Ok,so when I say lose,I mean something like a item or something that makes,say,a ancient red dragons fire immunity to become Resistance?

Does such a thing exist,or will I have to homebrew?

EDIT:in other words,I want to be able to have:any spell or effect you create ignores resistance to fire damage and treats immunity to fire damage as resistance to fire damage

(Taken from the not available on here pyromancy sorcerer 18th lv ability) is there anything that can do that,like a item or subclass (other then the above,cause I need something makeable in the dndbeyond character maker)

Edit2:ok so I found a version of the pyromancy sorcerer in the homebrew section,and unless something better/official comes up will use that

you'll have to homebrew. I recommend doing it in a way that treats immunity as resistance.

That's what I thought,but I wanted to make sure,thanks.

You will have to homebrew it. There isn't a game feature that can be used to counter immunity to a damage type.

However, there are several ways you could go about home brewing it. The easiest would be either to enhance the Elemental Affinity feat so that it reduces immunity to an element  to resistance to an element in addition to its current effect of taking away resistance to the specific damage type. If you wanted more of a feat tax you could add a half feat that requires elemental affinity as a prerequisite that would reduce immunity to resistance. (I suggest half feat since just reducing immunity to resistance seems like an expensive price considering it takes 2 feats to get there but I could see it either way).

Homebrew for sure; immunity is immunity.

I know you can overcome resistances in a number of ways such as Elemental Bane, but I don’t know of a single way to cancel immunity to a damage type.

Other than casting Wish I don't think there's anything that can let you remove or downgrade a creature's immunity to something. You're definitely in homebrew territory, as others have said.

Quote from Cyb3rM1nd >>

Other than casting Wish I don't think there's anything that can let you remove or downgrade a creature's immunity to something. You're definitely in homebrew territory, as others have said.

That's a good point. We should also note that the closest "offically accepted" usage (granting immunity) is limited to a singular, specific spell or magical effect, not an entire damage type.

I think any DM could agree to the inverse of removing immunity to a singular, specific spell or magical effect. Stripping immunity to an entire damage type would be a solid no-go from me, but your mileage may vary. With Wish it's always possible™.

[edit]Solid runner-up status to Polymorph/True Polymorph transforming a target into a creature without that immunity.

The Feat Elemental Adept will let you ignore Resistance.  The spell Elemental Bane, as mentioned above, also does this.

The only thing I know of that ignores Immunity is the penalty a CASTER gets when they use "Overpower" ability of an Evocation Wizard.

That is, even if you are immune to Necrotic damage, you still take necrotic damage when you Overpower more than a single spell before resting.

Quote from Cyb3rM1nd >>

Other than casting Wish I don't think there's anything that can let you remove or downgrade a creature's immunity to something.

 Polymorph it into something with 1 hp, then follow up with the elemental damage; the overflow damage will not be affected by resistance or immunity.

Quote from Pantagruel666 >>

 Polymorph it into something with 1 hp, then follow up with the elemental damage; the overflow damage will not be affected by resistance or immunity.

Uhhhhhh... since when? Any RAW quote to back that up?
I can't imagine many DM's letting that slide.

Quote from Emmber >>

Quote from Pantagruel666 >>

 Polymorph it into something with 1 hp, then follow up with the elemental damage; the overflow damage will not be affected by resistance or immunity.

Uhhhhhh... since when? Any RAW quote to back that up?

 "If it reverts as a result of dropping to 0 hit points, any excess damage carries over to its normal form." Though on reflection that's not clear, it depends on whether the overflow is 'fire damage' or just 'damage' and whether resistances can potentially be applied more than once.

That's just it's way of saying the excess damage isn't wasted by being lost into the void. It's still going to be applicable to any resistances or immunities or vulnerabilities the normal form has.

Quote from Pantagruel666 >>

Quote from Cyb3rM1nd >>

Other than casting Wish I don't think there's anything that can let you remove or downgrade a creature's immunity to something.

 Polymorph it into something with 1 hp, then follow up with the elemental damage; the overflow damage will not be affected by resistance or immunity.

 
"The transformation lasts for the duration, or until the target drops to 0 hit points or dies. "

"When it reverts to its normal form, the creature returns to the number of hit points it had before it transformed. If it reverts as a result of dropping to 0 hit points, any excess damage carries over to its normal form. "

If you polymorphed a fire elemental into a 1 hit point bunny then did 100 points of fire damage then 1 damage would be used to the bunny form which drops to 0 and instantly returns to its normal form, a fire elemental, then you apply the 99 remaining fire damage but since a fire elemental is immune to fire, the damage has no effect. That's 99 fire damage utterly wasted.

I'm not sure why you think "carried over to normal form" means we ignore everything about that normal form.

Quote from Pantagruel666 >>

Quote from Emmber >>

Quote from Pantagruel666 >>

 Polymorph it into something with 1 hp, then follow up with the elemental damage; the overflow damage will not be affected by resistance or immunity.

Uhhhhhh... since when? Any RAW quote to back that up?

 "If it reverts as a result of dropping to 0 hit points, any excess damage carries over to its normal form." Though on reflection that's not clear, it depends on whether the overflow is 'fire damage' or just 'damage' and whether resistances can potentially be applied more than once.

It is always damage of the relevant type. Remember that the full damage is always rolled without regard to whether the recipient will reduce it or not.

If the attack you are hit with deals 80 fire damage, and your shifted form hits 0 HP after taking 30 damage, you have 50 fire damage carrying over. If your normal form has resistance to fire damage, it kicks in and reduces the carried over damage down to 25.

If you have the same situation with swapped resistances (resistance in shifted form & none normally), then that 80 fire damage is immediately cut down to 40 damage, and only 10 damage carries over. It's a one-way bottleneck for order of operations.

Reverting while still taking damage. Damage is damage, and the only thing rollover damage represents is the fact that you took all the damage of the attack. If the damage were representative of 'system shock', then that would be representative of being on the receiving end of an ability that explicitly causes it, and it would explicitly say so.

Damage rolled over from Wild Shape/Polymorph is simply the result of you taking all of the applicable damage. Your transformed HP soaks as much as possible with the resistance/immunity/vulnerability of the specific form & specific attack. If the attack drops your transformed HP to 0, you revert to your original form, and you resolve taking the remaining damage with your original form's resistance/immunity/vulnerability.

Damage normally is instantaneous - but Polymorph explicitly says:

If it reverts as a result of dropping to 0 hit points, any excess damage carries over to its normal form.

That stops it from being instantaneous and explicitly says the rest of the damage is then taken by the normal form. In that way it implies that the normal form takes the rest of the damage as it usually would - which means resistances immunities and vulnerabilities are factored in. Any other interpretation - I feel - is reading far too much outside the lines.

Quote from Emmber >>

Damage normally is instantaneous - but Polymorph explicitly says:

If it reverts as a result of dropping to 0 hit points, any excess damage carries over to its normal form.

That stops it from being instantaneous and explicitly says the rest of the damage is then taken by the normal form. In that way it implies that the normal form takes the rest of the damage as it usually would - which means resistances immunities and vulnerabilities are factored in. Any other interpretation - I feel - is reading far too much outside the lines.

It's overflow damage. I mean, if a halfling gets polymorphed into a bear and then killed with an arrow, the arrow isn't going to even hit the halfling so there's some other effect that is transferring the damage.

Yea. It's the Polymorph spell itself by explicitly stating that the excess damage is carried over. It gives no specific indication about what to do with that damage so the only reasonable conclusion is to follow the general rules of the game. Saying the excess damage ignores any immunities or resistances or vulnerabilities of the normal form would be extrapolating a new specific rule that isn't stated anywhere.

I mean - you have to admit that this is at least the RAI if not explicitly RAW - especially since there's nothing that can overcome immunities anywhere else.

Quote from Emmber >>

Yea. It's the Polymorph spell itself by explicitly stating that the excess damage is carried over. It gives no specific indication about what to do with that damage so the only reasonable conclusion is to follow the general rules of the game. Saying the excess damage ignores any immunities or resistances or vulnerabilities of the normal form would be extrapolating a new specific rule that isn't stated anywhere.

I mean - you have to admit that this is at least the RAI if not explicitly RAW - especially since there's nothing that can overcome immunities anywhere else.

 The normal flow of damage is "apply damage negation, apply damage resistance, apply what remains to hit points". The question is whether overflow damage restarts the entire cycle or just continues step 3.