For the Redemption paladin's Aura of the Guardian feature, can the damage the target would take be reduced before Aura of the Guardian triggers?

Question

The Oath of Redemption Paladin subclass gets the Aura of the Guardian feature at 7th level (XGtE, p. 39; emphasis mine):

[...] you can shield your allies from harm at the cost of your own health. When a creature within 10 feet of you takes damage, you can use your reaction to magically take that damage, instead of that creature taking it. This feature doesn't transfer any other effects that might accompany the damage, and this damage can't be reduced in any way.

It is clear that the Paladin cannot reduce the damage they take, but it is unclear whether the damage the the initial target would have taken could be reduced.

Is the rule that the damage cannot be reduced from its original damage roll, or that it cannot be reduced from what the target would have taken?

For example:

Farla the fighter has the feat Heavy Armor Master, whose description states (PHB, p. 167):

While you are wearing heavy armor, bludgeoning, piercing, and slashing damage that you take from nonmagical weapons is reduced by 3.

Farla is under the effect of warding bond, which grants her resistance to all damage.

Farla is hit by a Storm Giant's greatsword which rolls 30 damage.

Heavy Armor Master reduces this to 27, which is then reduced to 13 by resistance. Unfortunately, Farla only has 10 hp remaining, so Psi the Paladin uses Aura of the Guardian to take the damage instead.

How much damage does Psi take?

Answer 1

It is up to the GM

That said, Darth Psyeudoynm's answer is excellent and matches how I would rule in this case, so go read it. What follows is just things I would consider when thinking about this scenario, and I would consider quite a lot of things:

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What is the trigger for Aura of the Guardian's reaction?

There are at least two very different answers to this and they depend on the ideas of "damage dealt" and "damage taken". We already have a few questions that directly or indirectly ask about this distinction:

• How does Vampiric Touch interact with Resistance/Vulnerability?
• How much damage can the Guardian of Faith spell deal?
• Is the Undead Fortitude trait's DC based on damage dealt or damage taken?
• Is the value of "damage dealt/taken" calculated after resistances?

From these we can see that people do not agree whether these are different quantities or not; however, we can look to the Sage Advice Compendium (SAC) for some guidance:

Q. When a creature successfully saves against guardian of faith and takes 10 radiant damage, how much damage does that count against the total amount of damage the spell can deal? Is it 20 because that’s how much it dealt or 10 because that’s how much the target took?

A. It dealt 10 damage to the creature, so 10 is subtracted from the total. (guardian of faith)

The guardian of faith spell states:

[...] The guardian vanishes when it has dealt a total of 60 damage [...]

And from the SAC we can at least conclude that "damage dealt" is determined after resistances are applied. Notably, the answers to the above question arguing that damage dealt and damage taken are different quantities all argue that damage dealt would be the value from before resistances; so one of two possibilities remain:

1. Damage dealt and damage taken are the same quantity
2. Damage dealt is the value after resistances are applied while damage taken is the value before resistances are applied

I have not seen anybody argue for case 2, and I will not be doing so myself, but it is an option. I will now assume that damage dealt and damage taken are thus a single quantity. Therefore, the trigger for Aura of the Guardian occurs after resistances are applied.

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But does the reaction remove its own trigger? Is this a special case?

One thing we do know is that reactions occur after their triggers (from the DMG):

[...] If a reaction has no timing specified, or the timing is unclear, the reaction occurs after its trigger finishes [...]

• See Also: Do reactions interrupt their triggers or not?

Except when they don't! There are those strange cases of reactions such as shield and absorb elements, that either involve time-travel or some other odd, not-particularly-well-defined interaction with the rules. We have a lot of question about shield but I'll only link two here:

• Do you have to cast Shield prior to your opponent's attack roll?
• Does blocking an attack with the Shield spell still trigger Armor of Agathys?
• Does Absorb Elements give you resistance to damage from the triggering attack?

So we have to asks ourselves "Does Aura of the Guardian's reaction interrupt its trigger or come after its trigger?" Unfortunately, this isn't particularly clear, and it isn't even clear what happens in the cases where we know something does interrupt its trigger so unfortunately even getting the answer might not be helpful in this case.

The feature does not explicitly state that it interrupts its trigger, but I wouldn't say absorb elements and shield do this extremely explicitly either so I would really say that it is going to be up to the GM whether or not the reaction interrupts, cancels, or has some other interaction with its trigger.

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Regardless, one key sentence might make none of this matter

The Aura of the Guardian feature contains the following sentence:

[...] this damage can't be reduced in any way [...]

We cannot know how far this statements was meant to apply, or what exactly it meant when saying "this damage". Did it mean only the damage after it transfers, or the entire damage chain even from before the transfer when damage reduction and damage resistance would have applied? This, again, requires a GM's call, though I would rule that it was intended to only affect the damage from after the transfer but that is only my interpretation of the text, nor do I have any especially strong reasoning or evidence for why I would make that decision.

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Something else worth considering is what resistance and damage reduction are

These are things that express a specific character's ability to shrug off damage or their natural, well, resistance to something. This is something they benefit from due to their own circumstances, and these circumstances (likely) do not apply to the Paladin as well. However, this doesn't actually get us anywhere since things like warding bond have their damage transfer occur after resistances are applied and wading bond, just like Aura of the Guardian, involves that ever-logic-defying thing: magic.

And so, applying what I would call logic, results in the conclusion that damage reductions and resistances should not apply to damage that is transferred, and yet we have an example where they do. This can, of course be "logic'ed" away as the magic only transferring the actual felt damage, the actual harm dealt and not the actual force of the blow in its entirely, but again, we're in the world of magic and things are fuzzy...

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TL;DR: Ask the GM, magic is weird

Answer 2

Since the trigger for this reaction is "a creature [...] takes damage", I would think only the actual damage against their HP total counts. By the time the original target of the attack actually takes damage, any reductions that target has were already applied, so the damage taken that the Paladin is transfering is the reduced amount. The "cannot be reduced" clause is, to my mind, meant to say that effects like damage resistance that the Paladin has won't apply because they aren't actually being burned or cut or whatever.

This fits well with both the letter of the rules and with the fiction: The paladin is taking damage to their body in place of another creature's, but only actual injuries, not some theoretical pre-defense damage total.

I know the game does not consistently treat "damage dealt" and "damage taken" as two different things, but it does seem to be the original intent that they are different, at least going by the "Damage Resistance and Vulnerability" example in the Player's Handbook (p.197):

Resistance and then vulnerability are applied after all other modifiers to damage. For example, a creature has resistance to bludgeoning damage and is hit by an attack that deals 25 bludgeoning damage. The creature is also within a magical aura that reduces all damage by 5. The 25 damage is first reduced by 5 and then halved, so the creature takes 10 damage.

I believe the intent of this rule is to prevent double-dipping if both the original target and the Paladin have resistance to the damage -- if the Wizard and the Paladin both have resistance to Fire, and the Wizard gets hit by a blast of dragon-fire, the damage to the Wizard is reduced, then the Paladin takes that amount of damage and is not allowed to then further reduce the damage using their own resistance. Only the actual target's resistance applies. The alternative interpretation would mean that neither resistance applies, which doesn't seem right to me.

Conversely, if the original target were vulnerable to the damage, the Paladin would have to take that higher damage total -- the hit did more damage to the target because of their physiology (or a curse, or whatever), so the Paladin has more actual physical damage to absorb.

So here's the way I'd rule the scenario you laid out: Farla it hit for 30 damage. Her armor reduces it to 27, then warding bond reduces that to 13 (with the warding bond caster taking 13 as well). Psi then takes the 13 damage in Farla's place, and any effects that would normally reduce slashing damage dealt to Psi do not help.