If a Slow spell ends due to a failed concentration (CON) saving throw, does the attacker get to make any remaining attacks?

Question

The Slow spell (PHB 277) requires concentration and states the following:

Regardless of the creature's abilities or magic items, it can't make more than one melee or ranged attack during its turn.

Suppose a PC or monster who could normally make more than one attack is affected by the Slow spell, takes the Attack action, and hits the spell's caster, who then fails their concentration check, causing the Slow spell to end. Does the attacker then get to take their remaining attack(s) because the slow spell is no longer in effect, or do they still only get 1 because the Slow spell was in effect at the time they took the Attack action?

Answer 1

If the spell ends, all of its effects also end; the attacker could make more attacks on their turn

The slow spell states:

[...] Each target must succeed on a Wisdom saving throw or be affected by this spell for the duration.

An affected target's speed is halved, it takes a −2 penalty to AC and Dexterity saving throws, and it can't use reactions. On its turn, it can use either an action or a bonus action, not both. Regardless of the creature's abilities or magic items, it can't make more than one melee or ranged attack during its turn [...]

And from this text we can see that if the spell ends, then its targets are no longer affected by the spell. This means the restrictions put on them by the spell no longer exist. For this question, that means the attacker can now make more attacks during their turn.

Another way to think about this is to note that the spell didn't come into effect when you took the Attack action, it was always there, limiting your options. Removing the spell removed those limits. The additional attacks themselves were never removed, you simply weren't allowed to take them. That changed when the spell ended and they once again became available for use.

We know that the spell ends when the caster loses concentration not only intuitively but also from the rule on "Concentration":

Some spells require you to maintain concentration in order to keep their magic active. If you lose concentration, such a spell ends.

- Player's Handbook: Chapter 10 "Spellcasting": Concentration (page 203)

I honestly can't see another way to think about this because it would lead to the seemingly illogical conclusion that the spell is no longer affecting you (its duration has ended) and yet it still has an effect on you (you can't make more attacks).

Answer 2

Once a spell ends, it ends

It's easiest to think about it in a sequential order and what is "a turn". The spell is in effect when the monster starts its turn and therefore the spell effect is "it can't make more than one melee or ranged attack during its turn". This includes the entire monster's turn, as long as concentration remains. Once concentration is broken the spell ends, instantly (PHB, p.203)

If concentration is broken in any way the spell-effect ends, just as in the example with Dispel Magic: When using dispel magic to remove slow, when do the effects of slow end?.

Basically, the monster's turn has not yet "ended" at the point where the spell stops being in effect - this could be because it used its action to make one Attack which disrupted the concentration. Whether the monster/creature can attack more times afterh that depends on whether it can normally take more than one Attack action on its turn.

Similarly, if the Slow spell was broken in any way in the same round before the monster's turn -by one of the monster's allies breaking the caster's concentraion or removing the Slow effect somehow- then the monster would be able to make more than one attack, because the spell-effect ended before the monster's turn.

On the contrary, if the Slow spell is broken after the monster's turn, it would not get to use any further actions - with the exception of reactions.