In my last D&D session I thought I was very clever when, while in ranged combat with an enemy, I ducked behind some large boulders (providing full cover) and then targeted him with Dissonant Whispers.
You whisper a discordant melody that only one creature of your choice within range can hear, wracking it with terrible pain.
This implies I don't have to be able to see the target creature. Just that the target has to be able to hear and be within the 60 ft range.
Further interpretation of the quoted sentence says the chosen creature can hear the melody but not necessarily has to be able to hear you. A deafened creature succeeds at the saving throw but can still be targeted and take damage from it.
Is this valid in RAW?
Are there any rules against targeting single creatures when they are unseen but you can guess where they are when the spell does not rely on sight? (Concealed or invisible, but not hidden)?
Does the cover obstruct the creature's ability to hear me?
Best Answer
You don't need to see them for this spell but you still need a clear path.
You have total cover from each other in this case and therefore do not have a clear path to the target (PHB, p. 204) to the target. Take a listen to the Dragon Talk podcast for 1/19/2017 (specifically starting on 32:14 for your issue). Jeremy Crawford goes into more detail on targeting and how it is supposed to work.
He also tweeted the following clarification while leaving it to an individual DM:
Then the following tweet about the difference between visibility and cover: (emphasis mine)
The clear-path rule as a definition is widely accepted (although it is ambiguously defined in the PHB) as you cannot have total cover between the caster and the target in a straight line.
Certain spells like Sacred Flame do not require a clear path to the target by design, as detailed by JC on that podcast, since it descends upon them from on high.
Comparisons and the exceptional.
From Sacred Flame:
No such text exists for Dissonant Whispers.
The general rule for all spells is that you require a clear path to the target. Dissonant Whispers does not countermand that general rule, therefore it doesn't remove that requirement. Unlike Sacred Flame which specifically says the target can't benefit from cover.
It could go something like this:
You can target creatures you can't see with Dissonant Whispers but only if you have a clear path to it.
Credit to @Rubiksmoose for helping with the Tweets which are easier than going through the Podcast covering the same general material.