Suppose I was playing a wizard. I say to my DM, "I ready my Magic Missile. I cast it when I take damage, at the attacker."
When the time comes, do I need to make a Constitution saving throw immediately before I cast to avoid losing concentration?
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\$\begingroup\$ Why not rephrase to trigger 'immediately before something causes me to take damage'? \$\endgroup\$– KieveliCommented May 2, 2019 at 19:22
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\$\begingroup\$ @Kieveli I also thought off that but (in the [hypothetical] game) that was the wording I chose. The question is about that specific wording. \$\endgroup\$– MrHiTechCommented May 3, 2019 at 15:33
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Yes, you do
The Readied action doesn't trigger until after your trigger completes:
When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger.
When you ready a spell, you are concentrating on that spell as you hold it (PHB, 193)
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 (explained in chapter 10). If your concentration is broken, the spell dissipates without taking effect.
Since your trigger is when I take damage and you are concentrating, you will have to make a concentration save per the rules on concentration:
Taking damage. Whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw to maintain your concentration. The DC equals 10 or half the damage you take, whichever number is higher. If you take damage from multiple sources, such as an arrow and a dragon's breath, you make a separate saving throw for each source of damage.
If you fail, then you've lost your spell
Just to close the loop, if you do fail that concentration save, then you drop the readied spell and you won't be casting it.
