Another part of my houserules for the gentle reader to consider.
And another Hackmaster 4e relic
Arguably the best tools for a DM help him describing the situation the characters are in and happen to work by just looking at what's already there. I like my games as gritty and dirty as your next favourite Die Hard movie and I like giving my players the feeling they can shine in the most desparate situations without throwing encounter after encounter at them. So, I thought, an element of exhaution, like the rules for Fatigue in Hackmaster, would do just fine in my game.
How to implement Exhaustion in an D&D game (a proposal)
Take half your Con as Endurance. In a fight you loose one point per round, with a critical hit you loose as much Endurance as you've lost hp. If you play with skills, you may allow players with a related skill to have their full Con as Endurance (alternatively you could give only fighters full Con or whatever tickles your fancy). Ten minutes of resting and breathing hard after a fight regenerates all but one point of your Endurance. This is cumulative until you have a full rest.
What happens if you hit zero 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. After that the character goes down, breathing hard and only able to crawl.
Monsters and Endurance
As easy as: 4 + Monsters HD as written (example (Goblin): 4 + 1 HD-1 = 4 Endurance)
Next up: Die Hard 0e (or: Exhausted Characters and Skills)
There is a lot more to say about Endurance in a game, but this is another post entirely...