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]).