Once a character reaches 0hp, they are knocked out and gain the "Unconscious" condition. A creature cannot go bellow 0hp either. While knocked out, your initiative moves to right before the creature or effect that knocked you out.
Additionally, unless the attack was nonlethal, the character also starts dying. The character gains "Dying #" condition. The degree of dying depends on a few things
If you take damage while you have the dying condition, increase your dying condition value by 1 (or 2 if a critical hit or critical failure brought you to the dying condition)
If a dying character's HP is ever brought up to 1 or more HP, they lose both the "unconscious" and "dying" conditions. Alternatively, if a dying character is stabilized either through the spell, with the stabilize medical action, or by doing recover checks; they remain at 0 hp and keep the "unconscious" condition however they do lose the "dying" condition
Anytime (unless specifically noted) a character loses the dying condition, they get a "wounded 1" condition. If they already have the condition, the value increases by 1.At the start of the characters turn (which is moved to right before the triggering creature or effect's initiative that caused the dying condition), the character must make a flat check (no modifiers, just a straight roll) with a DC equal to 10 + their current dying value to see if they get better or worse. The effects of this check are:
Critical Success (nat 20): Your dying value is reduced by 2.
Success: Your dying value is reduced by 1.
Failure: Your dying value increases by 1.
Critical Failure: Your dying value increases by 2.
If you have at least 1 Hero Point (Core: pg 467), you can spend all of your remaining Hero Points at the start of your turn or when your dying value would increase in order to return to 1 HP, no matter how close to death you are. You lose the "dying" condition and become conscious. You DO NOT gain the wounded condition (or increase its value) when you perform a heroic recovery.
After you die from reaching your maximum "dying" or "doomed" condition, you cannot act or be affected by spells that target creatures (unless they specifically target dead cratures), and for all other purposes you are an object. When you die, you are reduced to 0 HP if you had more than 0 HP (ie: died from "doomed" condition), and you cannot be brought above 0 HP as long as you remain dead. Some magic can bring creatures back to life, such as the resurrect ritual or the raise dead spell.
Some spells and abilities can kill you immediately or bring you closer to death without needing to reduce you to 0 HP. These abilities have the death trait and usually involve negative energy. If you are reduced to 0 HP by a death effect, you are slain instantly without needing to ready "dying 4". If an effect states it kills you outright, you died without having to reach "ding 4" or 0 HP
If you ever take damage equal to or greater than double your maximum HP in one blow, you die instantly