What happens to a Ghost when the Possessed creature's polymorphed form reaches 0 HP?

Question

Consider the following:

The party's resident barbarian, Conan, is targeted by a Ghost's "Possession" ability. He fails the save and is therefore possessed.

Possession (Recharge 6): One humanoid that the ghost can see within 5 ft. of it must succeed on a DC 13 Charisma saving throw or be possessed by the ghost [...] The ghost now controls the body [...] The possession lasts until the body drops to 0 hit points, the ghost ends it as a Bonus Action, or the ghost is turned or forced out by an effect like the Dispel Evil and Good spell.

Jack Black, the bard, tries to polymorph his newly possessed friend into a mouse, to make him more manageable. Failed save, the Barbarian body is transformed into a mouse.

Assume that the possession is still occurring and that the mouse is controlled by the Ghost. (I've asked about that in What happens to a Ghost when the Possessed creature polymorphs?)

Jon the Fighter strikes the mouse, hits it and deals more than enough damage to get it below 0 HP. The creature changes back to a Barbarian.

Is the Barbarian still being possessed by the Ghost afterward?

Answer 1

The condition (Possession) will end when polymorph ends

A possessed mouse shouldn't happen in the first place, however, if we presuppose a houserule that conditions can persist to invalid targets...

The feature Possession (SRD p.311) provides these other conditions on which posession ends:

1. the body drops to 0 hit points,
2. the ghost ends it as a Bonus Action, or
3. the ghost is turned or forced out by an effect like the dispel evil and good spell

When Conan the possessed Mouse takes damage he meets condition 1 (the mouse body has dropped to 0 hit points) so he'll loses the Possessed condition and will revert back to Conan the unpossessed Barbarian.

As a barbarian Conan is a humanoid and once again a RAW valid target for Possession, however, his current Possession has already ended and he will be immune to being re-possessed for 24 hours (SRD p.311).

Conan's Possession shouldn't have persisted through polymorph but, if you houserule that it did, then when he drops to 0 hp as a mouse he should lose Possession that way instead.

Answer 2

The relevant part of the "Possession" quote needed to answer this question seems to be:

The possession lasts until the body drops to 0 hit points

Again, given that we are assuming that the answer to your other question is that the mouse is still possessed by the ghost, then the question becomes: does "the body" in the quote refer to the currently possessed body (i.e. the mouse) or the originally possessed body (i.e. the barbarian)?

Obviously, if the former, then killing the mouse will end the possession and Conan, now returned to his original form, can give the ghost a good whack for possessing him. If the latter, then since the originally possessed body is still not at 0 HP after turning from a mouse back into a barbarian (i.e. Conan is still conscious and able to take Actions and such), then the possession won't end.

It depends on the interpretation. I'd say that, since no such distinction is made, and no mention of conditions like Unconscious or Incapacitated are present, it makes sense to reinterpret "the body" as "the current body" as the strictest reading, in which case killing the mouse would end possession, since the mouse's HP was dropped to 0.

Saying that, I'd argue that this might go against the spirit (no pun intended) of the possession effect, so I'd say that the barbarian would remain possessed regardless of what forms he switches between, and only the barbarian reaching 0 HP in his original body would count with regards to ending possession, but that'd be my ruling, not necessarily what the wording of Possession says.

Answer 3

Possession clearly ends with 0HP, as the target dies and the ghost is expelled... together with the soul of the possessed target. Should polymorph stay on (it doesn't in RAW), then sorry Conan, last we saw you, you were a mouse with few HP that was also possessed by a ghost.

And because I put on the EvilGM-hat and hate using polymorph as a solve-all dispel - and this is at the time of writing NOT a rules-as-written question - my ruling would be:

Conan entered the state of a non-possessed dead mouse. With less than 0HP that reverts back... And because as a GM I would be pissed of the continuous attempts to misuse polymorph as an universal solution, I'd rule that in this specific case of (mis)using it as an exorcism/dispel, the end of polymorph would be a likewise mangled mess of Barbarian corpse instead of the expected pre-polymorph-HP Barbarian. Hope someone brought a spell to revive you, Conan.^{Nice Job Breaking It, Hero!}