My character has acquired a suit of Scorpion Armor. Unfortunately, the armor is cursed and deals 100 damage (or half on successful saving throw) when the character dons it. This will potentially kill my character instantly.
Would Protection from Poison help?
From the Protection from Poison spell description:
For the duration, the target has advantage on saving throws against being poisoned, and it has resistance to poison damage.
By RAW, you have resistance to poison damage (halving damage), so 50 poison damage on a failed save or 25 damage on a successful one. You also have advantage on saving throws that inflict the Poisoned condition. The curse on Scorpion Armor does not inflict this condition, so you would not receive advantage on the saving throw.
This very fine distinction is a valid consideration when you compare the Protection from Poison spell description and the two following similar effects:
Dwarven Resilience: You have advantage on saving throws against poison, and you have resistance against poison damage.
Antitoxin: A creature that drinks this vial of liquid gains advantage on saving throws against poison for 1 hour.
Note the wording difference: "advantage on saving throws against being poisoned" compared to "advantage on saving throws against poison." Since this distinction is in the official rules, by RAW we should follow suit and apply it.
(In this related question about green dragon's breath which causes poison damage and Protection from Poison, the heavily supported and accepted answer states that the saving throw advantage should apply even when the Poisoned condition is not caused by the dragon's breath.)
The spell protection from poison gives the following benefits when cast on you:
For the duration, [you have] advantage on saving throws against being poisoned, and [you have] resistance to poison damage.
The armor you are trying to wear does not poison you; it deals poison damage to you. This is an annoyance of terminology in 5e: there is a condition called "poisoned" and there is "poison damage." The two often show up together but they are distinct, because one is a condition and one is a damage type. If you pretend they were called the "sickened" condition and "toxic damage" the disparity might be a little more clear.
Since the armor is not trying to poison you (it can't give you the poisoned condition), you are not making a saving throw against being poisoned. You are making a saving throw against taking damage that happens to be of type poison. You do not have advantage on the saving throw.
However, whether you succeed and take half damage (about 50) or fail and take full damage (about 100) from the armor's curse, that amount of poison damage is then reduced by half (to about 25 or about 50, respectively). You do have resistance to the poison damage.
So, yes, you would be benefited by using that spell before putting on the armor, but only with the damage resistance, not with the saving throw.
The target has advantage on saving throws against being poisoned, and it has resistance to poison damage.
Clearly you would have resistance to the poison damage.
I also believe that you would have advantage on the saving throw, based on this related ruling.
After the feast, you have immunity to poison for 1 hour (which would be a good time to put your armor on or take it off).
The creature... becomes immune to poison.
Jeremy Crawford ruled on twitter that the immunity to poison part of Heroes' Feast applies to both poison damage and the poisoned condition.
Q: Does the Heroes Frast spell grant immunity to poison damage or just the poison condition?
A: The heroes' feast spell grants immunity to poison in any form—damage and the condition.
This can logically be applied to how protection is meant to work. Both spells are equally ambiguous about poison damage vs the poisoned condition (it seems as though the designers consider them to be very closely related). Since one spell has an official ruling, we should be able to use that for a Rules as Intended interpretation to help resolve the RAW ambiguity. The spells are not an exact match, but for me the important takeaway is how loosely the designers treat poison (seeming to conflate the damage type and condition in various places).
