Suppose Alex is a fighter with the Extra Attacks feature, so each time they take the Attack action they can make two attacks. Alex attacks Sam, a Fighter/Monk with access to the Riposte Battle Master maneuver and the Stunning Strike monk feature.
Here's what happens:
- Alex takes Attack as their action for the turn.
- Alex makes their first attack against Sam and misses.
- Sam uses their reaction and Riposte (spending a superiority die) to attack Alex. Sam hits.
- Sam spends a ki point to activate Stunning Strike.
- Alex attempts a Con save and fails. Alex is now stunned.
It's still Alex's turn. They are stunned and thus cannot take actions. Can they make their second attack?
On one hand, Alex isn't taking an action to make their second attack; they have already taken their action and still have part of it left to resolve.
On the other hand, the effects of being stunned could be read as meaning that a stunned character stops taking actions, so any in-progress actions don't continue.
When a creature misses you with a melee attack, you can use your reaction and expend one superiority die to make a melee weapon attack against the creature. If you hit, you add the superiority die to the attack's damage roll.
(PHB, p. 74)
Starting at 5th level, you can interfere with the flow of ki in an opponent’s body. When you hit another creature with a melee weapon attack, you can spend 1 ki point to attempt a stunning strike. The target must succeed on a Constitution saving throw or be stunned until the end of your next turn.
(PHB, p. 79)
- A stunned creature is incapacitated (see the condition), can’t move, and can speak only falteringly.
- The creature automatically fails Strength and Dexterity saving throws.
- Attack rolls against the creature have advantage.