When does an "on their next turn" spell cast as a reaction take effect?

Question

Several spells take effect on the target's next turn. Command is one example (PHB, p. 223):

You speak a one-word command to a creature you can see within range. The target must succeed on a Wisdom saving throw or follow the command on its next turn.

Compulsion has similar effects.

A spellcaster with the War Caster feat can cast these spells as a reaction (instead of a melee opportunity attack), during the target's turn. For example:

Harrek the Barbarian is within reach of Almira the War-Cleric. Harrek wants to go hit Almira's pet troll, so he moves away from her, figuring that he can shrug off any melee attack she might make. Almira casts command, with a command of "Halt". Harrek fails his Wisdom save.

When does Harrek "Halt"? Immediately, effectively ending his turn? Or on his next turn, after he has had a chance to move and attack the troll, and after Almira has had her turn and had a chance to cast another spell.

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This point was raised in this question, but wasn't the main focus.

Answer 1

"Their next turn" refers to the next immediate turn the target would be able to take their action.

Don't forget in 5e, a turn is defined as a single person's action. So each round consists of as many turns as their are characters (PC's and DM controlled monsters/characters) in the encounter.

A round represents about 6 seconds in the game world. During a round, each participant in a battle takes a turn. The order of turns is determined at the beginning of a combat encounter, when everyone rolls initiative. Once everyone has taken a turn, the fight continues to the next round if neither side has defeated the other. -Roll 20, Combat

So if a Cleric casts "Command" on a target that has already taken their action/turn during the current round, the spell will take effect during the target's action/turn on the next round. If the Cleric casts "Command" on a target that hasn't taken their action/turn during the current round, the spell will take effect on the same round the Cleric cast the spell, on the target's action/turn.

As far as the spell being cast as a reaction to the target's turn, the target will still be able to act during this turn as planned, but the spell will effect them on the next turn the target takes.

Answer 2

Simply to offer a dissenting opinion. RAW says that it wouldn't take place till next turn (not ending the current one). However, RAI could be a bit more flexible. The emphasis that it would take place on the targets next turn could distinguish it from other spells that force the the target to act as their reaction during the casters turn.

An example of this would be Dissonant Whispers

... On a failed save, it takes 3d6 psychic damage and must immediately use its reaction, if available, to move as far as its speed allows away from you. ...

Which states that the spell forces the target to use its reaction to act.

Given that the command spell was used on the targets turn it could be argued that the command would be followed at the targets next opportunity to follow it. And with the command to "Halt" the target could immediately follow that command. This would resulting in either a lost turn, or just losing his movement but still having an action/bonus action to play with) depending on how the command "halt" is interpreted. This would (as many of these discussions are) ultimately be decided by the GM.

Answer 3

Since the spells don't make the target do anything until their next turn, the target would be able to continue their desired action this turn.

Answer 4

Harrek "halt"s immediately, effectively ending his turn.

On the PHB page 190 it says:

If the reaction interrupts another creature's turn, that creature can continue its turn right after the reaction.

In 5e, specifics rules beat general rules, and the war caster feat lets regular action spells to be cast as a reaction, similar to an opportunity attack, which interrupts the target's movement.

So It can be interpreted as RAW that its "next turn" is the remainder of its current turn after the command spell was cast as a reaction, and thus ending the target's turn.

However, the target would act normally on its proper next turn.