Another D&D Thing to Use: Major Injury Table

If a player character of level 3 or above takes more than half of their hitpoint worth in one strike of damage, they must make a Constitution saving throw of DC 15 or suffer a potentially mortal wound. A failed save uses its total result to determine the wound sustained.

Major Injuries Table

All wounds sustained in this manner require extended recovery through non-magical means and are likely to leave permanent markers or scars.