When is damage considered to have "killed a creature"? [duplicate]

Question

The Narzugon's Hellfire Lance states:

Hellfire Lance. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 11 (1d12 + 5) piercing damage plus 16 (3d10) fire damage. If this damage kills a creature, the creature's soul rises from the River Styx as a lemure in Avernus in 1d4 hours. If the creature isn't revived before then, only a wish spell or killing the lemure and casting true resurrection on the creature's original body can restore it to life. Constructs and devils are immune to this effect.

Notably, this wording is different from "if this damage reduces a creature to 0 hit points" effects.

When is the Lance's damage considered to have "killed a creature"?

A few situations I can think of (intended as illustrative examples of boundary cases, I'm not expecting an answer to address all of these):

1. If the lance's damage renders the creature unconscious, and they then fail 3 death saves without taking further damage
2. If the lance causes a creature to fail its last death save
3. If something other than the lance's damage rendered a creature unconscious, but then an attack from the lance causes the creature to fail 1-2 death saves, and they fail the last one naturally
4. If the lance's damage rendered a creature unconscious, but then damage from another source causes the creature to fail two death saves, and they fail the last one naturally
5. If the lance caused massive damage to a creature, killing them instantly

Answer 1

Damage from the lance has to be the killing factor. There are three ways for this to happen.

1. A hit from the lance causes the third failed death save.
2. The excess damage from the lance is greater than the creatures max HP. Thus, causing instant death. (Current HP - Damage < neg max hp)
3. In the case of creatures that do not get a death save, death occurs when they are reduce to 0 HP.

If the creature dies from naturally failing the third death save, then it dies from bleeding out, not from the lances damage. If it was intended to work as in the other cases, it would say something like "if the target dies within an hour of taking damage.."

Note: A related tweet by Jeremy Crawford.

Answer 2

As you've noticed, the path from "damaging" to "killing a creature" isn't always clear. Certainly, if the damage causes the last death saving throw failure or causes death by massive damage, it would mean "this damage kills a creature". In the other cases you mention where there may be a bit more uncertainty, if the damage from the Lance caused the last blow to make the creature fall unconscious, or caused damage during the time it was unconscious, I would probably rule that the effect also applied, as I assume that's the intent. I certainly would understand if a different DM that tended to be a but more literal with such things ruled differently, though.

Taking a step back at the bigger picture, I assume the Lance's ability is intended to change the usual effect of death, that's described in the DMG p. 24, under "Bringing Back the Dead":

When a creature dies, its soul departs its body, leaves the Material Plane, travels through the Astral Plane, and goes to abide on the plane where the creature's deity resides. If the creature didn't worship a deity, its soul departs to the plane corresponding to its alignment.

This can be a bit vague (what exactly counts as "worship" of a deity?), and could certainly be changed depending on the details of your campaign's cosmology and theology. The Lance's ability changes that general rule, but it can be hard to give details because any of the details are dependent on how your campaign generally deals with death.

Hopefully, the DM would use the ability in whatever way helps tell the best story for the group, and that might including making a ruling on whether the effect applies at all based on the whole scenario that's involved. If the Lance causes the vast majority of the damage, but isn't quite the killing blow, it may make sense for the effect to apply anyway, depending on the direction the story should go. Or maybe, based on the legends the characters have heard, they know not to let it make the killing blow, and work to deal the killing blow themselves so that the Lance can't, which might make for a different interesting story. The rules and stat blocks that are given are there to serve your game, not the other way around.

Answer 3

Most DMs consider monsters and minor villains to die when reaching 0 hit points, as seen in the top answer to this question, and in Player's Basic pg 76

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.