Death Saving Throw
A Death Saving Throw is a special type of Saving Throw that a Downed creature must make to avoid dying. Typically, Death Saving Throws are only performed by player characters, Companions, and certain notable NPCs. Most enemy creatures simply die when reduced to 0 Hit Points.
Rolling Death Saving Throws
When an eligible creature is reduced to 0 Hit Points, they fall unconscious and become downed. While downed, the creature must use their full turn to roll one Death Saving Throw per round.
The die rolled is a 
d20 and the Difficulty Class of the roll is 10. A roll of 9 or lower results in one failed save, and 10 or higher results in one successful save. A roll of 1 results in failure, regardless of the modifiers to the roll.
- Bonuses to Saving Throws from conditions (e.g. Bless or Aura of Protection) and bonuses from items that specifically add to Death Saving Throws (e.g. Family Ring) are added to this roll.
- Proficiency Bonus is not added to the roll, as you cannot be proficient in Death Saving Throws.
- An Ability Score Modifier is not added to the roll as Death Saving Throws are not tied to any Ability Score.
- Bonuses to generic Saving Throws from items (e.g. Safeguard Shield) are not added to the roll, as of Hotfix #6.
If the downed creature rolls a total of three failures, they die. If the creature rolls three successes before rolling three failures, they become Stable - though still unconscious - and no longer need to make Death Saving Throws.
Be wary that the stabilized creature still retain their current amount of failed saves, meaning if they only failed once and stabilize from 3 successful saves, 2 ranged attacks or 1 melee attack are required to kill the stabilized creature, these death saving throw values are immediately reset to 0 upon restoring health to the stabilized creature in any means necessary since they are back in the fight.
Healing a Downed Creature
At any time while downed, if a creature is Helped or healed by another creature, they immediately lose the downed condition, return to consciousness, and no longer need to make Death Saving Throws, however, they can only take a Bonus Action for this turn.
Note that granting a creature Temporary Hit Points is not the same as healing them, and does not remove the downed condition.
Receiving Damage when Downed
Whilst downed, if a creature receives ranged damage (either magical or martial), they automatically fail one death saving throw per damage application (for example, 3 separate Magic Missile hits would cause the target to fail 3 saves and instantly die).
Critical hits against down targets results in two failed saves, melee attacks are always critical hits against a downed target.
Reviving the Dead
A dead creature cannot be healed by normal means and can only be resurrected by powerful magic, such as Revivify.
Gallery
An example of something impossible
