If the Sending spell is cast to communicate with a character whose body is inhabited by an Intellect Devourer, what happens?

Question

I have just had a PC become inhabited by an Intellect Devourer ("ID") via the ID's Body Thief ability. I am going to let the player continue using the character, but now the player will play as the ID controlling the character's body.

First, ignoring any context. Would a message sent by the caster of a Sending spell, attempting to contact the PC, reach the ID instead?

The Sending spell description says:

You send a short message of twenty-five words or less to a creature with which you are familiar. The creature hears the message in its mind, recognizes you as the sender if it knows you, and can answer in a like manner immediately. The spell enables creatures with Intelligence scores of at least 1 to understand the meaning of your message.

You can send the message across any distance and even to other planes of existence, but if the target is on a different plane than you, there is a 5 percent chance that the message doesn’t arrive.

I'm sure we can split hairs over the semantics of which mind is now "the creature's" mind. What I'm wondering is if there are there any hard mechanics that would prevent the ID from receiving the message intended for the PC? Does the character no longer exist if their brain is gone?

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Second, a the same question, with the context of the Waterdeep: Dragon Heist adventure (SPOILERS):

In Dragon Heist, Vajra Safahr is said to normally reach out to people with the Sending spell. Meloon Wardragon is a member of Force Grey and would report to Vajra. However, he is inhabited by an intellect devourer. Surely, Vajra would know something is up with Meloon if Sending exposes that he as no longer present on any plane of existence. Has she not tried to contact Meloon? Has she contacted him and reached the ID instead? Is this just poor oversight on the part of the writers?

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Third, a question of how to play this as a DM running Dragon Heist (SPOILERS):

In my game, the PC in question was originally a member of the City Watch, who would have been a candidate to join the Gray Hands. Vajra would normally use Sending to do this. It will be entertaining for the player to have the ID try to infiltrate the Gray Hands as a Xanathar Guild spy. However, this will not work out if the Sending spell cannot reach the character when they appear to be alive and well. One option is that I could have Vajra just use another means of communication.

Second option:

The other option is to elaborate on how Body Thief works. Narratively, I have said that the ID pulled itself and the PC's mind to a pocket of the outer planes, did some indescribable mingling with it, and reappeared in the Material Plane, inside of the PC's skull. Maybe enough of the PC's mind is there to contact and Sending just works? Or maybe Vajra chats with a non-physical mind in the outer planes (not what I want to do)?

Answer 1

Sending contacts the ID

This is going to be one of those answers where I try to prove a negative.

Reviewing the Intellect Devourer's Body Thief (BT) skill in detail, I noticed that at no point does the ability describe the victim as being dead until the end:

If the host body drops to 0 hit points, the intellect devourer must leave it. [...] The body then dies, unless its brain is restored within 1 round.

The ability describes the myriad of benefits that the ID gains from performing BT and the detriments to the victim, but it specifically never mentions that the victim dies until after the ID has left.

BT 'magically consumes' the victim's brain, but it does not say that it kills them, therefore it doesn't (at least not until the ID leaves). From the relevant sections you've identified, it seems the original creature's mind is simply inside the ID for the duration of BT.

Because the victim is a creature and does not initially die from BT they are still alive. Because they are alive an attempt at Sending to them will reach the creature (and the ID because the ID is occupying the creature). At this point, it's a DM decision as to whether the victim can respond when their brain has been 'magically consumed' or if the ID simply responds on their behalf. This DM would rule only the latter is permissible.

Answer 2

The sender contacts who they are familiar with

Body Thief states:

[...] the intellect devourer magically consumes the target's brain, teleports into the target's skull, and takes control of the target's body. While inside a creature, the intellect devourer has total cover against attacks and other effects originating outside its host. The intellect devourer retains its Intelligence, Wisdom, and Charisma scores, as well as its understanding of Deep Speech, its telepathy, and its traits. It otherwise adopts the target's statistics. It knows everything the creature knew, including spells and languages.

If the host body dies, the intellect devourer must leave it. [...] By spending 5 feet of its movement, the intellect devourer can voluntarily leave the body, teleporting to the nearest unoccupied space within 5 feet of it. The body then dies, unless its brain is restored within 1 round.

There are several facts which indicate that the host and the Intellect Devourer are separate creatures and therefore separate targets:

• The target of Body Thief, aka the host, is alive until the intellect devourer leaves it.
• The host can be killed without killing the intellect devourer
• The host offers full cover to intellect devourer.
• The intellect devourer adopts the stastistics of the host

Who is targetted?

Sending targets "a creature you are familiar with" so the question is, who is the sender familiar with?

If the sender became familiar with someone before the body theft, then the host is the target of sending.

An argument can be made that, given enough interractions after the body theft, the sender can become more familiar with the Intellect Devourer and unknowingly begin to target it instead of the host.

Whereas if the sender never became familiar with the host before the body theft, but has unknowingly become familiar with the Intellect Devourer thereafter, then the Intellect Devourer is the target of sending.

What happens to the target?

If the Intellect Devourer is the target, then the sending proceeds as normal: it hears the message and can reply.

If the host is the target, then the host reveives the message, but it doesn't hear it. Per the sending spell, the message is heard in the mind, but the host doesn't have a mind because their brain has been eaten.
The Intellect Devourer does not hear the message becuase it does not replace or control the host's mind, it only controls the host's body.
The sender has no way of knowing why they received no reply. The possibilities range from the target being dead, unconcious, on another plane, or even just taciturn.