Exhaustion is useful for reducing heroes to jelly without explicit damage. At each level of exhaustion, you incur a penalty to all d20 tests: attacks, skill checks, and saving throws. There are four levels of exhaustion:

1. -2 to d20 checks.
2. -4 to d20 checks. Movement limited to jog (2x).
3. -6 to d20 checks. Movement limited to walk (normal). Spellcasters: save vs spell to cast spells greater than cantrips.
4. -8 to d20 checks. Movement limited to creep (half). 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; each day of rest reduces your exhaustion by one level.