Dropping to 0 Hit Points
When a creature drops to 0 hit points, it is vulnerable to dying or might die outright. An attack that reduces you to 0 hit points, and any damage you suffer while at 0 hit points, reflects an actual injury that could lead to death.
Instant Death
Here are the main ways a creature can die instantly.
Monster or NPC Death. A monster or NPC dies the instant it drops to 0 hit points. Optionally, the DM can instead give the creature the dying condition.
Hit Point Maximum of 0. A creature dies if its hit point maximum reaches 0. Certain effects drain life energy, reducing a creature’s hit point maximum.
Death Saves. You die immediately if you gain a third failed death save.
Dying
If you are reduced to 0 hit points and are not killed, you are dying and must make a death save (described below). While you are dying, you can still act. However, additional damage can kill you and you need to make death saves. You are no longer dying if your hit points increase above 0. Alternatively, you can choose to fall unconscious.
Falling Unconscious. When you are reduced to 0 hit points, you can choose to fall unconscious due to the shock and pain. You are unconscious and dying until you complete a short rest. During that time you do not make death saves on your turn but still make death saves when you take damage. Only an effect that removes unconsciousness can bring you back to action. Healing works as normal but does not wake you up. Otherwise, you regain consciousness when you finish a short or long rest.
Death Saves. You make a death save in three situations:
When you take damage that reduces you to 0 hit points.
When you begin your turn with 0 hit points and are dying.
Each time you take damage while you have 0 hit points and are dying.
Roll a d20 to make a death save, a special type of saving throw that does not use an ability modifier.
If you roll a 20 on the d20, you regain hit points equal to half your maximum hit points.
If the roll is 10 or higher, you gain a successful death save.
If the roll is less than 10, you gain a failed death save.
If you roll a 1 on the d20, you gain a failed death save and are dazed on your next turn.
While you have any failed death saves, you are obviously injured. You move gingerly and wince when others jostle you.
Three Successful Death Saves. When you gain your third successful death save, you immediately regain hit points equal to your level.
Three Failed Death Saves. When you gain your third failed death save, you die. If you gain it during your turn, you die at the end of your turn but cannot regain hit points that turn.
Losing Successful Death Saves. You lose all successful death saves when you are no longer dying.
Losing Failed Death Saves. You lose some, but potentially not all, failed death saves when you finish a long rest.
Other Ways to Gain Death Saves. Some special effects, such as traps, might inflict failed death saves. These effects typically bypass your hit points by striking when you are helpless or otherwise unable to defend yourself.