Injuries are penalties imposed on the healing of hit points when a great deal of damage has been delivered to a combatant in a single attack.
When a combatant suffers sufficient damage to reduce them from more than half their hit points above zero to less than zero in one strike, then the combatant is considered to be injured - unless the total damage done is less than 12 points total. For example, if Caleb the mage has 6 hit points total, then takes 7 points of damage (sufficient to lower Caleb to -1 hp) then Caleb is not injured because the total damage done was less than 12.
However, if Bala the Druid has 17 hit points total and is presently at 9 due to damage she has already suffered, then takes 12 points of damage (sufficient to lower Bala to -3 hp), then Bala would be injured.
For every 12 points of damage received, the injury tally is counted at 1 point. Thus 24 points of damage would mean a 2-point injury, 36 points of damage would mean a 3-point injury and so on.
Note that 11 or more damage received in a given attack is sufficient to cause a wound. But injuries are not wounds! An injury does not occur merely because 12 damage has been done. For the injury to occur at all, the combatant must be hit hard enough to eliminate half their maximum hit points. For higher level characters, particularly fighters, it may require as much as 40, 50 or 60 damage for an injury to occur. The tally only refers to dividing this ultimate amount of damage by 12.
A combatant that has been injured does not sustain any additional damage due to that injury. However, the amount of time necessary to recover from the injury is dependent upon the injury tally. A 1-point injury requires that 10 points of equivalent healing must be gained before the first hit point after damage can be raised by one point.
In the example above, Bala has a 1-point injury that dropped her total hit points to -3. In order for Bala to restore her hit points to -2, Bala must heal 10 hit points of equivalent damage. In effect, the injury that Bala has sustained has stretched the healing distance between -3 and -2 hit points by a multiple of 10.
Let us take another example. Albert is a 7th level cleric with 42 hit points. While climbing up the side of a mountain, Albert slips and falls, taking a total of 46 points of damage. This reduces Albert's hit points to -4, all at once. And because the damage sustained is more than half his total, Albert is injured. Because he is at -4 hit points, he must roll to see if he is conscious (see Negative Hit Points). Albert makes a wisdom check, as described on the negative hit points page, and fails by rolling a 14.
From the fall, Albert is also wounded, which means that he has injured himself in some way that he is suffering -4 hp damage per round from the 46 damage he received. If he were laying on the ground all alone, he would bleed out and be dead in two rounds - and there is nothing Albert can do about this, because he is unconscious. However, lucky for Albert, he fell near to Erick, a 1st level paladin, who quickly runs to Albert's side and lays hands for 2 hp as soon as he can. He can't quite get there before Albert bleeds for one round, now reduced to -8 hit points. However, the bleeding is stopped and Albert's life is saved.
Albert is not, however, restored to -6 hit points by Erick's laying on of hands. Albert was injured. He sustained a 3-point injury . . . so to be restored to -7 hit points (one above the score he has right now), Albert must gain a total of 30 points of healing. Erick has given him 2. Albert needs 28 more. Once Albert has been restored to -7 hit points, he can begin healing damage normally.
If Albert had died, then been restored by death's door, then his body would still have to be healed those same 30 points, as the body would retain the injury, even though Albert would be restored to zero hit points.
Type of Injury
At this time, I have not created any specific tables for what is injured. The rule is still in its infancy - and since in most cases the injured person will be in the negatives and laid up, a flat rule that it is one or the other arm is sufficient. If the rule works and does not overly tax the patience of my players, then I will work up a injury table specifying what type of injury occurs.