Dissonant whispers targeting a mounted combatant

Question

When a mounted target of Dissonant Whispers fails its saving throw, can it use its reaction to move the mount and itself away from the caster, or does it use its reaction to move on its own, abandoning the mount? The relevant text from the spell is quoted below:

The target must make a Wisdom saving throw. 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. The creature doesn’t move into obviously dangerous ground, such as a fire or a pit.

When this came up in a game I was running, I ruled that the knight could use his reaction to ride the horse away from the caster and continued the combat. I think RAW, though, the knight would have had to use his reaction to dismount and flee.

I've already checked the Mounted Combat section of the PHB (p.198), but it doesn't seem to contain any relevant ruling on this subject, specifically not with regards to controlling the mount outside of the rider's initiative.

Answer 1

The target must dismount (costing half its speed) and use the rest to move away

Spells do what they say they do: the target “must immediately use its reaction, if available, to move as far as its speed allows away from you.“ It says nothing about the mount so it doesn’t affect the mount.

Even if the target had some ability that allowed it to use its reaction to move the mount, it can’t because:

1. That wouldn’t be using the target’s movement
2. The target no longer has a reaction - the spell forced them to use it.

The only exception would be if they were on a mount that was so big that they could use their full movement and still remain on the mount.

Answer 2

RAW, movement options are limited when it is not your turn

What does Dissonant Whispers do?

The target must make a Wisdom saving throw. 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. The creature doesn’t move into obviously dangerous ground, such as a fire or a pit.

In a natural world (that is, one not governed by the abstractions of simplified game rules), a mounted target that was forced into moving away from the source of its pain could ride its mount away, could dismount and move away, or could fall off the mount and then move away. Within the mechanics of 5e, however, these options are constrained by the rules regarding movement. The most important of these constraints is the fact that the target is not moving on its own turn.

In general, you cannot move when it is not your turn. If something permits you to move when it is not your turn, that thing itself must then spell out the conditions under which you may move, as an example of specific over general.

A creature mounted on another creature and controlling it, can also 'direct' the movement of the mount, but since your controlled mount moves on your initiative, this option applies only on your own turn.

Thus, if the knight is being affected by dissonant whispers on the turn of the caster, the knight can move only in the way that the spell itself explicitly permits. Since the spell does not explicitly allow you to direct a mount...

The knight cannot ride the mount away in response to the spell.

If the knight cannot ride away, can it then dismount before fleeing?

Dismounting is a game term, and is governed by the 'Mounting and Dismounting' rules in the Mounted Combat section, which (emphasis mine) say:

Once during your move, you can mount a creature that is within 5 feet of you or dismount. Doing so costs an amount of movement equal to half your speed.

What does "during your move" mean? I believe that a strict RAW approach suggests that "your move" is any movement that is undertaken on your turn, as described in 'Breaking up your move' (emphases mine):

You can break up your movement on your turn, using some of your speed before and after your action. For example, if you have a speed of 30 feet, you can move 10 feet, take your action, and then move 20 feet.

If the rules had intended to allow mounting or dismounting during forced movement, then rather than saying you may dismount once during your move, they would have said once when you move, while you are moving, or during a move.

[Note that my interpretation that 'your move' is movement that occurs on 'your turn' is not universally acknowledged. Dale M, for example, believes that 'your move' is any movement you make, even forced movement, as he says in the chat discussion for this answer and others. The rest of my answer here presupposes that your move is only movement you make on your turn]

Since dismounting is a movement option that may be undertaken only on your own turn, a knight affected by a spell on the turn of the caster may not dismount unless the spell itself explicitly permits dismounting. Dissonant whispers does not.

The knight cannot dismount the mount in response to the spell.

If the knight can neither ride the mount away nor dismount it, can the knight simply fall off the mount and then move away?

Suppose the afflicted knight moves off the mount without dismounting. This movement is explicitly permitted by the spell. Once the knight has moved out of the square of the mount, being unsupported, the knight would then fall to the ground. Note that falling is not a form of movement that you elect; rather, it is something that happens to you, and thus can occur regardless of whether or not it is your turn.

Further, falling from a mount has the consequence of "landing prone in a space within 5 feet of it" (note that this is a specific exception to the general falling rules, which state that you do not land prone if you have somehow avoided damage from a fall, so even if your fall from the mount does no damage, you still land prone).

The RAW response to dissonant whispers is for the knight to fall off the mount, landing prone in a square adjacent to the mount but away from the caster, and then continue their movement away.

Once the knight is off the mount and prone on the ground, the spell dictates that they must continue their movement away from the caster, as far as their speed allows. They can now do one of two things. They may crawl away from the caster, with each foot of crawling costing two feet of movement. Or, they may stand up at a cost of half their speed and then continue to move away from the caster.

One might reasonably question whether the knight can stand up if it is not their turn, given that they cannot, for example, dismount. The rules for standing up say:

Standing up takes more effort; doing so costs an amount of movement equal to half your speed. For example, if your speed is 30 feet, you must spend 15 feet of movement to stand up. You can't stand up if you don't have enough movement left or if your speed is 0.

Note that standing up does not take your movement, it simply requires an amount of movement. Dismounting happens only during your move (on your turn), but standing up is not so limited. Standing up requires only that you have movement, and in this case that movement is supplied by the dissonant whispers spell; it is not your movement, but it is movement you are permitted to spend.

Once the knight affected by dissonant whispers has fallen off their mount, they may then either crawl away, or stand up and move away, until they have exhausted the movement provided by the spell.

Answer 3

Sensibly you would imagine the afflicted target to dismount and run on foot, but there is an interesting potential loophole.

You ask:

can [the target] use its reaction to move the mount and itself away from the [source], or does [the source] use its reaction to move on its own, abandoning the mount?

The more sensible reading of the mounted combat rules, favours Dale M's answer. However, there is a potential loophole, that while perhaps not agreeing with the spells narrative, may allow the target to get further away from the source of the spell, and should be preferred.

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Firstly, laying some ground work:

Actions and movement aren't the same

Dissonant Whispers states the target:

must immediately use its reaction, if available, to move as far as its speed allows away from you.

However reactions are not the same as movement, they are a 'special action':

Certain special abilities, spells, and situations allow you to take a special action called a reaction

(emphasis added)

So the only way I can see to rectify this disparity, is that the specific reaction granted by the spell (and is immediately used) overrules the general rule that restricts movement to be on your turn:

On your turn, you can move a distance up to your speed.

(emphasis added)

So now, after this reaction is taken, the target's movement can be used on turns other than your own.

Mounting and dismounting:

Getting on and off the mount is not an action, but doing so costs you movement:

Once during your move, you can mount a creature that is within 5 feet of you or dismount. Doing so costs an amount of movement equal to half your speed.

So note: it may not be the most efficient use of your movement to dismount.

When you choose to use the mount as a controlled mount :

[its] initiative [...] changes to match yours when you mount it.

and

it can move and act even on the turn that you mount it.

(emphasis added)

These two passages together, to me, implies that the mount can move and act on your turn, even though it's initiative will have changed to be after or before your own, as a specific change to the general rule. My understanding is that if the mount's movement was only allowed to be used on the turn you mount it, then the word 'even' would not be needed.

Further:

It moves as you direct it, [...]

Without stipulating this requires an action (which would have only been possible on your turn normally), you may after being targeted 'direct' the mount to move (which is neither an action or movement), although it still cannot move or take any actions until it is your or it's own turn as you did not mount it this turn (i.e. the spellcaster's turn), and as it is not the target of the spell it can still only move on your initiative.

Loophole

The loophole comes with a, possibly extremely generous, reading that so long as the mount can move faster than you, you are compelled to dismount the steed, then remount it (using all your speed), thus allowing the steed to "move and act [...] on the turn that you mount it", and then you must direct it (which does not use an action or movement of your own) to move it's full movement away from the caster.

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This interpretation is definitely counter to the usual, and more sensible reading of the rules involved, but I don't think it's so far removed that this is not something worth considering. This is all despite the hilarious image of the rider hastily dismounting and remounting their steed before it gallops off. If you wanted to work around that image, I would simply advise you to rule that you can use all of your movement, not in dismounting and remounting the steed, but in directing it to move.