Does Shadow Lore require the Bard to physically whisper, and could the target or a nearby creature detect this?

Question

A 14th level College of Whispers Bard gains the following class feature:

Shadow Lore: As an action, you magically whisper a phrase that only one creature of your choice within 30 feet of you can hear. The target must make a Wisdom saving throw against your spell save DC. It automatically succeeds if it doesn’t share a language with you or if it can’t hear you. On a successful saving throw, your whisper sounds like unintelligible mumbling and has no effect.

If the target fails its saving throw, it is charmed by you for the next 8 hours or until you or your allies attack or damage it. It interprets the whispers as a description of its most mortifying secret.

While you gain no knowledge of this secret, the target is convinced you know it. While charmed in this way, the creature obeys your commands for fear that you will reveal its secret. It won’t risk its life for you or fight for you, unless it was already inclined to do so. It grants you favors and gifts it would offer to a close friend.

When the effect ends, the creature has no understanding of why it held you in such fear.

(pg 16, Xanathar's Guide to Everything)

When using Shadow Lore, would the Bard physically whisper something that only the Target can hear, or is it more like a telepathic whisper that the target hears?

Would the target, or a nearby creature, be able to tell that Shadow Lore was used and potentially who used it?

Answer 1

You do have to actually whisper, but nobody else will hear you; what others creatures notice would be up to a GM

The feature states:

As an action, you magically whisper a phrase that only one creature of your choice within 30 feet of you can hear...

This is a result of spells and features not having any sort of "fluff" text in fifth edition. This is discussed at length in the answer to the question: "Does Burning Hands really require touching thumbs?".

This text states that you do whisper the phrase however only one creature hears it and it is a creature of your choice within 30 feet of you.

All we know is that the spell is a magical whisper which nobody else can hear. The general English meaning of the word "whisper" would certainly point to it requiring you to actually speak, and thus move your mouth which would be visually noticeable, though inaudible.
What knowledge a creature may gain from seeing your lips move, if anything, is left up to a GM.

Similarly, even if a creature could read lips, this would be left up to the GM. Perhaps your lips don't even move in normal shapes and ways; after all, on a successful save "your whisper sounds like unintelligible mumbling", quite different from what it should sound like. So whether it also can look different would be up to your GM.