The discussion came up around the Oath of Redemption paladin's Aura of the Guardian feature.

This feature doesn’t transfer any other effects that might accompany the damage, and this damage can’t be reduced in any way.

That led me to look in Xanathar's guide to see how damage is applied.

Here’s the order that you apply modifiers to damage: (1) any relevant damage immunity, (2) any addition or subtraction to the damage, (3) one relevant damage resistance, and (4) one relevant damage vulnerability.

So on the one hand, if you could say immunity reduces damage to 0 so the feature still applies. But immunity is listed as happening before damage reduction in Xanathar's guide, so you could just as easily say it doesn't reduce the damage, it just ignores it. Believe it or not, immunity is never defined in the game, as far as I can tell. What do you think?

I would read the "can't be reduced in any way", so that immunity would also not negate the magically transfered damage.

Considering that immunity is applied to conditions and diseases in a way that makes them not affect you to begin with - it should behave the same for damage. (Note that I say "should" - not "does" - because as you say - immunity is never explicitly defined in the rules).

So I would say that if you are immune to damage - you straight up ignore it. Doesn't matter if the damage can't be reduced - it simply doesn't affect you to begin with.

The aura makes sure the paladin takes the damage no matter what.

So paladin, immune to fire. Another character hit by fireball. 30 damage. 

Paladin takes the 30 damage. 

If the other character was immune of course you wouldn't take the damage, but they are not and take the damage. You transfer the effect of that damage to yourself, but the fire doesn't hit you so it's irrelevant since you are not the target.

I'd argue that maybe, MAYBE, if you were immune to ALL damage one could argue that you could get away with it but imo even someone immune to all damage would take full damage in this case. 

I think your poll and question leaves out important parts of the ability which leads to possible abuse by seeing it does this compared to that... Its better to copy the full text, not your version of it, when asking questions like this 

If damage cannot be reduced in any way, then immunity will not work and the damage will be taken. You will find that this wording is almost exclusively used for the various sacrificial features and effects. If you are forced to give up your own life energy in order to protect a friend, you can't skip out on the bill. It's a damage that is taken willingly and in full - if you don't take the damage then the magic cannot protect your friend. 

The way I see it, Immunity reduces damage to zero.  It's important because there are ways to get around immunity.  For a classic example, Werewolves are damaged by silver.  Even a trivial amount of it, like when you have a silvered blade.  Commoners need that kind of thing, it's pretty much a given that player characters will have Magic items that will let them bypass immunity.  There is a subtle point.  Magic weapons in and of themselves don't exploit the vulnerability.  They will do normal damage unless they are silvered or have some other sort of magical property.  I rather like the idea of Moon-Touched weapons exploiting the vulnerability, even though those don't give a +1, for thematic reasons.

"This feature doesn’t transfer any other effects that might accompany the damage, and this damage can’t be reduced in any way."  Unless the target has a vulnerability to the type of damage.  It doesn't say what kind of damage the aura protects the Paladin from, but they have to sacrifice their own blood.  I'd rather expect that Radiant damage would overcome it, so right there in Step 1, wham, the Paladin takes double from Radiant damage effects.  It would be kind of silly, but that would probably mean they couldn't use Lay On Hands on themselves, or maybe it does make sense, depending on how you want to look at it.  Paladins are supposed to be pretty selfless.

Honestly, I'd never do anything like that to a Paladin.  It's fun from a theoretical point of view, but it's silly to expect that level of sacrifice for something a Paladin does to help people out.  I'd absolutely let them use Radiant damage on a Werewolf though, or when fighting the undead.

Immunity is not a damage reduction up to its total, it's a different buffer that ignores it entirely. Its reperate from  damage resistance, which reduce damage. XGtE shows its not a damage substraction. Its more like damage N/A (not applicable)

Resistance and Vulnerability: Here's the order that you apply modifiers to damage: (1) any relevant damage immunity, (2) any addition or subtraction to the damage, (3) one relevant damage resistance, and (4) one relevant damage vulnerability. 

Taking damage means losing HP. Taking 0 damage is the same thing as not taking damage (or "ignoring" it) - the net result is still a lack of HP loss. If damage is being applied to you and you negate it, that's the same as reducing it to 0. One way of phrasing it just happens to be more outcome-centric and the other more math-centric.

Semantic debates aside I agree with RegentCorreon that the intent of the feature is straightforward. The target takes damage, has an opportunity to reduce it as normal, and whatever the final number is gets applied to the paladin unmodified.

Aura of Guardian is also similar to damage immunity for the ally, in that effect that are directly tied to taking damage won't be suffered by the ally since it doesn't take damage ex Banishing Smite

I'm trying to make a habit of hitting the little up arrow on original posts at the very least.  If they posted a topic that caught my interest, it doesn't matter if I agree or not, it's worth at least that much to me.  I'm not going to up-vote anyone else whom I haven't already so with, because then, in all fairness, I'd have to leave the people out who I disagreed with, some of them I don't quite know if they did, and even the ones I was sure of, it wouldn't be nice to point it out.

Thanks to everyone, it's been an enjoyable thread.

There are no rules for Immunity in the PHB...The damage section discusses resistance and vulnerability, but not immunity.  It is mentioned in the MM intro, but not given a definition and is just mentioned in tandem with resistance and vulnerability.  resistance and vulnerability are specifically referenced as damage reduction (and increase), and the game does say damage can be reduced to 0, but the (I assume conscious) omission of immunity from the section seems to indicate it is not a damage modifier in the same way.

Without any definition, I'd fall back to the standard, which is (from Google, and ignoring the first result which is specific to disease)

"protection or exemption from something, especially an obligation or penalty."

I'd say if it is not mentioned in those sections, then it is not a damage reduction, it is damage exemption (as in, it's not reducing damage to 0, it is ignoring damage entirely).  While that may be semantics to damage reduction to 0 in 99% of cases, in this one I think the difference is meaningful and means that this rule would not force a creature immune to damage to take that damage.

About a year ago I looked rrreeal hard all over for the RAW definition of damage immunity, for a similar situation. Is it “reduced to 0” or is it “ignored.” I never found one. If anyone does please RE: me and lemme know. Thank you in advance.

I'd see it as "target gets hit with fireball and isn't immune" the Redemption paladin, even though immune to fire (for some reason) doesn't necessarily feels the fireball, but feels the damage of the chap taking the hit, who isn't immune, hence he feels the full pain. He isn't getting hit with fireball, he just gets pure pain.

Let's look at a real world example: disease. If your immune system is strong, it keeps you from getting sick. But it doesn't stop illnesses from entering your body, it just deals with them and dramatically reduces the risk they pose, potentially down to nothing.

By this logic, a character with immunity to a condition, damage type, or anything else simply reduces the effects down to 0, and does not prevent it from affecting them in the first place.

In the context of Aura of the Guardian, I would say the paladin is not protected by any immunities they have, just as they would not be protected by any resistances. The RAI of the feature seems pretty clear to me.

As others in this thread have correctly pointed out, damage immunity isn't defined anywhere in the rules, so the answer is whatever your DM wants it to be.  In an effort to provide actual guidance, though, we can compare the ability to Warding Bond, so your DM (or you, if you're the DM) can decide how powerful they want the Redemption aura to be relative to Warding Bond's transfer effect (bearing in mind Warding Bond also provides +1 AC).

Warding Bond deals typeless damage to the caster, much as a stirge or bearded devil can deal typeless damage.  As a result, flat immunity to all damage will protect the warder, if they have that somehow, but any given specific damage type immunity won't, and usually that's the only kind of damage immunity you can get.  Note that Warding Bond provides universal damage resistance - including to typeless - but since damage resistance is defined in the rules, this introduces no new headaches.

Note that both Warding Bond and the Aura render the damage magical - Warding Bond because a spell is dealing you damage, and the Aura because it says you magically take the damage - so there's no difference there.

So if you want the Aura to be stronger than Warding Bond's transfer effect, let typed damage immunities work on the transferred damage.  If you want the Aura to be weaker than Warding Bond's transfer effect, don't let typed damage immunities work on the transferred damage.

Any rules interpretation that can make a Fire Elemental take fire damage should be heavily scrutinized.

Edit: the ability strips away anything other than the magical damage, so I'd rule that it's some kind of direct life force drain,  not a "typed" damage that can be negated.

What's the more uncomfortable end point? That a player character who SHOULD have to make a sacrifice finds a way to cast a powerful spell without making that sacrifice? Or that a player character who SHOULD have sufficiently protected themself against a specific element using powerful temporary magics, is nevertheless harmed by that element? 

I dunno, I think that #1 is more likely to feel like a "cool trick" that makes a party member feel powerful, and #2 more likely to feel like a failure of the game system that can't be narratively justified. But also, I'm not really sure what all the sources of "can't be reduced" damage are, can someone list all of the spells or game features where this is an issue? Am I correct in assuming this is something that will only ever effect player characters, not monsters? 

This is the same (or similar) wording to Life Transference. For that reason immunity shouldn’t overcome this damage. 

Quote from quindraco >>

much as a stirge or bearded devil can deal typeless damage.

It might be nitpicking, but those creatures can create an effect that causes you to lose hit points. They aren't actually inflicting damage with that effect*, any more than a curse that reduces your max HP is inflicting damage.

The difference comes into play when you, say, have temporary HP, which can only be applied to damage. You can have temp HP and still fall unconscious from blood loss due to being sucked dry by a stirge.

* Obviously they do normal, "typed" damage on their initial hit.