Can a readied action be taken after a spell is cast, but before it deals damage?

Question

This question evolved from discussion in the comments from another Q/A: "Can you use Shape Water on Tidal Wave?".

I've become unsure about how spells with a duration of "instantaneous" work. Specifically, do their effects also happen instantaneously?

An example where it matters:
If one readies the spell wall of stone and has the trigger be "when I see a fireball hurtling towards me I will block it with the spell", when do they actually cast the wall of stone spell?

We know that readied actions occur after their triggers, but there are (at least) two possibilities.

1. The effects of fireball happen instantaneously, so you could cast wall of stone only after every part of the spell, including the fireball exploding and dealing damage.

2. The effects of fireball happen in a timed order, so you could cast wall of stone immediately after any part of the spell, including the fireball coming towards you.

Can a readied action be triggered and taken after a spell is cast, but before it deals damage?
Or rather, which (if any) of the two interpretations above applies to spells with the duration "instantaneous"?

Answer 1

Instantaneously means instantaneously

An instantaneous spell takes no time - by the time you realise it’s happening it already has.

The fireball shoots from the fingers, expands, deals damage and is gone instantaneously - your Wall of Stone is now good to go (assuming you made your Concentration check from the damage).

Now, we’re not talking instantaneously as a physicist would (except, of course a real physicist would talk about spacelike separated events synchronous from particular frames of reference), we’re talking instantaneously as in too quick to react to, like the time between a bomb detonating and you getting ripped apart by shrapnel.

Answer 2

The rules state no:

Readying an Action

First, you decide what perceivable circumstance will trigger your reaction. [...] When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger.

D&D does not account for things like projectile travel time or the time it takes for a spell effect to fully materialize, etc. Once an attack is made or spell is cast, it takes effect without any time interval.

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Rule Zero Considerations

A DM might, from time to time, bend this according to Rule Zero, but this is a slippery slope. If a spell (action) can be interrupted, why not an attack?

Many abilities have wording to the effect of "after the dice are rolled but before the results are known." Allowing reactions to interrupt actions would undermine the potency of these abilities.

It would also erode the distinction between spellcasting classes*, and tilt the advantage in a fight towards defense. In a game where combat is often already a slow affair, this would exacerbate the problem.

So, while I think there is a place for what you describe under Rule Zero, I would caution against making it a reliable homebrew rule.

^{*Only Sorcerers, Wizards, and Warlocks get Counterspell, which is essentially what Wall of Stone is being used to do against Fireball in your example.}

Answer 3

Mostly, no

Dale M's answer is mostly right - the effect of an instantaneous spell happens instantaneously upon the completion of casting - or, as he clarifies, "instantaneously" from a game mechanics perspective, in that the completion of the spell and the application of its effects are atomic operations (cannot be broken apart or separated). Of course, as with everything, this is merely the general rule - specific rules can violate it, such as the shield spell (not useful in this case because fireball does not target AC, but useful for instantaneous spells that do target AC). There are a handful of other interrupt style spells/actions that do likewise (Paladin's Aura of the Guardian, etc). But the general rule is that being hit by an effect and suffering the effects of that, um, effect, are inseparable and uninterruptible.

However

Things like Dexterity saves and counterspell work by reacting to the casting of the spell rather than the completion of it. In this way, a prepared spell can be used as a sort of counter to a spell of instantaneous duration. In this case, you would have to spend your action on your turn readying the spell (which entails casting it, but holding its energy). Then, on the enemy's turn when they begin to cast fireball, you can release the energy of your held spell, throwing up your wall just before the casting of the fireball spell is completed. To be pedantic, you could release the energy at any point of the casting of fireball, but in order to give the opposing caster as little time as possible to abort the spell or re-target it, you'd wait until the last possible fraction of a second.

This is risky (or at least generally sub-optimal) for a number of reasons:

• You have to anticipate the fireball on your turn prior to the enemy casting it. If you anticipate it too early, the spell could be wasted - though you can hold a spell for as long as you like, it requires concentration, and so limits your actions until you can release it. Anticipate too late, and well, you get a fireball to the face before you can prepare.
• Because holding a spell requires concentration, it can be disrupted between your turn and the trigger.
• It takes an action and a reaction to pull off, which is a worse action economy than counterspell
• GM fiat could work against you. Just because you waited until that last possible moment to foil the fireball's path doesn't mean that the caster doesn't get a chance to react. The GM could just assume the caster noticed the wall too late and the fireball is foiled as you expect, but he could give the caster a save or some other opportunity to change his mind as he notices your wall suddenly blocking his view.
• It's not a sure thing anyway (not that even counterspell would be either, but for different reasons). In this case, fireball goes around obstacles, so if you place the wall incorrectly (or misjudge where the fireball is targetted) it could end up being for naught, and your spell is wasted. Other factors could also contribute in the case of other spells.

The last point may be the most relevant to your question - you can't wait until the fireball is exploding to see where it was targeted before putting up your wall. This makes it a trickier proposition to properly place the wall to achieve the best effect.