https://monarchsfactory.wordpress.com/2021/09/29/major-injuries-update/

To use, select the "Major Injuries" extension when loading your campaign; then in the Options under House Rules, make sure the "Combat: Massive system shock" is Off, and "Combat: Major Injuries" is On. You will also need to activate and load the "Major Injuries" module.

When a PC takes a single hit for at least half their max HP, they must make a DC 15 save (usually Constitution, but some damage types use other attributes) or receive a major injury.  Injury effects are specified in damage-specific tables (e.g. "Major Injuries - Bludgeoning").