Most old-school systems didn’t have rules for exhaustion, but exhaustion is useful for reducing heroes to jelly without explicit damage. If your system uses hit points, characters will start with very few and eventually gain a lot, so just docking hp isn’t very helpful. Here’s a way to handle it.

At each level of exhaustion, you incur a penalty to all d20 checks: attacks, skill checks, and saving throws. There are four levels of exhaustion:

1. -2 to d20 checks (or whatever your core mechanics are).
2. -4 to d20 checks. Movement limited to jog (2x movement rate).
3. -6 to d20 checks. Movement limited to walk (normal MR). Spellcasters: save vs spell to cast spells greater than cantrips.
4. -8 to d20 checks. Movement limited to creep (half MR). Reduced to 1 hp. No spellcasting. Save vs death every hour until reaching a safe location or die.

You can be saved from exhaustion (one level each) by healing magic stronger than Cure Light Wounds or by reaching a safe location with air, food, water, etc. and resting. The reason for the exhaustion determines how long it takes to recover each one naturally:

• Lack of air (suffocation): 1 minute per level.
• Injury: 1 hour per level.
• Lack of water (thirst): 1 day per level.
• Lack of food (starvation): 1 week per level.