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<blockquote data-quote="Blue" data-source="post: 8590505" data-attributes="member: 20564">Under the Ready action, there are examples that break up what a target is doing, such as &quot;If the cultist steps on the trapdoor, I'll pull the level that opens it&quot;.&nbsp; As such it seems that the only limit to the trigger is &quot;perceivable circumstance&quot;, there is no explicit or implied limitation that it can not interrupt part of an action.&nbsp; And the trigger occurs after the perceivable experience completes, not the whole action completes.&nbsp; It happen when they step on the trapdoor, not when they have completed a move that takes them off the trapdoor.We know from the Identifying a Spell section of Xanathar's that a reaction can be used to identify a spell explicitly before it is cast.&nbsp; So there is a period after casting is started before the casting is finished that can accommodate a Reaction during that period.So the rules are clear that we can have a Ready action that goes off &quot;when the target begins casting&quot;.&nbsp; Begining casting occurs -&gt; Ready -&gt; casting (may) complete -&gt; spell goes off.Now, unless they are casting a multiple round spell (a special case listed in the PHB), there is no need for Concentration spells to cast.&nbsp; So triggering while casting would not require Concentration checks - the spell is not up yet to need it either.</blockquote>

[QUOTE="Blue, post: 8590505, member: 20564"] Under the Ready action, there are examples that break up what a target is doing, such as "If the cultist steps on the trapdoor, I'll pull the level that opens it". As such it seems that the only limit to the trigger is "perceivable circumstance", there is no explicit or implied limitation that it can not interrupt part of an action. And the trigger occurs after the perceivable experience completes, not the whole action completes. It happen when they step on the trapdoor, not when they have completed a move that takes them off the trapdoor. We know from the Identifying a Spell section of Xanathar's that a reaction can be used to identify a spell explicitly before it is cast. So there is a period after casting is started before the casting is finished that can accommodate a Reaction during that period. So the rules are clear that we can have a Ready action that goes off "when the target begins casting". Begining casting occurs -> Ready -> casting (may) complete -> spell goes off. Now, unless they are casting a multiple round spell (a special case listed in the PHB), there is no need for Concentration spells to cast. So triggering while casting would not require Concentration checks - the spell is not up yet to need it either. [/QUOTE]

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