Who is targeted when Scrying's target is ambiguous?

Question

The 5th level spell Scrying allows you to attempt to magically spy on a creature. The caster doesn't even need to have met the creature to attempt this.

What happens when the target the caster has in mind could be multiple creatures?

As an example, a PC attempts to Scry a vigilante they have heard tales of. Unknown to them, this vigilante is actually 3 different people alternating shifts as the vigilante. Who does the spell end up targeting? Any of the people are a valid target for the spell, but which one attempts the saving throw to resist?

Answer 1

It is up to the DM.

The ultimate answer here is "it's up to you," but let's explore it a bit since this could go a number of ways.

The description for Scrying includes (emphasis mine):

You can see and hear a particular creature you choose...

As a DM, I would interpret this as particular = specific, so in this case your NPC/persona wouldn't be a valid target. Xanathar's Guide to Everything offers an optional rule for spells with invalid targets as the following:

If you cast a spell on someone or something that can’t be affected by the spell, nothing happens to that target, but if you used a spell slot to cast the spell, the slot is still expended. If the spell normally has no effect on a target that succeeds on a saving throw, the invalid target appears to have succeeded on its saving throw, even though it didn’t attempt one (giving no hint that the creature is in fact an invalid target). Otherwise, you perceive that the spell did nothing to the target.

If we choose to use this rule, the straightforward "answer" in this case is that the spell simply fails since there isn't a single valid target.

However, as a DM, I might interpret this situation a number of other ways:

I might choose whomever is on duty as that particular person for that day, or a random member of this group.

Or, as PJRZ mentioned below, the PC might have to describe how they know of this vigilante (since scrying allows for a target "you have heard of"), and whichever person did what the PC describes is the target of the spell.

Answer 2

Invalid Target (group instead of individual)

The DM could rule that the vigilante is actually a group of people and is not a valid target. In this case caster is unwittingly trying to targeting a group of individuals. A group is not a valid target for scrying. In this case, the guidance in Xanathar's is that nothing happens to the target and the spell slot is expended. From the caster's point of view, it appears as though the target saved.

Valid Target (shared single identity)

If the DM has reason to rule that the vigilante is a valid target, e.g. because it is a single identity, then there are a few options they might go with.

The current user of the identity

If one of the persons behind the mask is currently using that identity as their own, then scrying should target them. They are currently the vigilante.

Particular Tales

If the caster is basing their target from a particular tale or set of tales, the target would be the person behind the mask in those stories.

Equal Number of Tales

If the caster is basing their target from tales that are equally distributed between the persons behind the mask, randomly select from one.

Answer 3

Who is targeted?

The spell has a target of "Self" so you are targeted.

Who is seen and heard?

You can see and hear a particular creature you choose ...

Particular means "of, relating to, or being a single person or thing". In order to do this you must have enough information to unambiguously identify that "single person or thing"; until you can do that then the spell doesn't allow you to see and hear them.

As a DM you can handle this by asking follow-up questions about precisely who they are thinking of - "The magic is having trouble locking on to a particular individual from your vague description - can you be more specific?" "The one who did [X]." Bingo!

Of course, if [X] never happened (stories are not always factual) you might have to keep drilling down.