In the past, I have alluded to the fact that I continue tinkering with rules to produce my own version of the ideal OSFRPG, drawing mostly on B/X, Holmes, and Swords & Wizardry, while adding in house rules I've used over the years, my own new ideas (wand spells!), and anything cool and relatively rules-lite from any OGL source I might find helpful or adaptable, including Pathfinder. Usually these rules documents get shared with friends, experimented with, and put away in files against The Day of the Great Integration. But A Paladin in Citadel recently complained about the lack of fatigue rules in D&D, so I was jarred into sharing what I had been working on, below.
Hit Points (HP) are an overall measure of how long one can stand up to the demands of combat. When your character reaches 0 hit points, your PC is exhausted. Exhausted characters (1) lose their Dexterity bonus to armor class, (2) begin taking Constitution damage, and (3) have to make a Con check at the beginning of each round to successfully act.
HP AS FIRST LINE OF DEFENSE, CONSTITUTION AS FINAL LINE OF DEFENSE
At 0 HP, the character may choose to continue the strife or retreat, but any additional damage taken will come directly off of the PC's Constitution score. On every round after reaching 0 HP, the player must roll a Constitution check. Let's say the player Cassie is playing her character Bork the Barbarian, Fighter 3. In combat, Bork reaches 0 HP and has a phenomenal Con of 18. The next round, Cassie rolls a d20 and the result is a 16. Having passed the check, Bork may take only one regular action: move or attack (or cast, if Bork had been a magic-user1). Bork attacks and deals damage to the opponent, but takes 5 points of damage in the ongoing melee, bringing the PC's Con score to 13. Next round, Cassie rolls the check and rolls really low: a 3! Bork continues the fight, but deals no damage. However, the monster deals two points of damage, bringing Bork's Con to 11. Next turn, Cassie rolls the check to see if the Bork can take an action. The roll is 12. This round, heaving from the strain, Bork is unable to successfully accomplish anything due to his exhaustion. (The GM might narrate Bork attempting the intended action something like: “The arc of Bork's great club is so sluggish, the bugbear chief has plenty of time to duck the swing.”) Unfortunately, Bork continues to take damage. This round, he gets smacked for 7 points of damage, bringing the PC's Con down to 4. Rolling a 4 at the beginning of the next round, Cassie decides Bork will get out of the fight while the getting is good! If Cassie had allowed Bork to reach Con 0, he would have died.
For every one hour of rest, the PC gains back the number of HP equal to one's current Con score. If the PC has undergone Con damage, one uses the temporary Con score. To continue our example from above, one hour of healing would get Bork 4 hit points.
Sleeping a solid 8 hours, in a relatively comfortable environment will heal 8 x Con hit points. If Con damage has been suffered, then those eight hours would also heal one lost Con point. So building on the example above, Bork would gain 24 HP back and have a Constitution of 5. However, the maximum number of HP than can be regained must take into account the number of those hit points that were Con bonus. In Bork's case, nine points of the character's original HP total remain out of reach until Con levels are regained that enable the restoration through Natural Healing of bonus HP tied to Con. Fourteen nights of sleep back at the Green Dragon Inn are in order, if they can be had. If the adventure presses and time is precious, the player will weigh the need for Con and HP restoration against the pressure of time and the threat of failure.
First level healing spells (i.e., Cure Light Wounds) heal only hit points and not Con damage. Higher level healing spells heal Con damage first, then any hit points.
1Note, however, the dangerous synergy of Con damage with the desperate Con for casting rule. If a magic-user is out of spells and chooses to cast a known spell from a spell-book by sacrificing Constitution points (one point per spell level), the magic-user can die very quickly.