You don’t revert to your original form, but beyond that it’s up to the DM
The rules for Combining Magical Effects (PHB pg 205) come into play here:
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap.
For example, if two clerics cast bless on the same target, that character gains the spell's benefit only once; he or she doesn't get to roll two bonus dice.
So what is important here is to determine which effect is the most “potent”.
There is no clear guidance in the rules on how to determine potency when two spells have the same effect. There are some reasonable options in this case:
- If the spells have the same effect, the one with the longest remaining duration is most potent (think a sorcerer who has used their extended spell meta magic to double the duration of the first casting)
- The spell cast last is the most potent
- The first spell effect remains the most potent until it ends, at which point the second effect takes over.
My ruling from the games I DM is that the players can choose when they cast the second spell if they want to use either #1 or #2, but only if they are casting it on an ally. If they are casting the spell on an enemy I apply #2.
In your case these two options are functionally equivalent, so you would look like an Elf that turned into a Dragonborn.
That it is not determined by the rules, but rather at GM's discretion.
There seem to be some controversy on this matter!
Mike Mearls: NO
On the somewhat related question shown up by Sdjz (Posession isn't exactly a spell), tim-grant quotes Mike Mearls:
Does Charm Person ends if polymorph is cast on the charmed humanoid, changing its type to beast?
Mike Mearls: nope, restriction applies to targeting - sticks after successful cast #wotcstaff
Since Charm Person requires a Humanoid target, the possession should not end by a successful Polymorph, according to Mike Mearls.
Note that Mearls' Tweets are not considered official but just how he would rule them at his table. Read them as advice from an experienced GM, but remember they are not "official" in the way that Jeremy Crawford's (cited below) are.
Jeremy Crawford: Yes
However the accepted answer on that thread specifies that changing shape into a non-humanoid form, will end form-dependent spells.
As specified by BlueMoon93, Jeremy Crawford specifies in a Sage Advice:
In #DnD, the exceptional trumps the general. (No longer being a valid target trumps condition carryover.)
As @Adam says in a related answer:
Dominate Person requires a humanoid target. All official PC races are humanoid, so while the druid is in its normal form it can be targeted and charmed by a Dominate Person spell. If the druid wildshapes into a beast, its type changes to match that beast. So, the druid is no longer a humanoid, the target of Dominate Person is invalid, and the spell ends. In this way, it works in reverse of the Dominate Beast spell.
Dominate Monster works on any creature. Whether in beast form, humanoid form, or even in a Circle of the Moon druid's elemental form, the druid is still a creature. Thus, even going in and out of wild shape won't make the druid an invalid target. Since the target is still valid, the Dominate Monster spell will still have full effect, even if the druid wild shapes.
Best Answer
According to the ghost attributes in the Fifth Edition System Reference Document, the possessed character may be targeted by the sending spell without any interference from the ghost.
Does the ghost receive/hear it?
No, the ghost does not receive or hear the sending message. The ghost's possession ability indicates it cannot access the mind of the possession target. The ghost also cannot be targeted by the sending spell, so its effects would not apply to the ghost.
Does the player receive the sending?
Yes, as the quote above indicates, the target of possession does not lose awareness. They remain a valid target for the sending spell.
Can the player or the ghost reply to it?
If the player casts the sending spell, they can send a message to the possessed character. The possessed character can reply. The ghost is not necessarily aware that the spell was even cast.
If they reply, what voice will my PC hear - the character's or the ghost's?
Sending never specifies what voice the message is sent in.
The target recognizes the caster as if it knows the caster. Thus, it would be reasonable to rule that the voices heard in the characters' minds would be like the characters remember each other, not necessarily as they actually sound. For example, if two characters had not met since childhood, their voices might have changed, but the way they remember each other's voices would not have changed to how they actually sound. This is entirely up to the discretion of the DM.