Can phenomenal amounts of damage still be non-lethal?

Question

As a follow up to this question: is it possible to deal non-lethal damage even after dealing an enormous amount of damage to a target in a single turn?

Answer 1

Yes.

There are a few rules that might apply here. The first is the Instant Death rule on p. 197 of the PHB:

Massive damage can kill you instantly. When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum.

Note that this is primarily a rule for player characters; the default rule for how damage affects monsters is on the next page:

Most DMs have a monster die the instant it drops to 0 hit points, rather than having it fall unconscious and make death saving throws.

Since most of the time monsters and NPCs are dead as soon as an attack takes them to 0 HP (recalling that there are no negative HP in 5e), the instant death rule doesn't apply to them.

Finally, there is the rule for Knocking a Creature Out, also on p.198:

Sometimes an attacker wants to incapacitate a foe, rather than deal a killing blow. When an attacker reduces a creature to 0 hit points with a melee attack, the attacker can knock the creature out. The attacker can make this choice the instant the damage is dealt. The creature falls unconscious and is stable.

So, normally when a monster takes damage that would reduce its HP to 0, it dies. If it's from a melee attack, you as the attacker have the option of just knocking them unconscious. The amount of damage isn't a factor at all, since it doesn't apply to monsters or NPCs.

Answer 2

If a character or monster is dealt massive damage the creature is dead, regardless of the attacker's intentions

The massive damage rule from the PHB is:

Massive damage can kill you instantly. When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum.

This rule is an exception to the normal death saving throw rules applied to creatures at 0HP.

Normally this massive damage rule wouldn't apply to monsters as they automatically die once they reach 0 HP. There are however exceptions to this, and when those exceptions apply so do the massive damage rules.

Monsters and Death

Most DMs have a monster die the instant it drops to 0 hit points, rather than having it fall unconscious and make death saving throws.

Mighty villains and special nonplayer characters are common exceptions; the DM might have them fall unconscious and follow the same rules as player characters.

So, if we are in a situation where a monster is unconscious at 0 HP, we are already at the situation where the normal PC rules would apply.

Now we come to the meat of the question, what happens if an attacker tries to knock a creature unconscious despite the fact that they have dealt massive damage?

The rules are:

Sometimes an attacker wants to incapacitate a foe, rather than deal a killing blow. When an attacker reduces a creature to 0 hit points with a melee attack, the attacker can knock the creature out. The attacker can make this choice the instant the damage is dealt. The creature falls unconscious and is stable.

This is also an exception to the dropping to 0 HP and making death saving throw rules.

D&D is an exceptions-based game:

Remember this: If a specific rule contradicts a general rule, the specific rule wins. (PHB)

When an exception and a general rule disagree, the exception wins. (XGtE)

So we have two exceptions to the same rule that we need to adjudicate between. To do that we need to decide which is more specific (or alternatively, which is more general). This is surprisingly straightforward.

We simply ask:

Which rule applies in a narrower amount of potential circumstances when a creature is at 0 HP?

The answer here is clear, the massive damage rules are more specific. It is entirely possible to bring a creature to 0HP without dealing massive damage.

To put it another way, situations where you cause a creature to be 0 HP and you have dealt massive damage is a strict subset of the situations where you cause a creature to be 0 HP. You can apply the incapacitation rules to the superset of situations, while you can only apply the massive damage to the subset.

In particular, you have the option to apply the non-lethal damage rule to every situation where a creature is brought to 0 HP, including situations where massive damage has been dealt. You cannot, however, apply the massive damage rule to every situation where a creature is brought to 0 HP.

Thus the massive damage is the more specific rule and supersedes the more general incapacitation rule.

Narratively this also makes sense. It is possible for a creature to intend to knock another creature out, but to misjudge that attack and still kill the creature they were attacking.

Answer 3

The existing answer from Marq is nice, but it doesn't deal with the problem if a PC is attacked nonlethally. So I want to adress this issue as it actually might come up in my campaign.

I would apply the rule that specific trumps general. The rules for knocking out a creature are more specific than the rules for taking damage, as it is a special way of dealing damage.

Another way to state this is, that the rule for dying outright are under the heading 'Dropping to 0 hit points' PHB 197. The rule about 'Knocking a create out' has its own definition of what happens when the creature falls to 0 hit points, which is therefore more specific.

So assuming the rule for knockout is more specific than the rule for dying outright, we see that this rule specifically states that:

The creature falls unconscious and is stable.

This implies to me that the creature is not dead, since a dead creature cannot be stable (and probably doesn't count as unconscious). So a PC would not die from nonlethal damage even if it reduced him to the negative of his max hitpoints.