Mage Slayer vs Hold Person

Question

Interesting scenario was posed to me by one of my players.

A fighter with the mage slayer feat is within 5 feet of a mage who casts Hold Person on the fighter. The fighter fails his save (despite advantage for being w/in 5 feet) and on his turn can't act. BUT the mage then gets his concentration disrupted (perhaps another character hits the mage for damage).

Can the fighter, now "free" from Hold Person, make his feat granted reaction attack against the mage (who didn't move out of melee range)?

I figure that if it is still the same round then the fighter should be able to react, but if it is a subsequent round then whatever spellcasting related phenomenon that gives the mage slayer feat a reaction attack has passed (i.e., the fighter can't hold on to that reaction attack over multiple rounds).

And what if the mage, still in melee range, casts Hold Person on the same fighter again? Could we get stuck in a loop where the fighter can't attack?

In this case, since the feat is called Mage Slayer I would probably allow the fighter to make his reaction during the casting of the spell (possibly interrupting it) but that is just DM fiat to allow for a thematic moment and isn't my question (unless Mage Slayer is supposed to interrupt the spell while it is being cast, instead of allowing for a reaction attack afterwards [which is how I interpret the feat]).

Answer 1

If you don't meet the conditions for a reaction when (and right after) the trigger occurs, then you don't get to take the reaction.

The case you describe seems to be as follows:

1. Fighter and Wizard are w/in 5' of each other; Wizard casts Hold Person
2. Fighter fails save, is held (thus unable to take his reaction attack from mage slayer)
3. Wizard does other things on his turn
4. Time passes
5. Fighter becomes unheld and attacks wizard for using Hold Person.

This is not permitted by the rules:

A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else’s (Player's Basic D&D p. 70, emphasis mine).

A reaction is instant, you can't sit on the trigger for some undetermined amount of time just because you don't qualify at that moment.

The text from Ready lends credence to this interpretation:

When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger (Players Basic D&D p. 72, emphasis mine).

Unless otherwise specified by an ability, a reaction does not interrupt or occur in the middle of its trigger.

The reaction happens after its trigger completes, unless the description of the reaction explicitly says otherwise (Xanathar's Guide to Everything p. 5).

So no, the scenario you describe is not possible.

Answer 2

The rules as written are not entirely clear on whether it is possible to interrupt casting. My interpretation, as seen in my answer to this question, is that it is possible.

Given that assumption, I would say that the Mage Slayer reaction should happen during casting.

The 5e feat states:

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.

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To get an idea of intent we can look at the 3.5 edition rules. Mage Slayer feat (from Complete Arcane):

Spellcasters you threaten may not cast defensively [...]

and Casting Defensively (from PHB):

If you want to cast a spell without provoking any attacks of opportunity, you must make a Concentration check [...] to succeed. You lose the spell if you fail.

So the intent seems to be, allowing an attack of opportunity (or equivalent) against an enemy casting a spell, to try and prevent them casting.

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If these are true, then the events will happen as follows:

• Fighter and Wizard are w/in 5' of each other
• Wizard starts to cast Hold Person
• Fighter makes Mage Slayer reaction attack
  • If Fighter hits, Wizard must make Concentration check or fail casting
  • If Fighter misses or Wizard succeeds on Concentration, Fighter is Held
• Combat continues

Answer 3

I'd say "no", he can't attack later in the turn. As I understand it, he has to make the attack when(after/while is more of a controversial point) the spell is being cast. As I understand the fluff: He's reacting to the spell-caster focussing on his magic and taking the chance to attack him. That doesn't allow him to attack the caster later on, when he's on high alert.

Personally I'd have to go with the "interrupts the casting".
But if you're having trouble with the timing, think of this scenario:
"Fighter with Mageslayer is in melee with a wizard; The wizard takes a ready action to cast Disintegrate at the proverbial halfling rogue (who keeps popping out and sneak-attacking the wizard) and the PHB clearly states that you start casting the spell the moment you ready the action.

Now how would it seem if the fighter wasn't allowed to use his mageslayer-feat while the wizard was standing there concentrating on casting his spell, for possibly a full 6 seconds?
To me this situation demonstrates that demanding the fighter wait for the spell to resolve is simply absurd.
As both a DM and as a player, I don't think I'd ever allow the "he has to wait"-reading of it. I'd probably call for a concentration save (but make it clear that we have to see if this makes it over the top.) and see if that isn't okay power-wise.