Fatigue is a measure of exhaustion. It's built up through actions that cause Encumbrance, such as attacks and spellcasting, but certain attacks or magical abilities can add Fatigue in large amounts. Most non-Divine spells also add large sums of Fatigue to the caster. Unlike damage to Hit Points, it is not innately lethal.

• Every 10 Fatigue a unit has, reduces their effective Defence Skill by 1.
• Every 20 Fatigue a unit has, reduces their effective Attack Skill by 1.
• At 50 Fatigue or more, a unit is three times more likely to suffer an Armor-defeating hit (unless they're already immobile).
• At 100 Fatigue or more, a unit is unconscious; they cannot move, and they are helpless.
• A unit cannot have more than 200 Fatigue. Any Fatigue beyond 200 is converted to HP Damage with a Fatigue * 2% chance. For example, say a 5 Tuatha of Eriu opens a battle by casting Dreamwild Legion. He has 7 Spellcasting Encumbrance. If he spends only 4 to cast the spell, the cost is 407 Fatigue; starting with zero Fatigue, he ends up with 200 Fatigue and takes 4 damage [(400 - 200) / 50] from the recoil, with a 14% chance (7 * 2%) to take 5 damage instead. If he uses 5 instead, the cost is 207 Fatigue; he still ends up with 200 Fatigue, with a 14% chance to take 1 damage, but there's no guaranteed damage.
  • For Communion or Sabbath Slaves, they always take 1 HP damage if they would take Fatigue beyond 200 when assisting in a spell. This replaces the standard damage formula, however. If they would take Fatigue beyond 250 through assistance (such as taking 56 Fatigue while at 195), the regular formula is used.

1 point of Fatigue is removed from each unit every combat round. Unconscious units lose 5 points of Fatigue instead. Units with points of Reinvigoration lose that much more Fatigue each round.

Drain Damage saps energy from the target and removes Fatigue from the attacker/spellcaster, equal to twice the damage done. Note that most Drain Damage from weapons is "partial", limited to removing only 10 Fatigue at most.