Immune: undead, constructs, elementals
A fatigued character can neither run nor charge and takes a –2 penalty to Strength and Dexterity. Doing anything that would normally cause fatigue causes the fatigued character to become exhausted. After 8 hours of complete rest, fatigued characters are no longer fatigued.
Marius, Scholai Armsmaster Says: Though the -2 to strength and dexterity can be problematic, it is the inability to run or charge that makes fatigue a potent weapon. Combined with difficult terrain and hit-and-run tactics, there are it is easy to outmaneuver a fatigued opponent. Necromancers excel at fatiguing their enemies with spells like Ray of Fatigue and Waves of Fatigue. As an area of effect spell with no saving throw, the latter spell can easily cripple an entire squad of enemies.