An example: An imp turns into a rat, and someone throws a fireball at it. Does he take the damage, like a rat, or does he ignore it, like an imp?
Firstly, the Imp doesn't turn into a rat. The ability says:
The imp can use its action to polymorph into a beast form that resembles a rat (speed 20 ft.), a raven (20 ft., fly 60 ft.), or a spider (20 ft., climb 20 ft.), or back into its true form.
And secondly, the ability also says:
Its statistics are the same in each form, except for the speed changes noted.
So the Imp would still be immune to Fire (and Poison) damage while looking like a rat.
However if the Imp where to be changed by some other effect, like a Polymorph spell, then it might work differently depending upon the effect that creates the change.
An example: An imp turns into a rat, and someone throws a fireball at it. Does he take the damage, like a rat, or does he ignore it, like an imp?
Thank you!
Firstly, the Imp doesn't turn into a rat. The ability says:
And secondly, the ability also says:
So the Imp would still be immune to Fire (and Poison) damage while looking like a rat.
However if the Imp where to be changed by some other effect, like a Polymorph spell, then it might work differently depending upon the effect that creates the change.
I agree with Thezzaruz a shapechanged Imp retains all it's resistance and Immunity.