Physical Damage

When a creature takes damage, they roll Fortitude against the damage taken.

Perfect success: ignore the damage.
Mixed success: no major effect, but add 1 Wound.
Failure: disabled (remember the DC).
Critical Failure: disabled and unstable.

If you're disabled, you can use Fortitude to become active again (although most minions won't). Each attempt adds 1 Wound.

Wounds

Wounds apply a penalty to Fortitude saves against damage (1 to 1). They take time to heal (1 per long rest, or more with medical care).

Disabled

If you're disabled, you can no longer take most actions. You can move at half speed (provoking), you can communicate, you can take purely mental actions. You can attempt other actions, but must pass a Fortitude check against your last DC, or else gain 1 Wound and become unstable.

Unstable

When unstable, you must pass a Fortitude check every round:

Critical Success: self-stabilize
Success: nothing happens
Mixed: gain 1 Wound
Failure: gain 1 Wound, become unconscious

After Combat

All wounds are unstable on long enough time scales. After combat ends, all disabled characters use the Unstable rules, but with 1 minute increments.

Stabilization

Medical care can stabilize an Unstable character. Each check takes 1 minute, and the Medicine check replaces the subject's Fortitude check (although the subject can Help Out). Equipment can add bonuses.

Attacking the Wounded

Cruel attackers may assault the disabled or dying. Apart from the obvious penalty of not being able to use normal defensive abilities, this doesn't change much about the process of suffering damage--the target still rolls Fortitude as normal. If they become disabled (or worse), and the new DC is higher than the old, that replaces the old DC.

Critical Hits

TBD. For now just assume double damage, which makes the Fortitude check much harder to pass.

Injuries

An alternative to becoming disabled at a bad time, PCs who have failed a Fortitude check can exchange the short-term penalty of being disabled (or worse) for a long-term problem that leaves them in fighting condition.

Quick guidelines:

Can exchange damage for Ability Damage at a rate of X per Y
Can exchange entire attack for single Injury; higher DC means more serious injury

Major villains can do this too.

Ability Damage

An alternate form of damage, ability damage directly damages your abilities. It's fairly simple: if you take 5 Strength damage, your strength decreases by 5, correspondingly altering your modifiers, etc.

Ability damage heals in the same way as Wounds.