Let's say a spellcaster is fighting a dragon. Can they ready their action to cast a Wall of Stone with a trigger of "If the dragon uses its breath weapon I block it with Wall of Stone"? If so, how high would the wall have to be made?
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1Related: How does the Ready action work? – Slagmoth Mar 27 '18 at 17:35
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1Related: Do reactions interrupt their triggers or not? – Rubiksmoose May 09 '18 at 14:04
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No, this won't work
The method for readying an action is that you use your action to hold the spell at the ready (pre-casting it), then use your reaction to cast it when a given trigger occurs.
But here's the catch (PHB, p. 193):
When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. Remember that you can take only one reaction per round.
If you have readied a spell to cast if a dragon uses its breath weapon, the only time you may unleash the spell is after the dragon has used its breath weapon and the outcome of that has been resolved (finished). Meaning you will cast Wall of Stone after the dragon uses its breath weapon, and thus after you've already saved/taken damage from it.
V2Blast
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guildsbounty
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4couldn't you just use "when the dragon shows signs that he's about to use his breath weapon" or something like that as the trigger? – PixelMaster Mar 27 '18 at 18:39
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12@PixelMaster Yes, but how confident are you that you will correctly interpret the "signs"? You might end up casting the spell after the dragon sharply inhales (not because it was going to use a breath weapon: it was just breathing. But it sure looked like it was showing signs that it would use it). If your trigger is that it looks like something might happen, it'll get triggered pretty often. – Gandalfmeansme Mar 27 '18 at 19:21
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3Additionally, the reason this doesn't work while a spell like Shield does is that Shield explicitly defines the exception that it blocks against the triggering attack. – Mwr247 Mar 27 '18 at 20:58
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4Shield also explicitly uses a reaction. Other spells that require the Ready action only do so because of that, so these are 2 completely different situations – PixelMaster Mar 27 '18 at 21:56
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2Also, if the dragon doesn't use its breath weapon that round, you've wasted your spell slot. – nick012000 Jul 08 '21 at 01:56
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Yes
reactions can be taken after a trigger which you state, just use the same trigger for counterspell but reword it, which is just "when you see a creature within (range of spell you're casting) of you, starts casting a breath attack"
I don't see why you can only take your reaction after the thing already happens because almost all reactions this is false and you are actively doing something right after your trigger.
Another Example would be can you hold action to cast globe of invulnerability to save a friend from a breath attack. if you couldn't that would just be dumb and screw support classes
Heyz
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2With that phrasing the dragon would still use its breath weapon before your character casts the spell. – Akixkisu Jul 06 '21 at 22:01
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2Unfortunately, Counterspell is a rules exception and the Ready action doesn't include the ability to use phrasing from other more specific options. The related links listed under the question go into some detail here. – NotArch Jul 07 '21 at 15:29
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I see what you are saying but "When the trigger occurs, you can either take your reaction right after the trigger finishes" Can literally mean anything. You decide what the trigger is. it doesn't say that the action has to finish. You decide what the trigger is and when you act on that triggers description. I could just be ministering this and seeing this more as a RAI instead of a RAW but thats what Crowford is for so i'll see if i can ask him – Heyz Jul 07 '21 at 16:10
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1Welcome to RPG.SE! Take the [tour] if you haven't already, and check out the [help] for more guidance. – V2Blast Jul 08 '21 at 05:39
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1“Starts casting a breath attack” is not a perceptible trigger. Triggers for the ready action must be perceptible to the character, and as explained in this three year old comment, the trigger you have proposed here is based on a future event that hasn’t happened yet, not what your character is perceiving at the time of the trigger. – Thomas Markov Jul 08 '21 at 07:24
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Yes the trigger could be more defined here is a better example"When the dragons starts to breath Attack cast WOS Infront of the creature" Or "Cast GOI On the creature" Reactions interrupt a creatures turn so this should work Depending on the DM. This kinda actions isn't really in the books. it falls under the DM decision – Heyz Jul 08 '21 at 15:03
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@Heyz How do you know "when the dragon starts to breath attack"? It seems you cannot know that it has start its breath attack until after it has started its breath attack, at which point it is too late. – Thomas Markov Jul 08 '21 at 16:43