When a character is reduced to 0 HP or fewer, but not to a negative value equal to or greater than their total HP, they are considered Incapacitated. If the character receives enough damage that they are reduced to a negative value equal to or lower than their max HP, they are killed outright.
For example, a character with a current and max health of 1 Heart receives a critical hit from an enemy that punches through their armor and does 18 damage. This takes their HP from 10 to -8. The character is not killed outright because the damage did not get to -10 or lower, but the character is considered to be Incapacitated.
When Incapacitated, the situation is dire. The character immediately makes a Death Roll using a d4. The result is the number of rounds remaining until they die. The player may choose to use their Water modifier as the number of rounds instead, but this must be decided before rolling the d4 and if they decide to roll, the roll must be used. As a last ditch effort to live, at the beginning of each of the Incapacitated character's turns after the Death Roll, they may attempt a Miraculous Recovery. Roll a d20 and if it comes up a natural 20, they recover with 1 HP. An ally may also perform a Stabilization Action on them. Once a character is Stable, they stop making Miraculous Recovery rolls.