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?
Best Answer
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:
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.