. It is obviously possible to be hit by an attack and take no damage from it.

Answer 3

Fire Shield works on an attack

Fire shield states that it works based on getting hit by an attack. I take this to mean the actual attack connection that the in-world enemy makes.

Cloud Rune works on an attack roll

Cloud Rune states this works based on getting hit by an attack roll. I take this to mean the dice roll that determines whether the attack hits.

The attack roll happens before the attack connects with the Wizard (because the attack connecting is a direct result of the attack roll hitting). Thus in this case, I interpret that the Fighter can choose to redirect the attack roll to another target, potentially hitting them. In case this happens, the attack doesn't connect with the Wizard but rather with the other target, and thus the Wizard's Fire Shield does not activate.

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Note: this is based on my own careful reading of the rules, and assumes that an attack roll is a distinct part of a "melee attack" that's different from the "attack connects" part of a melee attack.