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So, I was looking at this question, and was discussing it with my friend. In the question, they ask whether someone could block a breath weapon by readying a Wall of Stone with the trigger "If the dragon uses his breath weapon I block it with Wall of Stone". It could not, due to the Ready action saying

"When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger."

My friend argued that one could theoretically have the trigger be "whenever the dragon targets someone with their breath weapon", and thusly be able to block the dragonfire. I argued that one could not, due to the fact that the trigger must be a perceivable action.

First, you decide what perceivable circumstance will trigger your reaction.

So, would someone be able to have the trigger be "targeting" someone, and have it go off before the attack with which the enemy has targeted someone?

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No - targeting is not perceivable by the characters in-game

First, you decide what perceivable circumstance will trigger your reaction.

Targeting is something that only occurs outside the fiction of the game. To qualify as a trigger it must be observable by the character in the game.

When the dragon you are fighting rushes forward and starts to breath fire, how do you know who it is targeting? You don't. You can try to guess, but the fact that you have to guess means that it is not observable.

Though you did not ask about them, it is worth pointing out that the targeting of spells is even less observable and more clearly not a valid trigger: Can the target of a spell be identified before the spell is cast?

Targeting is a mechanic, is not observable and is thus not a valid trigger.

Triggers must be phrased in terms of what a character can see/hear/perceive

If I set my readied action as "I cast this spell if anybody targets me with an attack" this has a major issue. Because it is not phrased in terms of what my character can actually perceive, it could technically trigger off of things my character could not possibly be aware of. For example, with the above trigger if an invisible archer who I was not previously aware of shoots an arrow at me, my readied action would technically trigger.

Obviously, the DM in this case could (and should) just rule that the readied action did not apply. But the whole situation arose because the trigger that was set was not phrased to be based on what the character could perceive and thus should not have been allowed in the first place. Fortunately, there is an easy fix for this.

When my players use a trigger says "targeting" I clear it up very easily by asking them to rephrase it in terms of what the character is looking/listening for. For example, "when the archer targets me with an attack" becomes "when the archer points her bow at me". Thus an invalid trigger becomes a valid one extremely easily. It just has to be phrased in terms that the character can actually perceive.


Jeremy Crawford has commented on a similar situation:

The trigger you choose for the Ready action must be a "perceivable circumstance" (PH, 193). A caster doesn't perceive turns ending.

A "turn" is another mechanical concept which is also not perceivable by the characters. Jeremy Crawford correctly points out here that it is not a valid trigger for that reason.

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No (Just don’t)

Spells and abilities that can interrupt an attack specifically say they can do so. Other things cannot. (Choosing a target and resolving the effects are all part of the attack.)

For example, the spell Shield says its effect works "including against the triggering attack."

Interrupting an action is mechanically powerful

Considering play balace for a moment, the Shield spell has very short duration compared to other spells such as Barkskin or Shield of Faith. Shield’s main advantage over those other spells is that it can help you even after you are hit by an attack.

Letting other spells and abilities interrupt an action would often break play balance, even considering the cost of the readied action.

Stacks of things interrupting things could quickly become a huge hassle

Consider that red dragon — he’s an intelligent guy, and might be anticipating a tricky spellcaster. Say he readies an action to target a spellcaster who reached for an arcane focus or started chanting. If the spell can happen before the dragon’s (original) breath attack, it’s logical the dragon’s (readied) attack could occur before the spellcaster’s spell.

(But wait, the fighter has readied an action in case the dragon did that! And so forth. Now let’s do this dance at the beginning of half of combats.)

Stacks of actions that resolve in reverse order are enough complexity in a game like MTG. Adding them to D&D 5e could add frustrating hassles. (The "interrupt" mechanic from 4e was largely dropped.) I suspect the new D&D rules are written to avoid stack-associated headaches, and would’t suggest trying to cleverly "lawyer" them into the game.

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No:

Unless your opponent is some sort of comic book villian who yells Breath Weapon Fire Attack! before unleashing his breath weapon, the rules do not support such fine-tuning of events.

The best you can hope for is a trigger such as "move within breath weapon range" - but even that would merely put the wall up before he uses it, and he'd walk around before breath weaponing you.


That said, talk to your DM about this. They are fully allowed to rule that you can do exactly as you ask - and I would allow that as long as you're not going to abuse it by trying to interrupt actions every round.

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Yes

Sort of.

While a character can't perceive that a dragon is "targetting", a character could perceive signals that indicate targetting. Maybe the dragon needs to inhale first, maybe it's eyes get big, maybe it's nose glows red. Maybe none of those things. How does the character know? Does the character have experience with red dragons? So maybe it's a perception check, with a huge difficulty.

The best summary of DnD is on page 6 of The Player's Handbook:

  1. The DM describes the environment. 2. The players describe what they want to do. 3. The DM narrates the results of the adventurers' actions.

This is how I play this out in my head:

DM: You see a really big red reptilian thing with wings.

Randall the Magnificent: Ah, a red dragon! I ready Wall of Stone for when the dragon targets someone with their breath weapon.

DM: Interesting. Is there anything about Randall that suggests that he has any specific knowledge of really big red reptilian winged creatures? Let's say I stipulate for a moment that this is in fact a red dragon. Does Randall have any particular knowledge of red dragons that suggests he knows what it looks like when a red dragon is getting ready to breathe?

R the M: Err, well, doesn't everyone know what red dragons look like and how to tell when they're getting ready to blow?

DM: Okay, great. You ready your action based on common knowledge. If there's a breath attack, you can roll a perception check to see if you can detect it fast enough. Whatever "it" is. I wonder how sure you are that you know what to look for. I wonder if you're right. I wonder if you're fast enough.

At that point I would let the player roll a perception roll against a difficulty, depending on how experienced the dragon and how likely the character actually is to have heard something reliable that describes how to tell when a red dragon's getting ready to toast a marshmallow, or an adventurer.

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  • \$\begingroup\$ NB that you can ready a normal-action spell to be cast as a reaction. (last ¶ of “Ready”, PHB, p. 193) You might want to revise that last paragraph accordingly. \$\endgroup\$ – SevenSidedDie Mar 27 '18 at 22:23
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    \$\begingroup\$ If you're going to do this, I would recommend defining a failure mode that causes the caster to use his wall of stone even when no breath weapon was incoming. \$\endgroup\$ – Neil B Mar 28 '18 at 10:11

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