The party sorcerer polymorphed himself into a T-Rex and got his CHOMP-CHOMP on with the Mummy Lord, taking a few hits in the process. The T-Rex now has mummy rot which is described as:
The cursed target can't regain hit points, and its hit point maximum decreases by 10 (3d6) for every 24 hours that elapse.
There have been several questions relating to conditions, spell effects, and other happenings asking if their effects persist on a player who then polymorphs into something, but I haven't found anything related to what happens when it's the other way around.
If this were a condition, we know that it would persist on the player after leaving T-Rex form:
A condition lasts either until it is countered … or for a duration specified by the effect that imposed the condition.
However, mummy rot is not a condition, and none of the damage done by the mummy will carry over to the player's original form once Polymorph ends because:
When it reverts to its normal form, the creature returns to the number of hit points it had before it transformed.
If a polymorphed player gets mummy rot, does it persist when they leave their polymorphed form?
Best Answer
The mummy rot will persist
Polymorph says:
So, only the game statistics are replaced, and ongoing effects like curses, spells, conditions and diseases are not part of a creatures game statistics, they merely alter the creature's statistics while they influence it.
When polymorph ends, the effect of polymorph ends and the target's game statistics are no longer replaced by the statistics of the chosen beast, they again become the original creature's statistics, including the hit points that creature had.
Ongoing effects that are not part of the creature's statistics continue to apply to the creature. Here this means that the effect of mummy rot continues to affect it. The original creature's hit point maximum remains decreased by whatever amount of time has elapsed, and will continue to decrease until the creature is cured with remove curse.