For how long does a creature run/stay away after failing the saving throw against the Dissonant Whispers spell?

Question

I'm playing a Bard in a 5E campaign. One of my most useful spells has been Dissonant Whispers. It's saved the party quite a few times but the question has come up and I've not found any answers in the rules as written for the spell. On a failed save, the creature takes 3D6 psychic damage and the spell causes it "to move as far as its speed allows away from you." However, there's no explanation as to how far it goes or if it ever comes back.

How far and for how long does the target run away?

(At present, we have have house ruled that it is gone for the duration of the battle, but we're considering a dice roll with a minimum of 5 rounds.)

Answer 1

It runs away from you at its normal speed and can return immediately

Dissonant Whispers (PHB p.235) states

On a failed save... and must immediately use its reaction, if available, to move as far as its speed allows away from you. The creature doesn't move into obviously dangerous ground...

So if the creature has its reaction available it uses its reaction to move its speed away. The creature does this immediately so it may not dash or do anything else to increase its speed or make the area safer or... anything. It just moves its speed away if safe to do so. Even if the creature had Dashed on its turn it doesn't matter since dash only lasts for the current turn.

The spell is instantaneous duration and doesn't list any lasting effects. While the DM could make a ruling like, "The creature is afraid of getting hurt again and chooses to avoid you", there is nothing in the spell forcing this to happen. The creature can choose to run right back toward you on its turn.

Answer 2

It would move a distance equal to its normal speed (which for many humanoid creatures is about 30 feet, but the DM should check the Monster Manual for other creatures).

It's up to the DM if or how quickly the creature returns. But as this spell has a duration of "Instantaneous", it could use its basic movement next round and attack right away (assuming that the party haven't "legged it" in the other direction). I don't see any evidence that it lasts beyond the current round.

Remember, this only happens if it still has its reaction remaining. If it doesn't, it just takes the damage and remains where it is, and probably attacks next round if it hasn't had enough yet!

Answer 3

It’s instant on your turn as a reaction, so I would rule also that it wouldn’t be more than its regular movement (usually 30 feet). If the spell started on its turn, the targeted creature must move max distance, then it would waste its action to Dash and move 60 feet. As the spell is written, it only burns its reaction and forces it to waste its movement on following turn.

A good plan would be to cast it at the max range of 60 feet and then use your movement (usually 30 feet) to move away, leaving a gap of at least 90 feet. That would give you 3 turns if it moves normally - 2 turns, if it wastes one Dashing - before it can counterattack.