This question already has an answer here:
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):
- If the lance's damage renders the creature unconscious, and they then fail 3 death saves without taking further damage
- If the lance causes a creature to fail its last death save
- 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
- 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
- If the lance caused massive damage to a creature, killing them instantly