Ready

Choose a trigger and a response reaction
First, you decide what perceivable circumstance will trigger your reaction.

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Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it.

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

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When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration
PHB, pg. 193.

Type:
Action
Cost:
1/Turn