The Dissonant Whispers spell description (PHB, p. 234) states:
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 War Caster feat (PHB, p. 170) states:
When a hostile creature’s movement provokes an opportunity attack from
you, you can use your reaction to cast a spell at the creature, rather
than making an opportunity attack. The spell must have a casting time
of 1 action and must target only that creature.
If the creature failing the Wisdom saving throw from Dissonant Whispers moves past another character provoking an opportunity attack (as per this Q&A), and that character also has the War Caster feat; then they also cast Dissonant Whispers as an opportunity attack using the War Caster feat (and the creature also fails the Wisdom saving throw) what happens? Since the creature is already using their reaction to move:
- does it have no effect, or
- do they then move their speed minus the distance already moved from
the previous Dissonant Whispers?
Best Answer
This will not make the chain you're looking for.
While the movement from Dissonant Whispers does provoke opportunity attacks, and War Caster permits Dissonant Whispers to be cast in lieu of an opportunity attack, all of this is limited by the target's action economy:
A second casting on the same target will do damage as normal, but the target has already used their reaction (to move from the first Dissonant Whispers), so the second will have nothing to forcibly convert into movement and thus does not affect their movement from the first Dissonant Whispers at all.