On reaching 0 HP
Once a character reaches 0 HP, they are unconscious, and starts rolling death saving throws on their turn. (See page 197 of the Player's Handbook.)
When rolling Death Saving Throws
When making death saving throws, there are 4 categories of results that we care about: rolling a 1 on the d20, rolling a total between 2 and 9, rolling a total between 10 and 19, and rolling a natural 20 on the die.
- If you roll a 1 on the d20, you automatically mark 2 death save failures (when you reach 3, you die)
- If you roll a total between 2 and 9, you only mark a single failure
- If you roll a total between 10 and 19, you mark one success (once you have 3 successes, you become stable)
- If you roll a natural 20, you immediately regain 1 HP (causing you to regain consciousness)
Once a character becomes stable, they remain unconscious for 1d4 hours before regaining 1 HP.
If a character receives healing while unconscious (whether stable or not), they wake up immediately, though still prone.
Once a creature is stable or has received healing, they reset their number of death saving throw successes/failures.
Unconsciousness and combat
If an unconscious character is attacked, the attacker will have advantage on the attack (if 5 ft. away) from the prone condition; if it hits, it will be an automatic critical hit because of the unconscious condition which is 2 death save failures.
If a stable creature takes damage, it has to start making saving throws again.