I just want to give this a clear write-up, adding some of the newer ideas regarding the system. After all, writing rules in a concise way makes them so much more accessible (and I have to admit, it's something I have to work on). This is about Endurance and skills (which are connected). I'm pretty happy with how those house rules work by now, but I never came around to write a short summary with all the changes and connections like I did for Weapon Mastery (Basic) a few weeks ago. I'll also link to the older versions of this house rules, but this here will be complete. So let's try this again...
Endurance and Exertion
Characters have CON -4 as Endurance (the endurance skill changes that, see below). Monsters and NPCs have 4+HD as Endurance*.
Basically every action a character could make, costs 1 point Endurance (a fight, an ability check, a skill check, etc.). Travel costs Endurance per mile depending on terrain** and weather. Only a full nights sleep or some sort of magical healing will recover those lost Endurance points.
Strenuous actions that need consecutive die rolls (like fighting or fleeing) exert a character faster. During the strenuous action they loose 1 point Endurance every round due to exertion. As they reach 0 Endurance, they can force themselves further with a successful CON-check (-4) or a skill-check (Endurance).
With zero Endurance a character counts as Exhausted. He is last in initiative and (playing with Rules Cyclpedia) his Weapon Mastery is down to Basic. MUs have to make an Int-check when casting or the spell (mana, whatever) is lost. Every round after that gives a cumulative -1 to AC, to-hit and check until it reaches the characters negative Endurance value (so if a character had 7 Endurance, his negative Endurance value would be -7).
After that the character goes down, breathing hard and only able to crawl.
The accumulated Endurance loss counts as basic exertion (a character had a busy day with already 4 actions, so for the next strenuous action the Endurance countdown starts with a -4). As soon as the strenuous action is over, the Endurance loss due to exertion is regenerated and all the character lost, is the one point because of the action (so the busy character in the example above would be at a -5 after the next fight).
If a strenuous action were to happen immediately after another strenuous action (fleeing after a fight, etc.), the DM is to decide how much breath the characters were able to catch again. An easy method to simulate this would be a roll with a d8 (or higher) with a threshold of 6 (lower that threshold by 1 for every 2 points a character is below zero). If the character rolls below 6, their Exertion is already recovered, if they roll 6 or more, that's the number of Endurance they had recovered before they have to exert themselves again***.
[Edit]: In addition to the damage due to a critical hit, the victim also looses 1d12 Endurance.
It should be very easy for a DM to keep track of those numbers for the players.
Skill Resolution System
Gathering skills is handled like the Rules Cyclopedia suggests:
"All 1st level characters start with four "blank" skills, often called slots. (When a skill is chosen, it stops being a slot.) Characters who have an Intelligence of more than 12 start with more than four skill slots. If the character has an Intelligence of 13-15, he gets 1 additional skill slot; if his Intelligence is 16-17, he gets 2 additional skill slots; and if his Intelligence is 18, he gets 3 additional skill slots." (Rules Cyclopedia, p. 81)
Characters get another skill slot every 4 levels. Skills are connected to ability scores. Which ability score is relevant at the time depends on the circumstances the skill is used under (a magic-user could intimidate another magic-user using INT instead of STR, etc.). The basic skill resolution is:
ABILITY SCORE + 1d20 versus DIFFICULTY
Unlearned skill use will get a penalty of -4 or more. At best players are unaware of the difficulty they're rolling against.
Skill checks and Exertion
If a skill check is not successful a player might decide to exert his character to get there. The difference between the failed roll and the difficulty (say the difficulty was 30, the characters ability score was 13 and the rolled a 5, exerting himself will cost him 12 points of Endurance to make it). This will cost him one additional point Endurance to the one for the action itself. This applies to every skill check (even searching a library or reading can be exerting).
Here, too, a character can force himself below zero Endurance with a successful check on CON (as the skill, CON -4 without the skill). See above for further penalties.
Skill Mastery
The abilities/skills a Thief
already has are:
Backstab (melee),
Backstab (ranged),
Acrobatics, S
leight of Hand,
Stealth (moving, hiding, keeping a low profile or doing something like picking a lock in total silence...) and
Sabotage.Using the skill system of the Rules Cyclopedia, every class has access to all skills but backstab (which even for the thief is only activated if chosen for skill mastery).
At character creation a player of a thief may now choose 4 abilities/skills for Skill Mastery. So skills like Disguise or Alchemy are, too, relevant for Skill Mastery if the thief chooses to do so (but backstab is only activated if chosen, which might be done at higher levels, of course).
Basic Skill Mastery is one additional D6 either for the skill check or as backstab damage. Every 3 levels a Thief gets one more point Skill Mastery to distribute. Progression is: Basic +1D6, Skilled +1D8, Expert +1D10, Master+1D12 and Grand Master +1D20.
So I hope this helps in understanding how I integrated those rules into D&D without bloating the system. The example I wrote about killing a horse should be much more transparent now, too (it's still pretty accurate, the only changes I made here were for characters...). It should be compatible with most (if not all) D&D variants up to (but not including) 4E.
A complete list of skills is on my to-do list.
*As an alternative one could roll a d6 instead the 4, interpreting the result as something between -1 and 5 to simulate activities of the day of an encounter. So it'd be 1d6 + HD...
**Suggested Endurance loss per mile:
Travel Mode Trail Clear Hills Mountains Desert
Foot 1/3 1/2 1/1 1/1 1/1
On a horse 1/12 1/10 1/10 1/8 1/4
Horse, riding 1/6 1/4 1/3 1/2 1/1
Horse, war 1/5 1/3 1/2 1/2 1/1
*** Using a higher die (a d10, a d12 or a d20) would allow for a higher regeneration, but the chances to roll below 6 get worse the higher the die.