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D&D 5E question on Mage Slayer feat, invisibility and concentration


So the situation is a character with the mage slayer feat is standing next to an NPC who casts the invisibility spell.

Pertinent verbiage from the feat:
  • When a creature within 5 feet of you casts a spell, you can use your reaction to make a melee weapon attack against that creature.
  • When you damage a creature that is concentrating on a spell, that creature has disadvantage on the saving throw it makes to maintain its concentration.
So how is this resolved in terms of invisibility, attack and concentration. I see three possibilities:

1. You attack before the spell is complete. It is a normal attack and after the attack the enemy turns invisible regardless of the outcome (I really don't think this one is supported by the rules because he did not "cast a spell" and trigger the reaction)

2. Your attack happens at the exact same time as the spell finishes. It is a normal attack and the enemy turns invisible regardless of the outcome of the attack (as long as he is not dead from the attack).

3. Your attack happens at the exact same time as the spell finishes. It is a normal attack and if it hits the enemy must make a concentration save with disadvantage or he loses concentration and does not turn invisible at all.

4. Your attack happens right after the spell completes, the attack is with disadvantage because the enemy is invisible. If you hit the enemy makes a concentration save with disadvantage.

I am leaning towards #2 above.

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Since it doesn't say anything like "The attack occurs right before" or "against the triggering attack", I would rule 4. You have disadvantage, but can break concentration.

If you want to be more confused, then how would you rule a wizard getting killed when casting misty step?

Also the reaction attack is removed in the playtest. Which avoids the issue.



DMG, p 252: "If a reaction has no timing specified, or the timing is unclear, the reaction occurs after its trigger finishes, as in the Ready action."

The opportunity attack is an example of a reaction with a specified timing (it occurs before the triggering creature leaves your reach).

Since the Mage Slayer feat does not specific a timing for the reaction attack, it follows the general rule and occurs after the invisibility spell has kicked in, meaning you would have disadvantage on the attack.

As an aside, this means that teleportation spells like misty step are a great way to avoid getting whacked by a mage slayer because you won't be in range by the time they get to make their attack.
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