Death and Dying
Dropping to 0 HP
When you drop to 0 hit points, you either die outright or fall unconscious, as explained in the following sections. When an ability score drops to 0, you die.
Instant Death
Massive damage can kill you instantly. When damage reduces you to 0 hit points and there is damage remaining, you take negative HP. If your negative HP is ever -10, you are no more.
A Cleric with a maximum of 12 hit points currently has 6 HP. If she takes 18 damage from an attack, she is reduced to 0 hit points, with 12 damage remaining. Because the remaining damage is greater than 10, she dies immediately.
Dying
If damage reduces you to 0 hit points and fails to kill you, you fall unconscious and are Dying. Dying creatures cannot act, and if in combat or a dangerous situation, begin making death saving throws instead. Death saves continue until you are dead, alive, or stabilized.
Death Saving Throws
Roll a d20 and add (or subtract) your CON bonus. If the total is 10 or higher, you cling to life. If you fail, you have one more chance to avoid a long nap. On your second failure you die.
Rolling a 20: When you make a death saving throw and roll a 20, you take System Shock and regain 1 hit point.
Healing
The best way to save a creature with 0 hit points is to heal it. Any magical healing will automatically stabilize a dying creature and bring them up to 0 HP. If the amount of HP healed was enough to bring them above 0, they take System Shock but are able to act.
Stabilizing
Once out of combat, the party can take a turn to stabilize a dying ally. The ally remains at 0HP and unconscious for 1d6 turns. When they awaken, they have 1HP and take System Shock.
System Shock
Any PC who drops to 0HP and survives takes 1d6 - CON slots of System Shock.