Can non-preventable damage be circumvented by Relentless Endurance?

Question

Some spells and other effects say "this damage can't be reduced or prevented in any way." Wish, for example, says

The stress of casting this spell to produce any effect other than duplicating another spell weakens you. After enduring that stress, each time you cast a spell until you finish a long rest, you take 1d10 necrotic damage per level of that spell. This damage can’t be reduced or prevented in any way.

This answer clarifies that damage is still taken even if the excess damage doesn't reduce your HP any further.

Therefore, if I use Relentless Endurance to cause the damage to reduce me to 1 HP, I am still taking the damage (i.e. it is not being reduced by resistance or prevented by immunity).

If I get hit by an ability that says the damage can't be reduced or prevented would Relentless Endurance work to keep me at 1 HP?

Answer 1

The phrasing "damage can’t be reduced or prevented in any way" should supercede relentless endurance

Relentless Endurance. When you are reduced to 0 hit points but not killed outright, you can drop to 1 hit point instead. You can’t use this feature again until you finish a long rest.

The relevant part of wish says:

After enduring that stress, each time you cast a spell until you finish a long rest, you take 1d10 necrotic damage per level of that spell. This damage can’t be reduced or prevented in any way.

While this is a case of two specific effects conflicting, I think that the wording of wish makes it clear that it should supercede the wording of relentless endurance.

The damage from wish is explicitly and non-optionally excluded from any effects that would reduce or prevent the damage. However, the wording of relentless endurance makes it an optional ability. Requirements should supercede something that is optional.

The wording even says "in any way". This kind of emphasis (which is not necessary for the spell's wording and not usually put in there) also implies that it should be considered first and that it overrides any other effects.

There is also the fact that wish is an incredibly powerful 9th level spell. Allowing a racial feature to override it seems like a gross power imbalance. For example, if the orc wizard casting wish was at 1 hp at the time that the damaging effect of wish was triggered (and the effect did not outright kill them) then relentless endurance would allow essentially completely nullify the damage.

In the end, this is a specific vs specific effect that does not have a truly explicit resolution. Thus, the DM will have to make the final call. However, I think, all things considered, that relentless endurance should clearly not be allowed to reduce damage from effects like these.

Answer 2

Below I lay out two cases. Two different ways of looking at this situation. As a GM I would 100% let my Half-Orc Wizard player do this (see case 2). As a PC I would 100% my GM not to let me do this (see case 1). After work I will be able to do a lot more research on this and will plan to update my answer.

Case 1: Can't > must. There is an idea in game design that a rule saying you can't do something supersedes a rule stating that you can or must do something. With this in mind - if you are taking damage that can't be reduced you may not use your racial feature that replaces an amount of damage you take with another lesser amount (reduces damage).

Case 2: The damage from Wish reduces you to 0 hit points, and you are not killed outright. You use your racial feature "Relentless Endurance" which states - "When you are reduced to 0 hit points but not killed outright, you can drop to 1 hit point instead. You can’t use this feature again until you finish a long rest." Wish did its non-preventable, non-reducible damage that reduced you to 0 hit points (and did not kill you outright) which put you in a position where you met the criteria to use Relentless Endurance. Wish did it's thing, now Relentless Endurance can do it's thing.

A very fine line here is that Relentless Endurance does not say "When you would be reduced to 0 hit points [do something else]" but instead states "When you are reduced to 0 hit points" [do something]. I'm thinking that it goes like this: Krall the Destroyer has 10hp and takes 10 necrotic damage form Wish putting him at 0hp. Krall the Destroyer uses Relentless Endurance and is now at 1hp.