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I understand that the caster first completes the casting, then the Mage Slayer feat (PHB, p. 168) triggers and allows the character to attack as a reaction.

If the spell requires concentration, what will happen if the caster fails the concentration check? Specifically, does the spell fail before taking effect, or does it take effect briefly before getting cancelled?

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In the event that the concentration of the caster has been broken by a Mage Slayer reaction attack to the casting of a spell, then the spell takes effect briefly before dissipating

The Mage Slayer attack action takes place after the spell has been cast, and thus had its initial effects resolved.

The order of operations is:

  1. Caster casts spell
  2. Spell effects resolve (including any saving throws)
  3. Mage Slayer gets their reaction attack
  4. If it hits and the spell was a concentration spell then the caster performs a concentration check (with disadvantage from Mage Slayer)
  5. If that check fails then the spell dissipates
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  • \$\begingroup\$ Small note: if the caster attacks with something like Shocking Grasp, the Mage Slayer would have advantage against the save (negated if they are wearing metal), but if they fail their save, they'll lose their ability to use their Reaction (spell effect). \$\endgroup\$
    – Nick Brown
    Feb 22, 2018 at 19:21
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    \$\begingroup\$ @NickBrown: Shocking Grasp is a melee spell attack; the target doesn't have to make any saves. \$\endgroup\$
    – V2Blast
    Feb 22, 2018 at 20:22
  • \$\begingroup\$ Yeah. I need to play am arcane character one of these days. Maybe once my druid dies. I was really trying to emphasize the loss of reaction more than the advantage on the save though. So Shocking Grasp might be very effective against a metal wearing Mage Slayer, since if you hit them with advantage they can't hit back. \$\endgroup\$
    – Nick Brown
    Feb 22, 2018 at 21:14
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    \$\begingroup\$ You may also want to note that the feat does impose disadvantage on Con saves to maintain concentration when you damage a creature concentrating on a spell. \$\endgroup\$
    – V2Blast
    Mar 19, 2019 at 6:10

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