How much damage is dealt/taken when that damage also reduces a creature to 0 hit points?

Question

The particular case where this question came up was when a Hobgoblin Warlord attacked my ally, who had 7 hit points remaining. The attack's damage was rolled for a total of 13, meaning they would fall unconscious. However, I am a Redemption Paladin which means I can Rebuke the Violent:

[...] Immediately after an attacker within 30 feet of you deals damage with an attack against a creature other than you, you can use your reaction to force the attacker to make a Wisdom saving throw. On a failed save, the attacker takes radiant damage equal to the damage it just dealt. [...]

The question that arose was how to calculate "the damage it just dealt". Did the Hobgoblin Warlord deal 7 or 13 damage to my ally?

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The only thing I found that somewhat answered this was the section on "Instant Death" which includes the following (emphasis mine):

[...] For example, a cleric with a maximum of 12 hit points currently has 6 hit points. If she takes 18 damage from an attack, she is reduced to 0 hit points, but 12 damage remains. Because the remaining damage equals her hit point maximum, the cleric dies.

This seems to state that the Cleric still takes 18 damage, even if her points drop to 0, but then it also states that 12 damage "remains". It's not what I would call definitive and it doesn't explicitly state that that is how damage works so I'm wondering if there's anything that makes this even clearer.

Answer 1

Going to 0 hit points does not change the damage dealt

This isn't necessarily the most explicitly defined interaction (as far as I've found) but some parts of the rules don't make sense otherwise. First, from the rules on Hit Points:

A creature's current hit points (usually just called hit points) can be any number from the creature's hit point maximum down to 0.

In other words, as you are aware, current hit points cannot be lower than 0. In addition to the example you found which states a creature with 6 current hit points can take 18 damage, there is another example nearby in the rules on Death Saving Throws:

If you take any damage while you have 0 hit points, you suffer a death saving throw failure.

If damage were reduced to the amount of hit points that were lost, then it would be impossible to take damage while you have 0 hit points.

This makes sense from another part of the rules

Also from the rules on Hit Points:

Whenever a creature takes damage, that damage is subtracted from its hit points.

This line sets up a clear argument that losing hit points is a result of damage, not the definition of damage. Thus changing the amount of hit points lost won't go back and change the damage taken.

Answer 2

Instructions Unclear. Use the ruling that makes the player's ability better.

Let's consider Rebuke the Violent:

Immediately after an attacker within 30 feet of you deals damage with an attack against a creature other than you, you can use your reaction to force the attacker to make a Wisdom saving throw. On a failed save, the attacker takes radiant damage equal to the damage it just dealt.

The idea here is that if you hurt my friend, I scream holy fury at you and hurt you back. This is a neat ability, but ruling that it does less damage the closer my friends are to death is lame.

Thematically, it makes more sense (to me) that this feature should not be weaker when the bad guy lands a coup de grâce on my friend who was barely alive already. My holy fury should be amplified in these moments, not diminished. So for this feature, definitely rule in favor of the player, that the damage potential of Rebuke the Violent is not diminished when my friends are almost dead.

Character's almost dying is tense enough. Don't make it worse by nerfing features that thematically make the most sense to use in these moments.

Answer 3

Damage is different from loss of hit points

You are correct that it isn't very clearly stated anywhere however the rules on Hit Points state:

A creature's current hit points (usually just called hit points) can be any number from the creature's hit point maximum down to 0. This number changes frequently as a creature takes damage or receives healing.
Whenever a creature takes damage, that damage is subtracted from its hit points. The loss of hit points has no effect on a creature's capabilities until the creature drops to 0 hit points.

This sets up HP reduction and damage as two different things. The loss of HP is defined by the damage and the minimum hp but the damage itself is independent.
Δhp = Minimum of d and hp₀

All sources of damage clearly define how much damage they do. For example Fire Bolt:

You hurl a mote of fire at a creature or object within range. Make a ranged spell attack against the target. On a hit, the target takes 1d10 fire damage. A flammable object hit by this spell ignites if it isn't being worn or carried.

The damage of Fire Bolt is not defined in any way by the remaining hp of the target.

It is also important to consider that if damage were the same as the hp reduction then resistance would make a creature invulnerable to the applicable damage type once they are reduced to 1 hp.

The most hp reduction they could experience would be 1 since their hp minimum is 0. That would be halved and rounded down resulting in 0 reduction in hp.

Instead the rules for resistance clearly apply to the damage which is then subtracted from the remaining hp.

If a creature or an object has resistance to a damage type, damage of that type is halved against it.

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So for your use of Rebuke the Violent the Hobgoblin Warlord would take 13 points of damage.

Answer 4

You do the full amount of the damage

Not the difference between the creature's current hp and 0.

In the basic rules, when talking about hp:

A creature's current hit points (usually just called hit points) can be any number from the creature's hit point maximum down to 0.

As you noted in the question, the total amount of damage dealt is a factor. After deducting the amount of damage necessary to bring a creature to 0 hp, if the remainder is equal to or greater than the the creature's maximum hp (not current hp) then they sufferer instant death.

Let's examine Bob, who has a maximum hp of 15, but currently has 5 remaining. Bob picks a fight with a dragon that uses a bite attack against Bob. If the damage is 1-4, then Bob should play the lottery as he is very lucky and still standing. If the damage is 5-19, then Bob goes "down to 0" hp. If the damage is 20+, then Bob is instantly killed; after deducting the 5 points to bring Bob to 0 hp, the remainder is equal to or greater than Bob's maximum hp. So there is a precedence in 5th edition for counting all damage not just going to 0 hp.

In previous editions there was a concept of "negative hit points". Basically, characters would take damage as normal. Once they reached 0 hp, they were considered unconscious. But you would still track the hp down to -10, and once you hit -10 hp you were completely dead. On top of this, every round that you remained under 0 hp and did not receive medical help, you would lose one more hp; going from -1 to -2 to -3, down to -10; effectively bleeding out. This was the old form of "death saving throws", or a mechanic to prolong the time before a character died outright.

So back to Bob and the dragon; in old school gaming, from 1-4 damage would still be lucky, 5-14 would be unconscious, and 15+ would be instant death. It doesn't matter Bob's maximum hp, just that he was now at -10.

I bring up the past edition rules only because it shows that D&D has included the concept of counting damage exceeding what is required to drop a character for quite some time.

Back to your case; your Paladin can rebuke the full 13 damage.