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Let's say a spellcaster is fighting a dragon. Can a they ready their action to put up a Wall of Stone with a trigger of "If the dragon uses his breath weapon I block it with Wall of Stone"? If so, how high would it have to be made?

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No

The method for readying an action is that you hold the spell at the ready (pre-casting it) then use your Reaction to cast it under a given trigger.

But here's the catch:

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.

Emphasis Mine from PHB193

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 you've already saved/taken damage from it.

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    \$\begingroup\$ couldn'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? \$\endgroup\$ – PixelMaster Mar 27 '18 at 18:39
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    \$\begingroup\$ @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. \$\endgroup\$ – Gandalfmeansme Mar 27 '18 at 19:21
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    \$\begingroup\$ Additionally, 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. \$\endgroup\$ – Mwr247 Mar 27 '18 at 20:58
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    \$\begingroup\$ Shield 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 \$\endgroup\$ – PixelMaster Mar 27 '18 at 21:56

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