If an attack causes additional damage after a saving throw, is it counted as a separate source of damage for the purpose of failed death saves?

Question

If an attack causes additional damage after a saving throw, is it counted as a separate source of damage for the purpose of failed death saving throws? In other words, does it cause another failed death save, in addition to the failed death save(s) resulting from the initial damage?

For instance, consider the wyvern. Wyverns have a Stinger attack:

Stinger. Melee Weapon Attack: +7 to hit, reach 10 ft., one creature. Hit: 11 (2d6 + 4) piercing damage. The target must make a DC 15 Constitution saving throw, taking 24 (7d6) poison damage on a failed save, or half as much damage on a successful one.

If a character at 0 HP is hit by the Stinger attack, they will immediately take some piercing damage. Then, they would ostensibly have to make a Constitution saving throw, taking some amount of poison damage either way.

If the wyvern is within 5 feet of the unconscious (and presumably prone) character, then its attack will be made at advantage. If the wyvern's attack hits the character from within 5 feet, it will automatically be a critical hit, which will cause 2 failed death saving throws just from the initial damage.

Then, if the poison damage from after the Con save is considered a separate instance of damage, it will cause a third failed death save, allowing the wyvern to kill a character that is at 0 HP with a single attack. (The wyvern can also use Multiattack to attack with its Bite and then its Stinger, meaning even a character that hasn't gone down yet could be rapidly killed in this way.)

Am I correctly interpreting the rules?

I was unable to find anything in the Sage Advice Compendium commenting on how damage done after a saving throw (as a result of an attack) is treated for the purpose of death saving throws.

Answer 1

According to the lead rules designer, it seems the answer is that they are different sources.

The general rules on how to apply damage from an attack are a little vague in this situation:

3. Resolve the attack. You make the attack roll. On a hit, you roll damage, unless the particular attack has rules that specify otherwise. Some attacks cause special effects in addition to or instead of damage.

The Wyvern does have special attack rules and, as Dale M has pointed out there is an extra conditional "gate" on the second poison damage which would pertain to the "unless" clause of the rule above (while the first half of the damage does not).

---

It is also illustrative to note how the DMG talks about poisons. Wyvern Poison is an injury poison (meaning it is applied in conjunction with a weapon attack, not inhaled, ingested, or applied topically):

Injury. A creature that takes slashing or piercing damage from a weapon or piece of ammunition coated with injury poison is exposed to its effects.

This description of how the poison works talks about its damage as separate from the weapon's damage which indicates (to me at least) that it is intended to be treated as a separate source.

---

In an unofficial tweet on a similar subject, lead rules designer Jeremy Crawford discussed the process of resolving the save on a Solar's Slaying Longbow which also features a two-part damage/effect combination:

Slaying Longbow. Ranged Weapon Attack: +13 to hit, range 150/600 ft., one target. Hit: 15 (2d8 + 6) piercing damage plus 27 (6d8) radiant damage. If the target is a creature that has 100 hit points or fewer, it must succeed on a DC 15 Constitution saving throw or die.

Crawford explains the way to handle the extra effect rider on the bow attack thus:

Slaying Longbow sequence:

1. Hit

2. Deal damage (apply any resistance)

3. Make saving throw if the target has hit points ≤ 100

Extrapolating this interpretation to the Wyvern or other compound damage attacks requiring a save: if the target is reduced to zero hit points by the first set of damage and then fails the save, the additional damage would cause a failed death saving throw because it happens after the first set of damage is calculated and applied as a result of the poison getting into the wound created by the weapon (or claw/fang, whatever).

---

However, I think it would also be a reasonable house rule to have the player make the Con save before damage was announced and then present all the damage at once to obviate the failed death save.

Answer 2

They are different sources of damage and cause separate death saving throw failures

This is because the piercing damage happens on a successful hit and the poison follows a saving throw - two separate mechanics so two separate sources of damage.

In contrast, a spell like Meteor Swarm is a single source despite doing two types of damage.

So, yes, unless the unconscious creature is immune to piercing or poison damage one critical hit from a wyvern will kill them if they are already at 0 HP.

Answer 3

The damage from a Wyvern's Stinger attack counts as one source of damage

Per the Basic rules on Damage:

Each weapon, spell, and harmful monster ability specifies the damage it deals. You roll the damage die or dice, add any modifiers, and apply the damage to your target.

The damage is dealt upon a hit by the attack. The constitution save merely allows a creature to mitigate the effect, reducing the poison damage by half. This is a modifier to damage (in that it modifies the result of a dice roll), and therefore is factored in before the damage is applied (all at once).

As a similar example of multiplicative modifiers, Damage and Resistance (emphasis mine) reads:

Resistance and then vulnerability are applied after all other modifiers to damage.

The fact that the modifier is conditional (and causes a roll) does not cause any change to the overall sequence to Resolve the attack:

You make the attack roll. On a hit, you roll damage, unless the particular attack has rules that specify otherwise. Some attacks cause special effects in addition to or instead of damage.

The attack does not make any exception to rolling damage, it simply dictates that there is a modifier to some of that damage contingent on a saving throw. All of the effects of this attack are resolved simultaneously and applied to the target, as one source of damage.