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.
Best Answer
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:
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):
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:
Crawford explains the way to handle the extra effect rider on the bow attack thus:
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.